Article

Exchange bias identifies lamellar magnetism as the origin of the natural remanent magnetization in titanohematite with ilmenite exsolution from Modum, Norway

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Abstract

Large and stable negative magnetic anomalies in southwestern Sweden, southern Norway, the Adirondacks, USA, and Quebec, Canada, are related to rock units with a magnetic fraction consisting primarily of ilmeno-hematite or hemo-ilmenite. It has been suggested that the unusual magnetic stability of these rocks results from lamellar magnetism. This is a type of magnetic remanence, carried by uncompensated magnetic layers at interfaces between nanoscale exsolution structures of antiferromagnetic (AFM) hematite and paramagnetic ilmenite. Here we present the first direct proof that this lamellar magnetism indeed is responsible for the natural remanent magnetization (NRM) of a rock from Modum, Norway. Our argument expands a previous observation, that, in mineral grains from this rock, the cooling of a positive-induced remanence from room temperature to 5 K – which is well below the ordering temperature of ilmenite (57 K) – leads to a large negative shift of the low-temperature (LT) hysteresis loop. This can only be explained by exchange bias due to exchange coupling across the hematite–ilmenite interfaces. In a different experiment, we now have cooled the original NRM of untreated grains to 5 K, and then measured the hysteresis loop. Again, in several separate grains we observed large shifts of the hysteresis curves. This shows that exchange bias develops also from the untreated NRM. This observation proves that the moments, which carry the NRM, also participate in the exchange coupling at the hematite–ilmenite interfaces. Therefore, the NRM is not carried by defect moments or stress-induced moments, which occur in normal bulk hematite. A closer look at the NRM-induced LT loops shows that exchange bias acts in both field directions, though one direction is clearly predominant. This observation can be interpreted as a frozen equilibrium of different proportions of oppositely directed lamellar moments, a key feature of the original lamellar magnetism hypothesis. We discuss lamellar aggregation, and the formation of exchange-coupled clusters to explain the observed high efficiency of lamellar NRM. We also conclude that remanence carried by lamellar moments should not be used for paleointensity estimates of terrestrial or extraterrestrial material.

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... The 1 billion-yr-old natural remanent magnetization (NRM) of metamorphic rocks from Modum, Norway, creates magnetic exchange bias when cooled in zero field below the ordering temperature of ilmenite Fabian et al. 2008). In two preceding papers Robinson et al. 2016), here referred to respectively as Papers I and II, we studied mineralogy, rock magnetism and NRM-induced exchange bias of a new set of samples (MOD22) from the Modum area. ...
... When the same samples are cooled in a field of +5 T from RT to 5 K before the hysteresis experiment, M(H) has a unimodal peak at negative fields. These experiments, and the vanishing of exchange bias above T N Fabian et al. 2008), suggest that the conditions under which the samples were cooled below T N of ilmenite, and thus acquired an ilmenite remanent magnetism coupled to the room-T lamellar magnetism, are critical in determining the nature and degree of the exchange bias . This indicates that the exchange coupling to the high anisotropy of low-T ilmenite magnetism is responsible for the exchange bias, which agrees with quantitative models (Shcherbakov et al. 2009). ...
... This indicates that the exchange coupling to the high anisotropy of low-T ilmenite magnetism is responsible for the exchange bias, which agrees with quantitative models (Shcherbakov et al. 2009). This link between RT NRM and LT exchange bias proves that the same magnetic units that create the exchange coupling at the hematite-ilmenite interfaces also carry the NRM (Fabian et al. 2008). ...
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Lamellar magnetism is a source of remanent magnetization in natural rocks different from common bulk magnetic moments in ferrimagnetic minerals. It has been found to be a source for a wide class of magnetic anomalies with extremely high Koenigsberger ratio. Its physical origin are uncompensated interface moments in contact layers of nanoscale ilmenite lamellae inside an hematite host, which also generate unusual low-temperature (low-T) magnetic properties, such as shifted low-T hysteresis loops due to exchange bias. The atomic-magnetic basis for the exchange bias discovered in the hematite-ilmenite system is explored in a series of articles. In this third article of the series, simple models are developed for lamellae interactions of different structures when samples are either cooled in zero-field, or field-cooled in 5 T to temperatures below the ordering temperature of ilmenite. These models are built on the low-temperature measurements described earlier in Paper II. The important observations include: a) the effects of lamellar shapes on magnetic coupling, b) the high-T acquisition of lamellar magnetism and low-T acquisition of magnetization of ilmenite lamellae, c) the intensity of lamellar magnetism and the consequent ilmenite magnetism in populations of randomly oriented crystals, d) lattice-preferred orientation of the titanohematite host crystal populations, and e) the effects of magnetic domain walls in the host on hysteresis properties. Based on exemplary growth models of lamellae with different geometries and surface couplings we here provide simple models to assess and explain the different observations listed above. Already the simplified models show that the shapes of the edges of ilmenite lamellae against their hematite hosts can control the degree of low-T coupling between ilmenite, and the lamellar magnetic moments. The models also explain certain features of the low-T exchange bias in the natural samples and emphasize the role of lattice-preferred orientation upon the intensity of remanent magnetization. The inverse link between ilmenite remanence and exchange-bias shift in bimodal low-T ilmenite lamellae is related to different densities of hematite domain walls induced by the clusters of ilmenite lamellae.
... Inquiry into the physical origin of these strong remanences led to the concept of lamellar magnetism (LM; Robinson et al., 2002), which has been theoretically investigated by atomic Monte Carlo and density functional simulations of the magnetic properties of lamellar interfaces (Harrison and Becker, 2001;Robinson et al., 2002Robinson et al., , 2004Pentcheva and Nabi, 2008). The experimental proof that the natural magnetic remanence originates from nanoscale lamellae formed during slow cooling (Fabian et al., 2008) is based on the discovery of giant exchange bias below 50 K in some ilmeno-hematite bearing rocks (McEnroe et al., 2007a;Harrison et al., 2007;Fabian et al., 2008). Exchange bias requires the magnetic moment to be linked to exchange-coupled interfaces which here occur between the host and the lamellae. ...
... Inquiry into the physical origin of these strong remanences led to the concept of lamellar magnetism (LM; Robinson et al., 2002), which has been theoretically investigated by atomic Monte Carlo and density functional simulations of the magnetic properties of lamellar interfaces (Harrison and Becker, 2001;Robinson et al., 2002Robinson et al., , 2004Pentcheva and Nabi, 2008). The experimental proof that the natural magnetic remanence originates from nanoscale lamellae formed during slow cooling (Fabian et al., 2008) is based on the discovery of giant exchange bias below 50 K in some ilmeno-hematite bearing rocks (McEnroe et al., 2007a;Harrison et al., 2007;Fabian et al., 2008). Exchange bias requires the magnetic moment to be linked to exchange-coupled interfaces which here occur between the host and the lamellae. ...
... In combination with bulk magnetic measurements, this allows us to calculate the efficiency of NRM acquisition for a lamellar magnetic material. In agreement with a previous estimate from low-temperature exchange bias (Fabian et al., 2008), this efficiency turns out to be an order of magnitude larger than the typical efficiency of thermoremanent-magnetisation acquisition in single-domain (SD) magnetite. This explains the occurrence of large remanent magnetic anomalies over rocks containing lamellar magnetism oxides. ...
Article
Nanosize lamellar structures that form in slowly cooled igneous and metamorphic rocks can have unusually high natural magnetic remanence that is stable over timescales of one billion years or more. This behavior has been attributed to lamellar magnetism in the system hematite-ilmenite, where ferrimagnetic contact layers form between paramagnetic ilmenite and antiferromagnetic hematite. Such stable magnetic memory is of importance for planetary magnetism, particularly for planets such as Mars where the magnetic field is no longer present, and for potential industrial applications such as the development of highly stable magnetic storage units. Mössbauer spectroscopy provides a unique probe of the iron environment, enabling the quantitative determination of iron distribution and abundance, with insight into parameters that determine the magnetic properties. To investigate the role of iron in lamellar magnetism, we have undertaken a study of rocks from several different regions using Mössbauer spectroscopy. Titanohaematite grains were identified optically on polished thin sections of slowly cooled rocks from Lerhuvud and Gödestad (both in southern Sweden) and the Russell Belt (Adirondack Mountains, USA). Grains were removed from thin sections with a microdrill, and mounted on a Mössbauer spectrometer fitted with a point source. Room-temperature Mössbauer spectra are dominated by magnetically ordered Fe3+ in hematite, with a smaller absorption corresponding to paramagnetic Fe2+ in ilmenite. Minor absorption is also observed from magnetite and pyrite in some grains. There is no evidence for superparamagnetic hematite in any of the spectra. Comparison of Mössbauer spectra of the natural samples with those from synthetic hematite-rich titanohematite solid solutions provides a measure of the iron environment in natural titanohematite, showing that there is only moderate deviation from the ideal hematite local environment. The absence of Fe3+ in ilmenite indicates that ilmenite lamellae are close to the endmember composition. All grains taken from the same thin sections show similar ilmenite:hematite area ratios, and the two different samples from Gödestad also show similar ratios, suggesting a similar bulk composition. Based on models of cation and magnetic ordering, the proportion of iron involved in the contact layers can be determined, and combined with information from the Mössbauer spectra, provide insight into the density of lamellae, which effectively controls the magnetization.
... Inquiry into the physical origin of these strong remanences led to the concept of lamellar magnetism (LM; Robinson et al., 2002), which has been theoretically investigated by atomic Monte Carlo and density functional simulations of the magnetic properties of lamellar interfaces (Harrison and Becker, 2001;Robinson et al., 2002Robinson et al., , 2004Pentcheva and Nabi, 2008). The experimental proof that the natural magnetic remanence originates from nanoscale lamellae formed during slow cooling (Fabian et al., 2008) is based on the discovery of giant exchange bias below 50 K in some ilmeno-hematite bearing rocks (McEnroe et al., 2007a;Harrison et al., 2007;Fabian et al., 2008). Exchange bias requires the magnetic moment to be linked to exchange-coupled interfaces which here occur between the host and the lamellae. ...
... Inquiry into the physical origin of these strong remanences led to the concept of lamellar magnetism (LM; Robinson et al., 2002), which has been theoretically investigated by atomic Monte Carlo and density functional simulations of the magnetic properties of lamellar interfaces (Harrison and Becker, 2001;Robinson et al., 2002Robinson et al., , 2004Pentcheva and Nabi, 2008). The experimental proof that the natural magnetic remanence originates from nanoscale lamellae formed during slow cooling (Fabian et al., 2008) is based on the discovery of giant exchange bias below 50 K in some ilmeno-hematite bearing rocks (McEnroe et al., 2007a;Harrison et al., 2007;Fabian et al., 2008). Exchange bias requires the magnetic moment to be linked to exchange-coupled interfaces which here occur between the host and the lamellae. ...
... In combination with bulk magnetic measurements, this allows us to calculate the efficiency of NRM acquisition for a lamellar magnetic material. In agreement with a previous estimate from low-temperature exchange bias (Fabian et al., 2008), this efficiency turns out to be an order of magnitude larger than the typical efficiency of thermoremanent-magnetisation acquisition in single-domain (SD) magnetite. This explains the occurrence of large remanent magnetic anomalies over rocks containing lamellar magnetism oxides. ...
Article
Rocks from large remanent magnetic anomalies on Earth have been found to contain exsolved rhombohedral oxides, which also have been proposed as earth analogues for the rocks creating the observed large remanent anomalies on Mars. Theoretical considerations and previous case studies of natural rocks have shown that the natural magnetisation is carried by lamellar magnetism due to uncompensated moments at the interface between antiferromagnetic hematite and paramagnetic ilmenite. Here, single grains (≈ 250 µm) of titanohematite with ferri-ilmenite exsolution lamellae from Mesoproterozoic metamorphic rock samples in southwest Sweden and the Adirondack Mountains, USA, are studied using room-temperature Mössbauer spectroscopy to identify possible characteristic magnetic signatures of lamellar magnetism. Mössbauer spectra of synthetic samples of titanohematite (Fe0.95Ti0.05O3 and Fe0.9Ti0.1O3) were collected for comparison and showed a dominant six-line magnetic spectrum due to Fe3+ in titanohematite with a weak sextet due to Fe2+–Fe3+ charge transfer. Mössbauer spectra of the natural ilmeno-hematite grains are similar to those for synthetic titanohematite, but contain additionally two paramagnetic doublets corresponding to Fe2+ and Fe3+ in ilmenite. However, several grains also contain an additional weak, broad magnetic component that we assign to iron in contact layers according to the lamellar magnetism model. Similar to the previous Mössbauer results for natural hemo-ilmenite, there is no evidence for superparamagnetic behaviour of the nanoscale titanohematite lamellae contained within coarser ferri-ilmenite lamellae, and no evidence for single-domain or superparamagnetic magnetite. The compositions of titanohematite and ferri-ilmenite in the individual ilmeno-hematite grains calculated from the Mössbauer area ratios show that compositions are closer to end-member values than the compositions inferred from electron microprobe and transmission electron microscopy on the same grains, consistent with observations that lamella thicknesses are as small as a few nm. Bulk compositions of ilmeno-hematite grains calculated from the Mössbauer data confirm that the Swedish samples are significantly more ilmenite-rich than the Adirondack samples, and agree with an estimation of bulk composition through point counting of electron backscatter images. The Mössbauer data allow a quantitative estimation of contact layer abundance, which is used to determine the efficiency of lamellar natural remanent magnetisation acquisition. Because minerals with a high natural remanent magnetisation efficiency are expected to create larger remanent magnetic anomalies, contact layer abundance determination by Mössbauer spectroscopy provides a valuable new tool for mineral-based magnetic anomaly interpretation on Earth and other planetary bodies.
... June 2021 | Volume 9 | Article 601401 from ∼90 K to ∼40 K , which is first evident in the raw-data from Paleosol 3. This feature is tentatively interpreted to be caused by ilmenite, which can be magnetized by the surrounding magnetic phases after the former is cooled towards its ordering temperature of ∼57 K (Fabian et al., 2008). Nevertheless, other possible causes such as finer-grained hematite or highly oxidized magnetite cannot be completely ruled out (Özdemir and Dunlop, 2010;Jiang et al., 2014;Dunlop and Özdemir, 2018). ...
... Similar to the detrended RT-SIRM LTD curves, each specimen displays ∼150 K -∼40 K humps which are likewise interpreted to represent ilmenite (Fabian et al., 2008). Nevertheless, it should be noted that these features may also be reflective of high-oxidation or domain-wall of a higher-coercivity magnetite that was insufficiently demagnetized by the AF-procedure (Özdemir and Dunlop, 2010;Dunlop and Özdemir, 2018). ...
Article
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The type-section of the Blackwater Draw Formation (BDF) consists of a series of five paleosol horizons developed on eolian deposits and an overlying surficial soil. Previous work has shown that magnetic properties (e.g., χ, ARM, and IRM) as a function of depth in this type-section, display both magnetically enhanced and magnetically depleted signals for different paleosols. To better understand the magnetic mineralogy responsible for these varying responses, various rock-magnetic experiments, scanning electron microscopy, and Mössbauer spectroscopy were conducted on representative samples from the six soil units which constitute the BDF type-section. Our results show that sub-micron hematite [with a minor contribution from single-domain sized hematite (Hc = ∼500 mT) dominates all the soils in terms of weight percent concentration. Whereas, low coercivity (Hc = ∼35 mT or less) magnetite/maghemitized-magnetite grains, largely in the PSD state (Mr/Ms=∼0.14 +/– 0.03588, Hcr/Hc=∼2.68 +/– 0.298789), dominate the magnetic signal. Magnetically depleted soils show a relatively higher proportion of goethite, while magnetically enhanced soils show an increased contribution from SP/SSD magnetite/maghemite phases.By combining our data-set with geochemically-derived climofunctions, we have correlated the magnetically preserved, depleted, and enhanced sections of the type-section to three distinct environmental phases (I-III). The basal sediments of Phase I displays relatively homogenous (neither enhanced nor depleted) magnetic properties due to relatively arid conditions and minimal alteration of southerly derive eolian sands. Conversely, Phase II-III represents a change in weathering intensities and provenance, resulting in a mix of southerly derived sands and northerly derived silts. Phase II, experienced greater precipitation levels, resulting in the dissolution of Fe-oxide phases and thus magnetic depletion. The uppermost Phase III experienced intermediate precipitation intensities resulting in magnetic enhancement.Using previously published age models we tentatively interpret these changing environmental conditions to be influenced by the Middle-Pleistocene Transition (1.2-0.7 Ma), where the Earth’s climatic cycles shifted from a ∼41 kyr to ∼100 kyr cycles. However, ambiguities persist due to uncertainties in the currently published age model. Due to the complexity of the magnetic signal, we recommend future studies utilize a holistic approach, incorporating rock-magnetic, geochemical, and microscopy observations for more accurate reconstruction of regional paleoenvironments.
... Lamellar magnetizations have been proposed to result from ferrous-ferric contact layers that reduce charge imbalance along lamellar contacts between canted antiferromagnetic hematite and paramagnetic ilmenite intergrowths to produce strong M s and high coercivity (Robinson et al., 2002). Such a possibility is tested readily for pyrrhotite because lamellar magnetizations are characterized by shifted negatively low-temperature hysteresis loops due to strong exchange bias associated with exchange coupling across hematite-ilmenite interfaces (Fabian et al., 2008). The low-temperature magnetic properties of authigenic and monoclinic pyrrhotite are discussed in detail by Horng and Roberts (2018), who demonstrate that low-temperature hysteresis loops are symmetrical and lack the large negative shifts observed by Fabian et al. (2008) in association with lamellar magnetizations. ...
... Such a possibility is tested readily for pyrrhotite because lamellar magnetizations are characterized by shifted negatively low-temperature hysteresis loops due to strong exchange bias associated with exchange coupling across hematite-ilmenite interfaces (Fabian et al., 2008). The low-temperature magnetic properties of authigenic and monoclinic pyrrhotite are discussed in detail by Horng and Roberts (2018), who demonstrate that low-temperature hysteresis loops are symmetrical and lack the large negative shifts observed by Fabian et al. (2008) in association with lamellar magnetizations. Thus, a lamellar-type magnetization can be ruled out as a cause of the room temperature magnetization in hexagonal (3 T) pyrrhotite. ...
... The preservation of strong remanent magnetization requires high magnetic stability and coercivity. The problem is that natural remanent magnetization cannot be explained by the properties of individual magnetic minerals only because none of them are high coercivity phases [1,4,7,8]. ...
... Previous studies have attributed natural remanent magnetization to the common interface between the (0001) planes of the rhombohedral oxide and the (111) planes of the cubic oxide [1,4,7]. Interestingly, the interface between cubic and rhombohedral oxides can produce luogufengite-like 2-D crystals or domains with a doubled hexagonal structure (ABAC packing sequence) (Figure 8a). ...
Article
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A natural ε-Fe2 O3 nano-mineral (luogufengite) has been discovered in young basaltic rocks around the world. Transmission electron microscopy (TEM) observed euhedral or subhedral luogufengite nano-minerals with crystal sizes ranging from 10 to 120 nm in the basaltic rocks. The magnetic property of treated scoria sample (containing 75.3(5) wt % luogufengite) showed a saturation remanence of 11.3 emu g⁻¹ with a coercive field of 0.17 tesla (T) at room temperature. Luogufengite-like nano-domains were also observed in natural permanent magnets (lodestone) and Fe-Ti oxides (ilmenite-magnetite series) with strong remanent magnetization. The structure of luogufengite-like domains (double hexagonal close-packing) is associated with the interfaces between the (111) plane of cubic magnetite and the (0001) plane of rhombohedral hematite or ilmenite. Stacking faults and twin boundaries of magnetite/maghemite can also produce the luogufengite-like domains. The nano-domains oriented along the magnetic easy axis play an essential role in enhancing the magnetic coercivity of lodestone and Fe-Ti oxide. We conclude that the luogufengite nano-minerals and nano-domains provide an explanation for coercivity and strong remanent magnetization in igneous, metamorphic rocks and even some reported Martian rocks. These nano-scaled multilayer structures extend our knowledge of magnetism and help us to understand the diverse magnetic anomalies occurring on Earth and other planetary bodies.
... The FeTiO 3 -Fe 2 O 3 solid-solution series further serves as a model system for natural ilmenite-hematite minerals, which are studied because of their remarkable magnetic properties and because of their importance as a source of anomalies in the magnetic field of the Earth (McEnroe et al. 2001;Kletetschka et al. 2002), and possibly of other planets like Mars (McEnroe et al. 2004). Interesting magnetic properties of natural ilmeno-hematite samples include giant exchange bias Fabian et al. 2008) and large and stable remanent magnetization that cannot be explained by the individual properties of hematite and ilmenite alone Robinson et al. 2002Robinson et al. , 2004. There is strong evidence that these properties are associated with fine scale exsolution structures observed in natural hematite-ilmenite samples Fabian et al. 2008;Brok et al. 2014). ...
... Interesting magnetic properties of natural ilmeno-hematite samples include giant exchange bias Fabian et al. 2008) and large and stable remanent magnetization that cannot be explained by the individual properties of hematite and ilmenite alone Robinson et al. 2002Robinson et al. , 2004. There is strong evidence that these properties are associated with fine scale exsolution structures observed in natural hematite-ilmenite samples Fabian et al. 2008;Brok et al. 2014). Solid solution FeTiO 3 -Fe 2 O 3 does not exhibit these welldeveloped exsolution structures and can therefore be used as a baseline for determining if the observed properties are tied to the exsolution structure. ...
Article
Full-text available
The spin orientation in synthetic hematite-ilmenite samples and in a sample of natural hematite was studied from room temperature to above the antiferromagnetic-paramagnetic phase transition (the Néel temperature; TN ≈ 600-950 K) by neutron powder diffraction and at room temperature by Mössbauer spectroscopy. The usually assumed magnetic structure of hematite within this temperature range is antiferromagnetic with the spins confined to the basal plane of the hexagonal structure; however, an out-of-plane spin component is allowed by the symmetry of the system and has been observed in recent studies of synthetic hematite samples. We find the spins in the antiferromagnetic sublattices to be rotated out of the basal plane by an angle between 11(2)° and 22.7(5)° in both synthetic hematite-ilmenite samples and in the natural hematite sample. The spin angle remains tilted out of the basal plane in the entire temperature range below the Néel temperature and does not depend systematically on Ti-content. The results indicate that the out-of-plane spin component is an intrinsic feature of hematite itself, with an origin not yet fully understood, but consistent with group theory. This represents a major shift in understanding of one of the two main mineral systems responsible for rock magnetism.
... The conclusions of previous studies on samples here referred to as MOD 2, which showed extreme exchange bias of ∼1.3 T (Harrison et al. 2007;McEnroe et al. 2007a;Fabian et al. 2008) andMOD22 (McEnroe et al. 2016), relating both to lamellar thickness and to the out-of-plane component of lamellar magnetism, were employed in the design of more sophisticated magnetic experiments. These were initiated using the cryogenic magnetometer at the Institute of Rock Magnetism. ...
... Here, the negative loop with the smaller area shows a larger absolute shift in median field than the positive loop with the larger area. Three of the negative separation peaks in Table 6(B, C, D) {note that C is not shown in Fig. 3} have shifts at -1.41, -1.64 and -1.65 T, and two of the positive separation peaks (C, D) have shifts at +1.46 and +1.48 T, and show larger absolute shifts than were measured in the same way on a MOD2 sample at -1.34 T (McEnroe et al. 2007a;Fabian et al. 2008). ...
Article
This is the second of three papers investigating properties of titanohematite-bearing quartzofeldspathic rocks that create a significant remanent magnetic anomaly in the Modum District, South Norway. The first paper provided initial magnetic results, mineralogical characterization and evidence for the presence of lamellar magnetism. In this paper, knowledge of lamellar magnetic properties is explored through experimentswhere ilmenite lamellaewere magnetized below 57 K, and interact magnetically along interfaces with the titanohematite host. Samples with known NRM directions were placed in specific orientations in an MPMS then cooled in zero field to 5 K, where hysteresis loops were measured in fields up to 5 Tesla. This assured that results were ultimately related to the natural lamellar magnetism produced during cooling ~1 billion years ago. In a second set of experiments the same oriented samples, were subjected to a +5 Tesla (T) field then field cooled to 5 K before hysteresis experiments. The first experiments consistently produced asymmetric shifted hysteresis loops with two loop separations, one in a positive field and one in a negative field.Without exception,when theNRMwas oriented toward the negative field end of the MPMS, the bimodal loop showed a dominant loop separation in a positive field. By contrast, when the NRM was oriented toward the positive field end of the MPMS, the bimodal loop showed a dominant loop separation in a negative field. Both observations are consistent with antiferromagnetic coupling between the hard magnetization of ilmenite and the more easily shifted lamellar magnetism of the hematite. The bimodal nature of the loops indicates that the NRMs are vector sums of natural lamellar moments, which are oriented both positively and negatively, and that these opposite moments control the orientations of ilmenite magnetizations when cooling through 57 K. Here, extreme exchange biases up to 1.68 Twere measured. The second set of experiments produced asymmetric shifted hysteresis loops with one opening always in the negative field. These observations indicate that the +5 T field applied at room temperature rotated the hematite lamellar magnetism in a positive direction, so that upon cooling all the ilmenite lamellae acquired negative magnetic moments, thus causing unimodal negatively shifted loops. Here, the largest exchange bias among the unimodal loops was only 0.7 T. These results will be used in paper III to build a better understanding of lamellar magnetism at the atomic layer scale. © The Authors 2016. Published by Oxford University Press on behalf of The Royal Astronomical Society.
... The FeTiO 3 -Fe 2 O 3 solid-solution series further serves as a model system for natural ilmenite-hematite minerals, which are studied because of their remarkable magnetic properties and because of their importance as a source of anomalies in the magnetic field of the Earth (McEnroe et al. 2001;Kletetschka et al. 2002), and possibly of other planets like Mars (McEnroe et al. 2004). Interesting magnetic properties of natural ilmeno-hematite samples include giant exchange bias Fabian et al. 2008) and large and stable remanent magnetization that cannot be explained by the individual properties of hematite and ilmenite alone Robinson et al. 2002Robinson et al. , 2004. There is strong evidence that these properties are associated with fine scale exsolution structures observed in natural hematite-ilmenite samples Fabian et al. 2008;Brok et al. 2014). ...
... Interesting magnetic properties of natural ilmeno-hematite samples include giant exchange bias Fabian et al. 2008) and large and stable remanent magnetization that cannot be explained by the individual properties of hematite and ilmenite alone Robinson et al. 2002Robinson et al. , 2004. There is strong evidence that these properties are associated with fine scale exsolution structures observed in natural hematite-ilmenite samples Fabian et al. 2008;Brok et al. 2014). Solid solution FeTiO 3 -Fe 2 O 3 does not exhibit these welldeveloped exsolution structures and can therefore be used as a baseline for determining if the observed properties are tied to the exsolution structure. ...
Article
The spin orientation in synthetic hematite-ilmenite samples and in a sample of natural hematite was studied from room temperature to above the antiferromagnetic-paramagnetic phase transition (the Néel temperature; T\textit{T}N_{N} ≈ 600 − 950 K ) by neutron powder diffraction and at room temperature by Mö ssbauer spectroscopy. The usually assumed magnetic structure of hematite within this temperature range is antiferromagnetic with the spins confined to the basal plane of the hexagonal structure , however, an out-of-plane spin component is allowed by the symmetry of the system and has been observed in recent studies of synthetic hematite samples. We find the spins in the antifer romagnetic sublattices to be rotated out of the basal plane by an angle between 11 (2)° and 22.7(5)° in both synthetic hematite-ilmenite samples and in the natural hematite sample. The spin angle remains tilted out of the basal plane in the entire temperature range below the Néel temperature and does not depend systematically on Ti-content. The results indicate that the out-of-plane spin component is an intrinsic feature of hematite itself, with an origin not yet fully understood, but consistent with group theory. This represents a major shift in understanding of one of the two main mineral systems responsible for rock magnetism.
... Recent studies have demonstrated that nanoscale microstructures are extremely common in magnetic minerals, and that they have a significant impact on their macroscopic magnetic properties (Harrison and Becker 2001;Harrison et al. 2002;McEnroe et al. 2001 and2002;Robinson et al. 2002Robinson et al. , 2004Robinson et al. , and 2006Harrison et al. 2005;Feinberg et al. 2004 and2005;Kasama et al. 2010;Brownlee et al. 2010 and. These microstructures not only determine the intensity and stability of macroscopic magnetism recorded in rocks -thereby controlling the fidelity of paleomagnetic recordings at the global scale -but are often important in an industrial context, providing natural analogues of magnetic phenomena (such as exchange bias) that are central to the design of new magnetic recording materials (Skumryev et al. 2003;Puntes et al. 2004;McEnroe et al. 2007;Fabian et al. 2008). ...
... Slowly cooled rocks that contain finely exsolved hematite-ilmenite have strong and extremely stable magnetic remanence, which may account for some of the magnetic anomalies that are present in the deep crust and on planetary bodies that no longer retain a magnetic field, such as Mars (McEnroe et al. 2001(McEnroe et al. , 2004aMcEnroe et al. 2009;Brown and McEnroe 2012). This remanence has been attributed to the presence of a stable ferrimagnetic substructure, which is associated with the coherent interface between nanoscale ilmenite and hematite exsolution lamellae (the so-called 'lamellar magnetism hypothesis'; Harrison and Becker 2001;Robinson et al. 2002 and2004;Harrison 2006;Kasama et al. 2003Kasama et al. , 2004Kasama et al. and 2009Fabian et al. 2008;McCammon et al. 2009;Fig. 8a). ...
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This review describes the current state of the art in the field of computational and experimental mineral physics, as applied to the study of magnetic minerals. The review is divided into four sections, describing new developments in the study of mineral magnetism at the atomic, nanometer, micrometer, and macroscopic length scales. We begin with a description of how atomistic simulation techniques are being used to study the magnetic properties of minerals surfaces and interfaces, and to gain new insight into the coupling between cation and magnetic ordering in Fe-Ti-bearing solid solutions. Next, we review the theory of off-axis electron holography, and its application to the study of magnetotactic bacteria and minerals containing nanoscale transformation-induced microstructures. Then, we review the theory and application of micromagnetic simulations to the study of nonuniform magnetization states and magnetostatic interactions in minerals at the micrometer length scale. Finally, we review recent developments in the use of macroscopic magnetic measurements for characterizing and quantifying the microscopic spectrum of coercivities and interaction fields present in rocks and minerals.
... Particularly interesting with respect to spintronics are solid solutions of hematite-ilmenite, ␣-Fe 2 O 3 -FeTiO 3 . Both naturally occurring minerals are antiferromagnetic insulators, but when combined in a layered structure they exhibit very stable magnetization and large exchange bias due to a phenomenon known as lamellar magnetism [5,6]. With the coexistence of ferrimagnetic and semiconducting properties [7][8][9], these systems show great promise for applications in electronics and spintronics [10]. ...
... Iron oxide films deposited by ALD were first prepared more than 10 years ago, and since then a variety of different precursors have been utilized. Pure ␣-Fe 2 O 3 films have been obtained starting from various Fe(III) complexes, including Fe(acac) 3 (Hacac = 2,4-pentanedione) combined with O 2 [13], Fe(thd) 3 (Hthd = 2,2,6,6-tetramethyl-3,5-heptanedione) with O 3 [14,15], as well as iron(III) tert-butoxide (Fe 2 (O t Bu) 6 ) [16] and FeCl 3 [17], both with H 2 O as co-reactant. Depositions starting from Fe(II) precursors on the other hand require a more careful selection of the oxidizing agent and deposition temperature to produce pristine films. ...
... LM has been investigated further theoretically with density functional simulations of the magnetic properties of lamellar interfaces (Pentcheva and Nabi, 2008), with neutron powder diffraction studies (Harrison et al., 2010) and by room and low-temperature Mössbauer (Dyar et al., 2004; Frandsen et al., 2007; McEnroe et al., 2007; McCammon et al., 2009). The experimental proof that the NRM originates from nanoscale lamellae (Fabian et al., 2008) is based on the discovery of giant exchange bias below 57 K in ilmenohematite rocks (McEnroe et al., 2007; Harrison et al., 2007; Fabian et al., 2008). Exchange bias requires the magnetic moment to be linked to exchange-coupled interfaces that occur between the host and the lamellae. ...
... LM has been investigated further theoretically with density functional simulations of the magnetic properties of lamellar interfaces (Pentcheva and Nabi, 2008), with neutron powder diffraction studies (Harrison et al., 2010) and by room and low-temperature Mössbauer (Dyar et al., 2004; Frandsen et al., 2007; McEnroe et al., 2007; McCammon et al., 2009). The experimental proof that the NRM originates from nanoscale lamellae (Fabian et al., 2008) is based on the discovery of giant exchange bias below 57 K in ilmenohematite rocks (McEnroe et al., 2007; Harrison et al., 2007; Fabian et al., 2008). Exchange bias requires the magnetic moment to be linked to exchange-coupled interfaces that occur between the host and the lamellae. ...
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Paleomagnetic samples were collected from an east-west traverse across the Highlands region of the Adirondack Mountains, northern New York, which forms part of the southern most exposure of the Grenville Province of North America. Granulite facies metamorphism (T>650°C) at ∼1050Ma completely reset pre-existing magnetic directions in sampled microcline gneisses and metamorphosed anorthosites. Fourteen sites of microcline gneiss yield a mean direction of I=-62.8°, D=289.2° with α 95=7.6° and a corresponding pole at 18.4°S and 151.1°E. Metamorphic anorthosites and associated rocks (N=14) show a direction of I=-67.3°, D=283.9° with α 95=7.7° and a corresponding pole at 25.1°S and 149.0°E. Post-metamorphic fayalite granites possess a statistically different direction of I=-75.8° and D=297.0° (α 95=3.9°, N=8 sites) and a pole of 28.4°S and 132.7°E. Both normal and reverse polarities are recorded in all units, with reverse sites occurring on the eastern and western ends of the traverse, and normal polarities restricted to the central part of the massif. The remanence is carried by ilmeno-hematite in the microcline gneisses. In the metamorphosed anorthosites, and the unmetamorphosed fayalite granites, hemo-ilmenite and magnetite occur, though magnetite is the predominant oxide. Using cooling curves established for the Highlands, and blocking temperatures determined for ilmeno-hematite, hemo-ilmenite and magnetite, the age of remanence is determined to be ∼990Ma for the magnetite-bearing Wanakena granite, ∼970Ma for the metamorphosed anorthosite and related rocks, and ∼960Ma for the ilmeno-hematite rich microcline gneiss. The pole data from the Adirondacks, as well as selected studies from other areas of the Grenville Province on units with similar mineralogy and some age control, helps define the southerly part of the Grenville loop of the apparent polar wander path. The three units from the Adirondacks indicate counter-clockwise motion of the APWP between 990 and 960Ma.
... In general, inhomogeneous interacting materials can be far from saturation in the typical peak fields (1-2 T) used for VSM loop measurements, and are thus not guaranteed to yield loops that are symmetric about the origin, or even to have strong symmetry at all. For example, in exchange-coupled multiphase systems, loops may exhibit approximate symmetry about a point with a large positive or negative field coordinate, and furthermore they commonly exhibit a significant lack of symmetry [e.g., McEnroe et al., 2007;Harrison et al., 2007;Fabian et al., 2008;Lindgård, 2009]. Similarly, a center of (approximate) symmetry may be shifted vertically in multiphase systems after in-field cooling (FC), when one phase acquires a very hard FC remanence (M R,FC ) and the others remain soft enough to approach saturation in the applied fields of the low-temperature loop (e.g., siderite and magnetite [Housen et al., 1996]). ...
... [15] We have evaluated a number of alternative methods for robust calculation of both vertical and horizontal components of loop shifts (i.e., calculation of the center of symmetry (H 0 , M 0 )), and many of them work well under favorable conditions. For example, the M rh curve is independent of vertical loop offsets, and therefore the center of symmetry or center of mass of M rh (H) can be used to isolate the horizontal displacement of the loop [e.g., Fabian et al., 2008]. This approach fails, however, when the remanence ratio (M RS /M S ) is low (and M rh is consequently weak and noisy), or when drift is significant (and M rh is consequently asymmetric). ...
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Magnetic hysteresis data are centrally important in pure and applied rock magnetism, but to date, no objective quantitative methods have been developed for assessment of data quality and of the uncertainty in parameters calculated from imperfect data. We propose several initial steps toward such assessment, using loop symmetry as an important key. With a few notable exceptions (e.g., related to field cooling and exchange bias), magnetic hysteresis loops possess a high degree of inversion symmetry (M(H) = −M(−H)). This property enables us to treat the upper and lower half-loops as replicate measurements for quantification of random noise, drift, and offsets. This, in turn, makes it possible to evaluate the statistical significance of nonlinearity, either in the high-field region (due to nonsaturation of the ferromagnetic moment) or over the complete range of applied fields (due to nonnegligible contribution of ferromagnetic phases to the total magnetic signal). It also allows us to quantify the significance of fitting errors for model loops constructed from analytical basis functions. When a statistically significant high-field nonlinearity is found, magnetic parameters must be calculated by approach-to-saturation fitting, e.g., by a model of the form M(H) = Ms + χHFH + αHβ. This nonlinear high-field inverse modeling problem is strongly ill conditioned, resulting in large and strongly covariant uncertainties in the fitted parameters, which we characterize through bootstrap analyses. For a variety of materials, including ferrihydrite and mid-ocean ridge basalts, measured in applied fields up to about 1.5 T, we find that the calculated value of the exponent β is extremely sensitive to small differences in the data or in the method of processing and that the overall uncertainty exceeds the range of physically reasonable values. The “unknowability” of β is accompanied by relatively large uncertainties in the other parameters, which can be characterized, if not rigorously quantified, through the bootstrapped distribution of best fit models. Nevertheless, approach-to-saturation fitting yields much more accurate estimates of important parameters like Ms than those obtained by linear M(H) fitting and should be used when maximum available fields are insufficient to reach saturation.
... Dependent at which temperature the interface formed an interface magnetization could be acquired either as a TRM or TCRM. Interface related physical mechanisms for unusual efficiency of NRM acquisition have earlier been discussed by Fabian et al. (2008) and McCammon et al. (2009), for lamellar magnetism in the ilmenite-hematite system, and in titanohematite by McEnroe et al. (2001). In these cases the acquisition of NRM is considered to be synchronous with the formation of 1-100 nanometer sized lamellae. ...
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Quaternary lavas of the Stardalur Caldera, 20 km northeast of Reykjavik, Iceland, create a 27 300 nT magnetic anomaly visible in both ground- and aeromagnetic surveys. Here, we provide a comprehensive mineralogical and rock magnetic data set to analyze NRM intensities and Koenigsberger ratios of 57 drill-core samples from the critical zone (CZ) of the anomaly high at depths between 41 m and 131 m. This extends previous studies and verifies that the anomaly is due to an unusually high intensity of remanent magnetization carried by magnetite. The NRM of the CZ samples was acquired during the Olduvai subchron in a field of at most today’s strength. NRM intensities range from 20 A/m to 128 A/m with a median of 55 A/m, and an average of 61 A/m, respectively, approximately 13-15 times higher than in typical Icelandic basalts (AIB) with an NRM intensity of 4 A/m. Our new data set shows that the magnetite concentration throughout the CZ basalts is at most twofold higher than in AIB lavas. New data on domain state and TRM efficiency prove that these properties account for an additional factor of at most 2.3. Because magnetite is the most abundant remanence carrier in rocks on Earth, and its remanence acquisition is considered to be extremely well understood, we assert that the remaining discrepancy is a critical enigma in rock magnetism. Results from scanning electron microscopy show that a significant fraction of all CZ magnetite particles have dendritic shapes with grain sizes <1 μm, indicating rapid crystallization. Most large magnetite grains are heavily subdivided by very fine oxidation-exsolution lamellae of ilmenite, and subordinate amount of exsolved spinel as needles, blebs and blades. These common microstructures found throughout the CZ subdivide the initially homogeneous mineral into separate cubicles, here denoted as compartments. The magnetite compartments then have sizes below 1 μm. Hysteresis data, Preisach maps and FORC data consistently confirm that the coercivity distribution is dominated by values above 10 mT, such that multidomain behavior is of little relevance in the CZ. Between 5%-20% of the IRM is carried by coercivities above 100 mT, which for magnetite indicates unusually high anisotropy effects in the individual particles. Based on the quantitative analysis of all magnetic contributions to the NRM, we can demonstrate that the average efficiency of NRM acquisition in the CZ Stardalur basalts must be at least a factor 3 higher than in typical basalts. We speculate that this is related to the observed focused compartment size distribution <1 μm, and indicates thermochemical remanence acquisition below the Curie temperature of magnetite. Yet, a detailed physical mechanism for the extreme over-efficiency of NRM acquisition remains enigmatic.
... Self-reversal occurs when a mineral is magnetized antiparallel to the ambient magnetic field direction (e.g., Nagata, 1952;Nagata & Uyeda, 1959). This phenomenon has been studied widely in natural and synthetic samples (Burton et al., 2008;Doubrovine & Tarduno, 2004;Fabian et al., 2008;Garming et al., 2007;Heller et al., 1986;Hoffman, 1992 Yoshikazu & Yasuhiko, 1963). Titanohematite was studied intensively after discovery of reversed TRM in the Haruna dacite tuff from Japan (Nagata, 1952), which complicated early recognition of geomagnetic polarity reversals (A. ...
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Hematite is a canted antiferromagnet with reddish color that occurs widely on Earth and Mars. Identification and quantification of hematite is conveniently achieved through its magnetic and color properties. Hematite characteristics and content are indispensable ingredients in studies of the iron cycle, paleoenvironmental evolution, paleogeographic reconstructions, and comparative planetology (e.g., Mars). However, the existing magnetic and color reflectance property framework for hematite is based largely on stoichiometric hematite and tends to neglect the effects of cation substitution, which occurs widely in natural hematite and influences the physical properties of hematite. Thus, magnetic parameters for stoichiometric hematite are insufficient for complete analysis of many natural hematite occurrences and can lead to ambiguous geological interpretations. Remagnetization, which occurs pervasively in red beds, is another ticklish problem involving hematite. Understanding red bed remagnetization requires investigation of hematite's formation and remanence recording mechanisms. We elaborate on the influence of cation substitution on the magnetic and color spectral properties of hematite, and on identifying hematite and quantifying its content in soils and sediments. Studies of remagnetization mechanisms are discussed, and we summarize methods to discriminate between primary and secondary remanences carried by hematite in natural samples to aid primary remanence extraction in partially remagnetized red beds. Although there remain unknown properties and unresolved issues that require future work, recognition of the properties of cation‐substituted hematite and remagnetization mechanisms for hematite will aid identification and interpretation of the magnetic signals that it carries, which is environmentally important and responsible for magnetic signals on Earth and Mars.
... Mineral magnetic properties can be affected by magnetostatic interactions and compositional or structural defects. Magnetic interactions, as is the case with compositional defects, suppress the Morin transition; for example, samples with lamellar magnetism lack a Morin transition (e.g., Fabian et al., 2008). The Marble Bar Chert has a clear alternating pattern with the largest difference between the HRS and LRS, and contains magnetite and siderite. ...
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Hematite is a commonly occurring magnetic mineral in nature that has numerous scientific and technological applications. A characteristic property of hematite is a low‐temperature spin‐flop transition called the Morin transition. Above the transition temperature, hematite is a canted antiferromagnet that can carry a remanent magnetization. Below this transition, spin canting disappears and hematite becomes a true antiferromagnet although a small defect moment is usually preserved. We observe Morin transition behavior in natural samples that has not been reported before for hematite. During repeated thermal cycling of a remanent magnetization acquired at room temperature, the remanence intensity at the end of the cycle oscillates between a high remanence state at the end of odd‐numbered cycles and a low remanence state (LRS) at the end of even‐numbered cycles. Alternation of the high and LRSs during repeated thermal cycling points to hysteretic behavior of the spin‐flop process, likely due to sublattice magnetization alignment switches along different easy magnetization axes in samples with preferred crystallographic orientations of hematite particles. We report these observations to seek to expand explanations of the magnetism of hematite.
... Hematite and ilmenite, when forming heterointerfaces with each other, have been proposed to give rise to a strongly ferrimagnetic contact layer [32], which plays a crucial role in explaining the high and stable natural remanence in massive rocks where nanoscale exsolution lamella of ilmenite occur in a hematite host (or vice versa) [33,34]. Rocks with abundant hematite-ilmenite exsolution lamella are known to have a saturation magnetization more than twenty times that of pure hematite (2.5 kAm -1 ) and are predicted to produce a giant exchange bias of 1 T [35,36]. The interface between a-Fe 2 O 3 (0001) and FeTi-O 3 (0001) has a polar discontinuity (disruption of charge neutrality), which is compensated by a disproportionated Fe 2? /Fe 3? contact layer [37]. ...
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The interface between hematite (α-Fe 2IIIO3) and ilmenite (FeIITiO3), a weak ferrimagnet and an antiferromagnet, respectively, has been suggested to be strongly ferrimagnetic due to the formation of a mixed valence layer of Fe2+/Fe3+ (1:1 ratio) caused by compensation of charge mismatch at the chemically abrupt boundary. Here, we report for the first time direct experimental evidence for a chemically distinct layer emerging at heterointerfaces in the hematite—Ti-doped-hematite system. Using molecular beam epitaxy, we have grown thin films (~25 nm thickness) of α-Fe2O3 on α-Al2O3 (0001) substrates, which were capped with a ~25 nm thick Fe2−x Tix O3 layer (x = 0.44). An additional 3 nm cap of α-Fe2O3 was deposited on top. The films were structurally characterized in situ with surface X-ray diffraction, which showed a partial low index orientation relationship between film and substrate in terms of the [0001] axis and revealed two predominant domains with (0001)Fe2O3    (0001)Al2O3, (0001) _{{{\text{Fe}}_{2} {\text{O}}_{3} }} \;||\;(0001) _{{{\text{Al}}_{2} {\text{O}}_{3} }}, one with [101ˉ0]Fe2O3    [101ˉ0]Al2O3, [10\bar{1}0]_{{{\text{Fe}}_{2} {\text{O}}_{3} }} \;||\;[10\bar{1}0]_{{{\text{Al}}_{2} {\text{O}}_{3} }}, and a twin domain with [011ˉ0]Fe2O3      [101ˉ0]Al2O3. [01\bar{1}0]_{{{\text{Fe}}_{2} {\text{O}}_{3} }} \;||\;\;[10\bar{1}0]_{{{\text{Al}}_{2} {\text{O}}_{3} }}. Electron energy loss spectroscopy profiles across the Fe2−x Tix O3/Fe2O3 interface show that Fe2+/Fe3+ ratios peak right at the interface. This strongly suggests the formation of a chemically distinct interface layer, which might also be magnetically distinct as indicated by the observed magnetic enhancement in the Fe2−x Tix O3/α-Fe2O3/Al2O3 system compared to the pure α-Fe2O3/Al2O3 system.
... The directions of the natural remanent magnetization (NRM) are mostly dominated by a viscous component (close to the present Earth field direction), especially in felsic and intermediate igneous rocks such as granites and orthogneisses. The high-amplitude magnetic anomalies in the Kongsberg-Bamble area, on the Fosen Peninsula (Roan) in mid Norway, Lofoten -Vesterålen and the northwestern part of Senja in northern Norway are caused by granulite-facies gneisses and intrusions (mangerites) (Schlinger 1985;Olesen et al. 1991;Skilbrei et al. 1991;Fabian et al. 2008). The orthopyroxene isograd in the Vesterålen area coincides with the boundary between the high-magnetic and low-magnetic gneisses. ...
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The Geological Survey of Norway (NGU) has produced new aeromagnetic and gravity maps from Norway and adjacent areas, compiled from ground, airborne and satellite data. Petrophysical measurements on core samples, hand specimens and on in situ bedrock exposures are essential for the interpretation of these maps. Onshore, the most prominent gravity and magnetic anomalies are attributed to lower crustal rocks that have been brought closer to the surface. The asymmetry of the gravity anomalies along the Lapland Granulite Belt and Kongsberg–Bamble Complex, combined with the steep gradient, points to the overthrusted highdensity granulites as being the main source of the observed anomalies. The Kongsberg–Bamble anomaly can be traced southwards through the Kattegat to southern Sweden. This concept of gravity field modelling can also be applied to the Mid-Norwegian continental shelf and could partially explain the observed high-density rocks occurring below the Møre and Vøring basins and in the Lofoten area. Extrapolations of Late-Caledonian detachment structures occurring on the mainland can be traced on aeromagnetic and gravimetric images towards the NW across the continental margin. Subcropping Late Palaeozoic to Cenozoic sedimentary units along the mid-Norwegian coast produce a conspicuous magnetic anomaly pattern. The asymmetry of the lowamplitude anomalies, with a steep gradient and a negative anomaly to the east and a gentler gradient to the west, relates the anomalies to gently westward dipping strata. Recent aeromagnetic surveys in the Barents Sea have revealed negative magnetic anomalies associated with shallow salt diapirs. Buried Quaternary channels partly filled with gravel and boulders of crystalline rocks generate magnetic anomalies in the North Sea. The new maps also show that the opening of the Norwegian–Greenland Sea occurred along stable continental margins without offsets across minor fracture zones, or involving jumps in the spreading axis. A triple junction formed at 48 Ma between the Lofoten and Norway Basins.
... The samples are characterized by strong and very stable remanent magnetization, resulting from uncompensated spins provided by magnetically interacting contact layers on two sides of nanometre-scale exsolution lamellae. The NRM is likely produced at the moment of creation of interfaces during exsolution within the thermo-chemical region where CAF hematite is stable (Robinson et al. , 2004Fabian et al. 2008). Hence, it is rather a chemical than a thermal remanent magnetization. ...
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New experimental and computational approaches to interpret orientation and intensity of natural remanent magnetization (NRM) carried by lamellar magnetism are applied to historic magnetic measurements on a collection of 82 massive hemo-ilmenite samples from the Allard Lake District in the Grenville Province, Quebec. The anisotropy of magnetic susceptibility (AMS), together with declination and inclination of NRM, indicate a systematic deflection β of the NRM vector away from the unit vector v that represents the Mesoproterozoic magnetizing field direction. The deflection β is caused by a statistical lattice-preferred orientation (LPO) of the individual (0001) basal planes, to which the NRM is confined in hemo-ilmenite crystals. Here, we study a second deflection ψ that is the angle the NRM makes with the statistical (0001) basal plane of the crystal assemblage, in relation to the angle α between the statistical (0001) basal plane and v. The relation between these two angles depends on the scatter of the distribution of crystal platelets, which also influences the AMS of the assemblage. For a Fisher distribution of basal planes, the distribution parameter K can be determined from ψ and α. It is then further possible to infer the single-crystal anisotropy of individual platelets. Typical crystals of hemo-ilmenite turn out to have a relatively weak AMS so that samples with a narrow Fisher distribution of platelets nevertheless can have a weak AMS. This has been confirmed in two samples by measurement of the (0001) basal plane distribution of crystals using electron backscatter diffraction, and in one of these two samples by measuring AMS and NRM of a single hemo-ilmenite crystal. Based on our estimated K values for selected samples, we calculate values of β, NRM intensity and ψ for any value of α. These data provide striking examples of the influence of the orientation of the crystal LPO on the intensity of lamellar magnetism, and explain the large variation of observed NRM intensities by varying orientation with respect to the magnetizing field, without requiring large variations of the paleomagnetic field intensity. This relation between NRM and LPO is also important for anomaly interpretation in areas with strong foliation.
... In Paper II (Robinson et al. 2012) an analysis was presented of the chemical phase relations and processes leading to the microstructures considered most favourable to produce these magnetic properties, as previously proposed by Harrison et al. (2005) and Harrison (2006). In Paper II it was shown that 'contact layers', previously shown to be the key to 'lamellar magnetism' in exsolved intergrowths of near end-member hematite and ilmenite (Harrison & Becker 2001;Robinson et al. 2002Robinson et al. , 2004Fabian et al. 2008;McCammon et al. 2009), also apply at interfaces between disordered, ordered and anti-ordered solid solutions of essentially the same composition. ...
Article
Paper II of this series described the chemical and microstructural evolution of ferri-ilmenite solid solutions during high-T quench and short-term annealing. Here we explore consequences of these Fe-Ti ordering-induced microstructures and show how they provide an explanation for both self-reversed thermoremanent magnetization and room-T magnetic exchange bias. The dominant antiferromagnetic interactions between (001) cation layers cause the net magnetic moments of ferrimagnetic ordered phases to be opposed across chemical antiphase domain boundaries. Magnetic consequences of these interactions are explored in conceptual models of four stages of microstructure evolution, all having in common that A-ordered and B-anti-ordered domains achieve different sizes, with smaller domains having higher Fe-content, lesser Fe-Ti order, and slightly higher Curie T than larger domains. Stage 1 contains small Fe-rich domains and larger Ti-rich domains separated by volumes of the disordered antiferromagnetic phase. Magnetic linkages in this conceptual model pass through disordered host, but self-reversed TRM could occur. In stage 2, ordered domains begin to impinge, but some disorder remains, creating complex magnetic interactions. In stages 3 and 4, all disordered phase is eliminated, with progressive shrinkage of Fe-rich domains, and growth of Ti-rich domains. Ordered and anti-ordered phases meet at chemical antiphase and synphase boundaries. Strong coupling across abundant antiphase boundaries provides the probable configuration for self-reversed thermoremanent magnetization. Taking the self-reversed state into strong positive fields provides a probable mechanism for room-temperature magnetic exchange bias.
... Their characteristics have been demonstrated by Monte Carlo simulations (Harrison & Becker 2001;Robinson et al. 2002Robinson et al. , 2004, by bond-valence calculations (Robinson et al. 2006), by density functional theory (Pentcheva & Nabi 2008) and most recently by Mössbauer spectroscopy (McCammon et al. 2009). Their critical role at phase interfaces between haematite and ilmenite has recently been demonstrated in studies of magnetic exchange bias in naturally exsolved samples at very low T (McEnroe et al. 2007a, b;Fabian et al. 2008), by Monte Carlo modelling of exchange bias (Harrison et al. 2007) and by neutron diffraction of a natural sample showing extreme exchange bias (Harrison et al. 2010). The Fe 2+ -Fe 3+ contact layers on lamellar interfaces are magnetically significant because they produce an uncompensated spin, as described in the above papers, but they are also significant in considering chemical constitution. . ...
Article
Chemical and microstructural evolution during quench and short-term annealing of a sample XFeTiO3= 0.61 is explored in the light of observations in Paper I. Ordering proceeds by (1) random appearance of ordered and anti-ordered domains within a disordered host, (2) coarsening of ordered and anti-ordered domains until they impinge along antiphase domain boundaries, (3) growth of regions where one ordered or anti-ordered phase becomes dominant over the other, with progressive reduction in surface area of antiphase domain boundaries and (4) dynamic development where antiphase boundaries migrate during annealing, leading to Fe enrichment of shrinking domains and Fe depletion of growing domains. These conclusions are supported by 2-D Monte Carlo simulations illustrating that Ti-Ti avoidance is a powerful driving force for Fe enrichment along antiphase boundaries, and by bond-valence calculations demonstrating that local charge balance is improved when antiphase domain boundaries contain a combination of Fe-rich contact layers and disordered boundary layers along (001). Chemical phase separation during quenching is driven by the disorder/order transition at temperatures above the tricritical point and by spinodal decomposition at temperatures below the tricritical point. The former explains microtextures and chemical features in samples quenched from high temperature; the latter produces textural and chemical evolution during subsequent annealing. All these features provide the atomic basis for self-reversed thermoremanent magnetization and room-temperature magnetic exchange bias as will be described in Paper III.
... These loops indicate the existence of two distinctive coercivity components, one saturated around 1 T but the other far from saturation. The higher coercivity component is probably caused by the surface effect or the existence of exchange bias, which may be attributed to a strong unidirectional anisotropy caused by the Al-ordering arrangement (Figure S2 in the auxiliary material) [Fabian et al., 2008; McEnroe et al., 2009]. Then, coercivity (B c ) and pseudo-saturated magnetization (PM s , as it is not saturated absolutely) used in the following text are obtained from the first saturated component. ...
Article
Hematite is an important carrier of natural remanence magnetization for sediments. In addition, the presence and destruction of hematite in natural environments bear great information of paleoenvironmental changes. To understand the magnetism of hematite, it is essential to determine the grain size dependence of magnetic properties for hematite. Previous studies have mostly focused on the hematite particles with grain size well above its superparamagnetic (SP) / single domain state (SD) threshold (~27.5 nm for pure hematite) up to several tends of microns. However, less attention has been made to the determination of the SP/SD threshold, typically for Aluminum-substituted hematite. In this study, a series of Al substituted hematite (Al-hematite) were synthesized with different particle sizes, which range from several tens to several nanometers (nm). On the basis of comprehensive rock magnetic (including hysteresis loops, low temperature AC magnetic susceptibility at several frequencies, etc.) and non-magnetic measurements (XRD and TEM), we observed that as the particles size decreases, hysteresis loops change from standard SD shape to wasp waist shape. Coercivity (Hc) and magnetic remanence (Mr) decrease nearly to zero at 15.4±3.2 nm. In addition, low temperature hysteresis loops show that sample with particle size 15.4±3.2 nm display SD properties at low temperatures, while as temperature increases, Hc and Mr decrease to zero, which further confirm its SP behavior. Therefore, the SP/SD threshold for Al-hematite is estimated to be ~15.4±3.2 nm, lower than that of pure hematite. This new estimation greatly improves our understanding of the complicated magnetic properties of Al-hematites in natural samples.
... These loops indicate the existence of two distinctive coercivity components, one saturated around 1 T but the other far from saturation. The higher coercivity component is probably caused by the surface effect or the existence of exchange bias, which may be attributed to a strong unidirectional anisotropy caused by the Al-ordering arrangement (Figure S2 in the auxiliary material) [Fabian et al., 2008; McEnroe et al., 2009]. Then, coercivity (B c ) and pseudo-saturated magnetization (PM s , as it is not saturated absolutely) used in the following text are obtained from the first saturated component. ...
Article
Hematite, a ubiquitous mineral in aerobic sediments and soils of temperate and warm areas, is weakly magnetic. However, it carries a stable natural remanent magnetization, and thus can reflect paleoenvironment changes. To quantify the influence of Al content in hematite on its magnetic properties, two series of hematite particles were prepared by hydrothermal transformation of ferrihydrite in aqueous suspension (HFh* series) and by thermal dehydration of goethite (HG* series). Crystal morphological and mineral magnetic properties of these two types of hematites differ distinctively. More specifically, the HFh* series samples display oblate (plate-like) morphologies, while the HG* series samples are prolate (highly acicular). HFh* series samples display higher saturation magnetization but lower magnetic coercivity than that of the HG* series. It is tenable that a better lattice ordering of Al substitution occurs during the process of dehydration of goethite than after transformation from ferrihydrite, resulting in weaker saturation magnetization for HG* series samples. The origin of single domain (SD) hematite in nature can be diagnosed by the correlation of unblocking temperature and magnetic coercivity: a positive correlation indicates the presence of pure (Al-free) SD hematite, while a negative correlation indicates a chemical origin of SD Al-substituted hematite. These results bear new information on decoding the complex magnetic properties of SD Al-hematite in nature environments, and thus deepen our understanding of the mechanism of variations in both paleomagnetic and paleoenvironmental signals carried by Al-hematite.
... We propose that the anomaly is probably caused by a metamorphic complex situated in the upper crust. Rogaland, Norway (McEnroe et al. 2004and Robinson et al. 2002 and in the Modum district of Southern Norway (Fabian et al. 2008). These results suggest that the stabile remanent magnetization is produced by the exsolution of the hematite-ilmenite minerals. ...
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In this study we interpret the magnetic anomalies at satellite altitude over a part of Europe and the Pannonian Basin. These anomalies are derived from the total magnetic measurements from the CHAMP satellite. The anomalies are reduced to an elevation of 324 km. An inversion method is used to interpret the total magnetic anomalies over the Pannonian Basin. A three dimensional triangular model is used in the inversion. Two parameter distributions, Laplacian and Gaussian are investigated. The regularized inversion is numerically calculated with the Simplex and Simulated Annealing methods and the anomalous source is located in the upper crust. A probable source of the magnetization is due to the exsolution of the hematite-ilmenite minerals.
... Following discovery of highly magnetized lithosphere on Mars in the absence of a Martian magnetic field, it was realized that hematite can contribute significantly to remanent magnetization of the earth's deeper crust ( Kletetschka et al., 2000a;Dunlop and Kletetschka, 2001). Further studies have shown that multidomain hematite ( Kletetschka et al., 2000b), slowly cooled fine lamellae of hematite-ilmenite intergrowth (Robinson et al., 2002;Kasama et al., 2004;Harrison et al., 2006;McEnroe et al., 2007;Fabian et al., 2008), and hematite with fine intergrowths of magnetite and/or maghemite ( Schmidt et al., 2007) can retain remanent magnetizations with intensities comparable to magnetite. These hematite magnetizations can be very stable. ...
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Recent and new paleomagnetic data from ignimbrite-rich Carboniferous successions of the western Tamworth Belt, Southern New England Orogen, eastern Australia, show a northward excursion over ~ 30°. Paleozoic data from the Australian craton and Tasman Orogenic System (TOS) suggest an Early Devonian start. At the middle-late Visean peak, the central New Guinean promontory of the Australian craton reached 30°–40°N, within the latitude range of the western Central Asian Orogenic Belt (CAOB). Devonian–Carboniferous convergence of Australia/northeastern Gondwana with the CAOB, across the Paleoasian–Rheic Ocean, is proposed as a major driver for contemporaneous tectonism throughout Australia and the CAOB. This implies a substantial Variscan, Pangea-forming, influence on Australian Devonian–Carboniferous tectonics — Alice Springs Orogeny (ASO) and Quilpie and Kanimblan Orogenies. Convergence-related compressional deformation of Australia is largely confined to a “compression box”, extending southward from the New Guinean promontory and bounded westward by the Lasseter Shear Zone and eastward by the East Australian Rift System. Comparable characteristics of Paleozoic Australia–Asia and Cenozoic India–Asia convergence — north–south compression, weak and heated crust (Larapintine Graben and TOS/Tibetan Plateau), eastern “free oceanic boundary” (Paleopacific/Pacific) — do link Paleozoic Australia–Asia convergence to Cenozoic tectonic extrusion of Tibet. Tectonic extrusion of ductile lower crust from the Larapintine Graben led to eastward displacement of the Thomson and Northern New England Orogens, with upper crustal displacement bounded northward by Arunta Block shear zones, the Diamantina River Lineament, the Clarke River Fault Zone and the Townsville Trough, and southward by the Darling River/Cobar-Inglewood Lineaments and Cato Fracture Zone with the Lake Blanche-Olepoloko Fault Zones and Lachlan Transverse Zone as a subsidiary. Recognition of ASO-related tectonic extrusion opens novel, provocative, insights into puzzling aspects of Australian Middle–Late Paleozoic evolution: (i) The different tectonic grains of the Alice Springs Orogeny (ASO) (E-W) and the Quilpie Orogeny (N-S) represent different effects of Australia-Asia convergence on brittle upper crust and ductile lower crust. In the upper crust, direct transmission of north-south compression led to the east-west tectonic grain of the ASO. In the lower crust, hydraulic re-alignment of north-south compression along the east-west pressure gradient near the “free boundary” of the Paleopacific, led to the north-south tectonic grain of the Quilpie Orogeny; (ii) Seismic tomography of the Australian uppermost mantle/lower crust shows east-west fanning out of azimuthal anisotropy in eastern Australia and continental-like velocities in the internal TOS — features reflecting lower crustal tectonic extrusion from the Larapintine Graben; (iii) Prominent negative magnetic anomalies across the Larapintine Graben and the internal TOS represent reverse polarity overprinting of deeper crust, of post-tectonic-extrusion, Kiaman, origin; (iv) Devonian-Early Carboniferous paleomagnetic overprints, widely present across the upper crust of the “compression box”, represent syn-tectonic-extrusion, mainly pre-Kiaman, hydrothermal imprints emanating from lower crustal ductile flow; (v) The dual pattern of pervasive deeper crustal Kiaman remagnetisation and upper crustal Middle-Late Paleozoic paleomagnetic overprinting resembles remagnetisation patterns for European Variscan massifs; (vi) Extent of lower crustal flow may be outlined by seismic mapping of highly reflective lower crust, assuming that its planar anisotropy reflects horizontal ductile flow rather than vertical magmatic underplating; (vii) The Namurian sedimentary lacunae reflect thermal expansion from heated lower crustal ductile flow masses and consequent upper crustal erosional transport from the thermally elevated “compression box” into the tectonically less affected New England Orogen (NEO) and western Australian basins; (viii) Lower crustal thermal expansion led to Late Carboniferous development of the Kanimblan Highlands (TOS), with latest Carboniferous and Permian thermal relaxation leading to their demise; (ix) Enhanced middle - Late Carboniferous lower crustal heat flux may explain the fission track and low-temperature isotopic disturbance record without the disputed requirement for extensive burial and denudation; (x) Stephanian (~305 Ma) start of clockwise rotation of Gondwana caused northward telescoping of the SNEO against the eastward-displaced buttress of the NNEO. Telescoping was facilitated by detachment of the SNEO continental ribbon from the Lachlan Orogen along the East Australian Rift System and led to formation of the Texas-Coffs Harbour and Manning Oroclines.
... attributed to a strong unidirectional anisotropy caused by the Al-ordering arrangement ( Figure S2 in the auxiliary material) [Fabian et al., 2008;McEnroe et al., 2009]. Then, coercivity (B c ) and pseudo-saturated magnetization (PM s , as it is not saturated absolutely) used in the following text are obtained from the first saturated component. ...
Article
Hematite is the eventual magnetic oxide that forms in the aerobic and moist heat environments (e.g., tropical and sub-tropical areas). Although hematite is weakly magnetic compared to the other ferromagnetic oxides like magnetite and titanomagnetite, it is usually abundant in natural environment, and is one of the most important remanent carriers. In addition, the formation and preservation of hematite is sensitive to the surrounding environments, so the indexes related to hematite can be used as paleoenvironmental proxies. In this study, we synthesized hematite by two major pathways: hydrothermal transformation of ferrihydrite in aqueous suspension for several days (series I), and the thermal dehydration of different goethites which were aged at 60 centidegree and room-temperature, respectively (series II). These samples were undertaken a body of experiments, including Atomic Absorption Spectrometry (AAS), X-ray Diffraction (XRD), Transmission Electron Microscope (TEM), magnetic susceptibility, hysteresis curves and low-temperature remanence measurements. The XRD data confirm the purity of all samples, and the unit cell edge lengths of hematites decrease as Al substitution increases. The TEM photos display that the series I samples are platy-shape, and become larger and thinner with the increase of Al concentration. While the series II samples preserve the outline of their precursors (goethite). Furthermore, its particle sizes become smaller as the Al substitution increases. For all of samples, the magnetic susceptibility decrease with the increase of Al substitution until the Al concentration reach 6-8 mol%, where the curves increase suddenly. The low-temperature remanence curves show that there is unambiguous Morin transition for hematite transformed from goethite dehydration reaction with particle size larger than 100 nm, which is absent for the series I samples. In conclusion, we can distinguish these two kinds of hematite with the following pieces of evidence: (1) hematite with the Al substitution higher than 16 mol% is transformed from goethite; (2) the particle size for hematite by hydrothermal transformation of ferrihydrite is larger; (3) hematite synthesized by hydrothermal method has higher magnetic susceptibility and magnetization than those transformed from goethite. These new results provide new insights into the formation environment of hematite, and they are important for both paleomagnetic and paleoenvironment studies.
... It is the abundance of these lamellae that will control the amount of magnetization in the hemo-ilmenite samples , 2004, McCammon et al. 2009). Though these structures are very small they are stable due to the exchange coupling between the lamellae, the contact layer and the host (Fabian et al. 2008. ...
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Magnetic anomalies provide information about location, size and composition of earth structures, ore bodies and tectonic features even in bodies containing only a few percent magnetic minerals. Here we investigate the magnetic properties and oxide mineralogy of anorthosites, rocks rich in plagioclase (>90%), and compare their magnetic signatures to aeromagnetic anomaly maps of the regions. Two of the anorthosite complexes have large negative anomalies associated with them; both have low susceptibility and high remanence related to hemo-ilmenite mineralogy and remanent directions antiparallel to the present field. One complex has appreciable natural remanent magnetization quasi-parallel to the present field, and strong susceptibility, creating an enhanced positive anomaly. The fourth anorthosite has little or no magnetic anomaly over much of its area, in accordance with the weak remanence, low susceptibility and variable magnetic mineralogy observed. The anorthosite samples producing significant anomalies, and maintaining strong and stable natural remanent magnetization over geologic time all contain oxides of the hematite-ilmenite series. This study adds support to ‘lamellar magnetization’ whereby exsolved phases in the ilmenite-hematite system produce strong and stable magnetization with only minor amounts of oxide material.
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This chapter describes the current state of the art in the field of computational and experimental mineral physics, as applied to the study of magnetic minerals. This chapter is divided into four sections, describing new developments in the study of mineral magnetism at the atomic, nanometer, micrometer, and macroscopic length scales. We begin with a description of how atomistic simulation techniques are being used to study the magnetic properties of mineral surfaces and interfaces and to gain new insight into the coupling between cation and magnetic ordering in Fe-Ti-bearing solid solutions. Next, we review the theory of off-axis electron holography and its application to the study of magnetotactic bacteria and minerals containing nanoscale transformation-induced microstructures. Then, we review the theory and application of micromagnetic simulations to the study of nonuniform magnetization states and magnetostatic interactions in minerals at the micrometer length scale. Finally, we review recent developments in the use of macroscopic magnetic measurements for characterizing and quantifying the microscopic spectrum of coercivities and interaction fields present in rocks and minerals.
Article
Recent high-resolution aeromagnetic surveys in South Norway have revealed numerous remanent anomalies over Mesoproterozoic metamorphic rocks. Studies on the nature of the minerals that are the remanent carriers has led to discoveries of titanohematite samples with unusual magnetic properties caused by nanoscale exsolution lamellae with their related lamellar magnetism. Here we focus on a rock unit dominated by quartz-plagioclase-biotite granulite containing titanohematite grains with a strong lattice-preferred orientation parallel to regional foliation. When samples with their natural remanent magnetization (NRM), acquired nearly 1 billion years ago, are cooled to 10 K and hysteresis loops measured, these loops show bi-modal exchange bias caused by the magnetism induced within the ilmenite by antiferromagnetic coupling with the adjacent lamellar NRM. By contrast when the samples are cooled in a strong magnetic field (1.5 Tesla), this results in unimodal lamellar magnetism, and, below the T-N of ilmenite it adopts a consistent negative orientation, giving rise to unimodal negative exchange bias of >500 mT. The results presented here cover the chemical and magnetic properties, Mossbauer results and transmission electron microscopy of the titanohematite and ilmenite lamellae. Initial magnetic experiments indicated the shifts found in the exchange-bias experiments were directly related to the orientation of the sample to the applied field and the initial state of the NRM. In most samples with these unusual magnetic properties, ilmenite lamellae could not be seen in an optical or a scanning electron microscope. However magnetic experiments gave proof of the presence of ilmenite, later confirmed by Mossbauer spectroscopy. Several attempts were made to identify ilmenite in TEM studies, finally successful in showing ilmenite lamellae parallel to (001) of hematite with thicknesses similar to 1.2 to 1.7 nmand aspect ratios 7-13. Here we compare new TEM images and the magnetic behaviour of these samples to the MOD2 samples that previously showed extraordinary exchange bias properties, and investigate further the nature of these magnetic minerals.
Article
The Pannonian Basin is a deep intra-continental basin formed as part of the Alpine orogeny. In order to study the nature of the crustal basement we used the long-wavelength magnetic anomalies acquired by the CHAMP. Some 165522 data distributed in a spherical shell were available, which were recorded between January 1 and December 31 of 2008. They covered the Pannonian Basin and its vicinity. Those magnetic data were interpolated into a spherical grid of 0.5° × 0.5°, at the elevation of 324 km by the Gaussian weight function. The vertical gradient of the total magnetic anomalies were also computed and mapped on the surface of a sphere at 324 km elevation. The former spherical anomaly data at 425 km altitude were downward continued to 324 km. To interpret those data we used an inversion method based on a polygonal prism model. The minimum problem was solved numerically by the simplex and simulated annealing methods. To solve the problem L1 norm was used in the case of Gaussian distribution parameters and L1 norm in the case of Laplace distribution parameters. We suppose that the magnetic anomaly is produced by the exsolution of hemo-ilmenite minerals in metamorphic rocks of the upper crust.
Article
Extensive negative aeromagnetic anomalies in the Modum area, south Norway, derive from rocks containing ilmenite with hematite exsolution, or hematite with ilmenite exsolution, carrying strong/stable reversed remanence. Here we describe a 2.5 cm thick high-temperature metamorphic vein of exsolved titanohematite. Reflected-light and EMP analyses show it contains three types of exsolution: spinel plates on (001); rutile blade satellites on spinel oriented at angles of similar to 60-90 degrees to titanohematite (001); and lamellae 0.1-0.3 mu m thick too fine for EMP analyses, also parallel to (001). Powder XRD gave a = 5.0393 angstrom, c = 13.7687 angstrom, V = 302.81 angstrom(3) for titanohematite (approximate to Ilm9), and unrefined reflections of rutile and geikielite. Overlap EMP analyses showed enrichment in MgO, TiO2, and lack of Al2O3, indicating a mixture of titanohematite and geikielite. Non-overlap analyses showed the titanohematite is 6%Fe2+TiO3, 2%MgTiO3, 92% Fe2O3, generally confirmed by TEM-EDX analyses that also showed the geikielite is 30%Fe2+TiO3, 70%MgTiO3. Orientation and interface relationships between exsolutions and host titanohematite were characterized with TEM, using conventional and high-resolution imaging complemented by selected-area electron diffraction. Spinel shares (111) with (001) of titanohematite and geikielite (001) the same. The epitactic relationship between rutile and titanohematite, previously not well constrained, was estimated from reflected-light and TEM images and lattice-fit studies. The a(1) axis of rutile is parallel to a(1) of hematite and c of rutile is normal to a(2) of hematite, all in the hematite basal plane, which, however is not a phase interface. The rutile appears to occur in blades within prism planes in titanohematite located similar to 69 degrees from a axes of hematite, with long axes of the blades oriented in a minimum strain direction within the planes at similar to 63 degrees from the (001) basal plane. Spinel and rutile, analyzed by EMP, exsolved first. Spinel gave 96%MgAl2O4, 3%FeFe2O4, Mg/total R2+ = 0.98. Magnesian/aluminous spinet lacking Ti exsolved from titanohematite in coupled exsolution with ferrian rutile, where combined components were dissolved as corundum/geikielite components in high-T aluminous magnesian titanohematite. Early exsolution lowered geikielite, and eliminated the corundum component. Later fine exsolution of ferroan geikielite moved the titanohematite closer to Fe2O3. Mg2+ has no magnetic moment, but breaks up linkages between Fe atoms, lowers Neel Ts, and produces unusual low-T properties. This titanohematite has Neel T, 873 K (600 degrees C). Geikielite at 70%MgTiO3, is far beyond its theoretical nearest-neighbor percolation threshold at 30.3%MgTiO3. However, the sample shows a negative magnetic exchange bias below 25 K and low-T remanence lost above similar to 40 K. Such properties are reported in samples containing thin ilmenite lamellae in titanohematite, in theory with odd numbers of Fe layers, where exchange bias is linked to lamellar magnetism at the phase interfaces, when the ilmenite becomes a high-anisotropy magnet in a magnetically softer host. Potential explanations for the behavior of ferroan geikileite are discussed.
Article
Large local anomalies in the Earth's magnetic field have been observed in Norway, Sweden, and Canada. These anomalies have been attributed to the unusual magnetic properties of naturally occurring hemo-ilmenite, consisting of a paramagnetic ilmenite host (α-Fe2O3-bearingFeTiO3) with exsolution lamellae (≈3μm thick) of canted antiferromagnetic hematite (FeTiO3-bearingα-Fe2O3) and the mutual exsolutions of the same phases on the micron to nanometer scale. The origin of stable natural remanent magnetization (NRM) in this system has been proposed to be uncompensated magnetic moments in the contact layers between the exsolution lamellae. This lamellar magnetism hypothesis is tested here by using polarized neutron diffraction to measure the orientation of hematite spins as a function of an applied magnetic field in a natural single crystal of hemo-ilmenite from South Rogaland, Norway. Polarized neutron diffraction clearly shows that the ilmenite spins do not contribute to the NRM and that hematite spins account for the full magnetization at ambient temperature. Hematite sublattice spins are shown to adopt an average angle of 56∘ with respect to a saturating magnetic field, which is intermediate between the angle of 90∘ predicted for a pure canted moment and the angle of 0∘ predicted for a pure lamellar moment. The observed NRM is consistent with the vector sum of lamellar magnetism and canted antiferromagnetic contributions. The relative importance of the two contributions varies with the length scale of the microstructure, with the lamellar contribution increasing when exsolution occurs predominantly at the nanometer rather than the micrometer scale.
Article
Single magnetic nanodots, exchange coupled to an antiferromagnetic (AF) matrix, can produce large exchange bias, while superparamagnetic behavior of the nanodots is suppressed. The exchange bias originates from the formation of a (quasi)spherical domain wall inside the AF matrix when the particle moment rotates under the influence of an external magnetic field. Micromagnetic calculations show that for isolated nanodots the energy of this domain wall increases nearly quadratically with the deflection angle of the nanodot moment. By introducing the corresponding quadratic energy term in a modified Stoner-Wohlfarth model, a two-parameter family of hysteresis loops is obtained, depending on scaled anisotropy energy and field direction. The loops are represented in a phase diagram with three main regions, containing (1) reversible loops, (2) irreversible loops with a metastable 180° AF domain wall, and (3) loops with metastable AF domain walls with 360° or higher rotation angles. According to this model, isolated nanodots display reversible negatively biased loops for all field directions, if their anisotropy energy is small in comparison to the AF domain-wall energy. For higher anisotropy, irreversible, mostly negatively biased, loops result from switching between the ground state and an higher-energy inverse state with a 180° AF domain wall. At even higher anisotropy energy, the loops can show positive exchange bias after an initial “training branch.” Switching after “training” takes place between states having a 180° and a 360° AF domain wall, respectively. While for thin films, the bias field increases in inverse proportion to thickness, for nanodots it increases in inverse proportion to the square of particle diameter. Therefore, nanodots can show significantly larger exchange bias than thin films of similar dimension. Hysteresis loops, obtained from averaging over directions and sizes using the modified Stoner-Wohlfarth model, were compared to measurements from a natural sample with nanometer-scale ilmenite-exsolution lamellae in a hematite matrix. The shapes of the hysteresis difference, the difference between upper and lower branches, are similar for model and experiment, whereby increasing temperature in the measurement corresponds qualitatively to decreasing the relative anisotropy energy in the model.
Article
The Pannonian Basin is a deep intra-continental basin that formed as part of the Alpine orogeny. In order to study the nature of the crustal basement we used the long-wavelength magnetic anomalies acquired by the CHAMP satellite. The anomalies were distributed in a spherical shell, some 107,927 data recorded between January 1 and December 31 of 2008. They covered the Pannonian Basin and its vicinity. These anomaly data were interpolated into a spherical grid of 0.5°×0.5°, at the elevation of 324km by the Gaussian weight function. The vertical gradient of these total magnetic anomalies was also computed and mapped to the surface of a sphere at 324km elevation. The former spherical anomaly data at 425km altitude continued downward to 324km. To interpret these data at the elevation of 324km we used an inversion method. A polygonal prism forward model was used for the inversion. The minimum problem was solved numerically by the Simplex and Simulated annealing methods; a L2 norm in the case of Gaussian distribution parameters and a L1 norm was used in the case of Laplace distribution parameters. We interpret that the magnetic anomaly was produced by several sources and the effect of the sable magnetization of the exsolution of hemo-ilmenite minerals in the upper crustal metamorphic rocks.
Article
The orientation of spins in a natural sample of Ti-bearing hematite (Fe2O3) has been measured from 2–300 K using time-of-flight neutron powder diffraction. It is shown that the antiferromagnetic alignment vector is tilted out of the basal plane by an average angle of 30°, independent of temperature, contrary to the normal expectation that all spins lie in the basal plane due to the suppression of the Morin transition by Ti. This unusual result is related to the non-uniform spatial distribution of Ti in this sample, which takes the form of ~1 nm exsolution lamellae of ilmenite (FeTiO3), observed using transmission electron microscopy. It is suggested that the exsolution lamellae lead to a localization of Fe2+ species within the lamellar interfaces, which cause tilting of some spins toward the crystallographic c axis. The presence of an out-of-plane component of spin at room temperature reconciles experimental and computational attempts to explain the phenomenon of “giant exchange bias” that appears when this sample is zero-field cooled below the ilmenite Néel temperature.
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An approximate low-temperature, metastable phase diagram is drawn for the system (1 - X) Fe2O3-(X)FeTiO3. It is based on published and new magnetic data from nine synthetic samples with bulk compositions in the range 0.6 <X< 1.0. Fields are plotted for(l) the paramagnetic phase (PM); the Fe2O3-rich ferrimagnetic phase (FM); (2) the FeTiO3-rich anti ferromagnetic phase (AF); and (3) a rc-entrant spin-glass phase (RSG). In addition, two subfields are plotted: (1) FM', a subfield of the FM-phase, which occurs below a characteristic temperature T-K, at which the magnetic susceptibility drops sharply on cooling, and (2) PM', a subfield of the PM-phase (traditionally called superparamagnetic) forms below a sharp rise in Susceptibility at T-S and exhibits measurable dispersion in the magnetic susceptibility at T< T-S. The diagram is drawn with a bicritical point, T-lambda lambda at X approximate to 0.87 T approximate to 39 K. which is the intersection of second-order magnetic phase boundaries for the paramagnetic -> ferrimagnetic [PM(PM') -> FM] transition, T-C(X), and the PM(PM') -> AF transition, T-N(X). In addition, the RSG phase is plotted as one of four stable phases at T-lambda lambda a construction that is not required by the phase rule, but is strongly favored by the physics of competition between the incompatible magnetically ordered Structures of the FM- and AF-phases. These phase relations are at Such low temperature as to be of little consequence for terrestrial magnetism, however, they may well be essential for interpreting the magnetism of the Moon, Mars, and other cold planets. These phase relations are also essential for the characterization of fine natural and synthetic intergrowths, and for understanding magnetic materials for low-temperature technological applications.
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The current CHAMP and the upcoming SWARM missions provide high-resolution magnetic data which for the first time, allow study of remanent crustal magnetization directly from space. SWARM promises nearly to close the resolution gap to aeromagnetic surveys, thereby opening new doors for validation of level 2 data products. By combining aeromagnetic data with mineralogical investigations on old crustal shields, we study the primary sources of large scale-remanent anomalies in continental crust. We observe that an important contribution in strong anomalies in old oxidized crust arises from lamellar magnetism. The mineralogic and magnetic investigation of this newly discovered remanence source on length scales from nanometer to aeromagnetic will help to understand and validate the crustal magnetic data inferred from the SWARM mission.
Article
Magnetic ordering in the ilmenite-hematite solid solution (Fe2-x Ti x O3) has been investigated using Monte Carlo simulations, with particular emphasis on the low-temperature spin glass region of the phase diagram. Complex magnetic behavior is observed due to the presence of two competing magnetic order parameters: a ``hematite-like'' ordering with a two-layer repeat (Q 2) and an ``ilmenite-like'' ordering with a four-layer repeat (Q 4). The susceptibility and degree of magnetic order were calculated from the Fourier transform of the layer-averaged spin distribution, allowing long-range and short-range contributions from Q 2 and Q 4 to be analyzed separately. For x = 0.8 a heterogeneous FM phase (HFM) followed by a modulated FM phase (MFM) develops. There is an increasing contribution from Q 4 with increasing x, and a pronounced cusp in both Q 2 and Q 4 susceptibilities develops at 30 K. The superposition of Q 2 and Q 4 leads to frustrated layers containing dynamically disordered spins. Freezing of this spin disorder below 30 K is responsible for the cusp in susceptibility, which can be classified as a reentrant spin glass (RSG) transition. A gradual loss of long-range FM order occurs as the percolation threshold is approached, resulting in a conventional spin glass (CSG) with no long-range order below 30 K for x >= 0.92. For x > 0.95, a transition to an antiferromagnetic (AF) phase occurs at 40-55 K, followed by an RSG transition at 20-30 K. Changes to the phase diagram caused by chemical clustering are determined using a preannealing algorithm. Clustering expands the AF field to x > 0.9 and the HFM field to x >= 0.55. The topology of the simulated phase diagram compares favorably with experiments but suggests that the nature of some phase boundaries should be reexamined from both experimental and computational perspectives.
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Magnetic anomalies are deviations from an internal planetary magnetic field produced by crustal materials. Crustal anomalies, measured over a wide range of vertical distances, from near-surface to satellites, are caused by magnetic minerals that respond to the changing planetary field. Previously, magnetism of continental crust was described In terms of the bulk ferrimagnetism of crustal minerals, which is mostly due to induced magnetization. the recent discovery of lamellar magnetism, a new interface-based remanence type, has changed our thinking about the contribution of remanent magnetization. lamellar magnetism may also be an important contributor to deep-seated nomallies in the crust of the Earth and in other planets with highly magnetic trusts, like Mars.
Article
Lorentz transmission electron microscopy has been used to study fine-scale exsolution microstructures in ilmenite-hematite, as part of a wider investigation of the lamellar magnetism hypothesis. Pronounced asymmetric contrast is visible in out-of-focus Lorentz images of ilmenite lamellae in hematite. The likelihood that lamellar magnetism may be responsible for this contrast is assessed using simulations that incorporate interfacial magnetic moments on the (001) basal planes of hematite and ilmenite. The simulations suggest qualitatively that the asymmetric contrast is magnetic in origin. However, the magnitude of the experimental contrast is higher than that in the simulations, suggesting that an alternative origin for the observed asymmetry cannot be ruled out. Electron tomography was used to show that the lamellae have lens-like shapes and that (001) planes make up a significant proportion of the interfacial surface that they share with their host.
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Hemo-ilmenite ores from Allard Lake, Quebec, were first studied over 50 years ago. Interest was renewed in these coarsely exsolved oxides, based on the theory of lamellar magnetism as an explanation for the high and stable natural remanent magnetizations (NRMs), 32 to 120 A/m, reported here. To understand the magnetism and evolution of the exsolution lamellae, the microstructures and nanostructures were studied using scanning electron microscopy and transmission electron microscopy (TEM), phase chemistry, and relations between mineral chemistry and the hematite-ilmenite phase diagram. Cycles of exsolution during slow cooling resulted in lamellae down to 1-2 nm thick. Combined electron microprobe, TEM, and X-ray diffraction (XRD) results indicate that hematite hosts reached a composition approximately ilmenite (Ilm) 14.4, and ilmenite hosts ~Ilm 98. The bulk of the very stable NRM, which shows thermal unblocking ~595-620°C, was acquired during final exsolution in the two-phase region canted antiferromagnetic R {\overline 3 c hematite + R {\overline 3 ilmenite. Hysteresis measurements show a very strong anisotropy, with a stronger coercivity normal to, than parallel to, the basal plane orientation of the lamellae. Magnetic saturation (Ms) values are up to 914 A/m, compared to 564 A/m predicted for a modally equivalent spin-canted hematite corrected for ~15% R2+TiO3 substitution. Low-temperature hysteresis, AC-susceptibility measurements, and Mössbauer results indicate a Néel temperature (TN) of the geikielite-substituted ilmenite at ~43 K. The low-temperature hysteresis and AC-susceptibility measurements also show a cluster-spin-glass-like transition near 20 K. Below TN of ilmenite an exchange bias occurs with a 40 mT shift at 10 K.
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A large, distinct negative aeromagnetic anomaly of over 2000 nT associated with microcline-sillimanite-quartz gneisses in the Russell area, northwest Adirondack Mountains, was previously shown to be remanence-dominated, although the carriers of remanence were not well documented. Russell Belt gneisses have a strong natural remanent magnetization with steep remanence directions, D=263°, I=-58°, an average intensity of 3.6 A/m, and typical susceptibilities of 10-4SI. The remanence is thermochemical in origin, acquired during cooling from peak metamorphic conditions of 650°-750°C during the Ottawan Orogen (1050-1080 Ma). The reversed polarity of remanence reflects a reversed paleofield, rather than self-reversed, contrary to earlier suggestions. The gneisses contain up to 3% oxide, predominantly metamorphic titanohematite, which accounts for the low susceptibility values and highly stable remanence. Optical observations show titanohematite grains with multiple generations of ilmenite, pyrophanite, rutile, and spinel exsolution lamellae. Microprobe analyses confirm titanohematite compositions ranging from 72 to 97%Fe2O3, with hematite83 being most typical. In rare samples, inclusions of magnetite were identified. The ubiquitous presence of titanohematite, and the rare occurrence of magnetite, is supported by thermal and alternating field demagnetization studies, saturation magnetization measurements, hysteresis properties, temperature-hysteresis studies, and low-temperature remanence measurements. Numerous crustal granulites have titanohematite as part of the oxide assemblage, and this may contribute a strong remanent component to what have previously been considered to be solely induced anomalies.
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A model of the process by which certain members of the ilmenite-hematite solid solution series acquire reverse therinoremanent magnetization (reverse TRM) is proposed. It is based on the observation that the cation order-disorder transformation at work in this mineral system produces cation-ordered regions, or domains, antiphase with respect to cation positions. Given the fundamental assumption that the so-called "x phase", the weakly magnetic phase known to control self-reversal, is in fact Fe-rich cation-disordered boundary material between the two types of cation-ordered domains, symmetry considerations help eliminate the possibility that the direction of magnetic remanence associated with the boundaries lies parallel to the line of Fe spin. Rather, the net magnetization of the boundary material need be essentially at right angles to the orientation of spin, as is the case for pure hematite. Results of thermomagnetic experiments further suggest that superexchange with the Fe-rich boundary material produces a ferdmagnetic structure within each cation-ordered domain such that its net moment lies along the direction of spin, that is, perpendicular to the net boundary moment. At this stage the moments of each pair of cation-ordered domains (antiphase in composition) cancel. We propose that reverse TRM arises during cooling in an applied field through rotation of particular Fe spins within each cation-ordered domain, causing the spin arrangement to become progressively noncollinear. Competing superexchange interactions within the cation-ordered regions may be responsible for such a spin rotation. According to this model, each cation domain provides a component of magnetization opposite to the net magnetization associated with the boundary material. Provided that there exists in a given grain equal fractions of each type of cation domain, the grain will fully self-reverse. Synthesized polycrystalline samples as well as rocks containing ferrari ilmenites having compositions in the self-reversing range will acquire reverse TRM.
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Magnetic exchange bias is a phenomenon whereby the hysteresis loop of a 'soft' magnetic phase is shifted by an amount H(E) along the applied field axis owing to its interaction with a 'hard' magnetic phase. Since the discovery of exchange bias fifty years ago, the development of a general theory has been hampered by the uncertain nature of the interfaces between the hard and soft phases, commonly between an antiferromagnetic phase and a ferro- or ferrimagnetic phase. Exchange bias continues to be the subject of investigation because of its technological applications and because it is now possible to manipulate magnetic materials at the nanoscale. Here we present the first documented example of exchange bias of significant magnitude (>1 T) in a natural mineral. We demonstrate that exchange bias in this system is due to the interaction between coherently intergrown magnetic phases formed through a natural process of phase separation during slow cooling over millions of years. Transmission electron microscopy studies show that these intergrowths have a known crystallographic orientation with a known crystallographic structure and that the interfaces are coherent.
Article
Grains consisting of finely exsolved members of the hematite-ilmenite solid-solution series, such as are present in some slowly cooled middle Proterozoic igneous and metamorphic rocks, impart unusually strong and stable remanent magnetization. TEM analysis shows multiple generations of ilmenite and hematite exsolution lamellae, with lamellar widths ranging from millimeters to nanometers. Rock-magnetic experiments suggest remanence is thermally locked to the antiferromagnetism of the hematite component of the intergrowths, yet is stronger than can be explained by canted antiferromagnetic (CAF) hematite or coexisting paramagnetic (PM) Fe-Ti ordered (R3̄) ilmenite alone. In alternating field demagnetization to 100 mT, many samples lose little remanence, indicating that the NRM is stable over billions of years. This feature has implications for understanding magnetism of deep rocks on Earth, or on planets like Mars that no longer have a magnetic field. Atomic-scale simulations of an R3̄ ilmenite lamella in a CAF hematite host, based on empirical cation-cation and spin-spin pair interaction parameters, show that contacts of the lamella are occupied by "contact layers" with a hybrid composition of Fe ions intermediate between Fe2+-rich layers in ilmenite and Fe3+-rich layers in hematite. Structural configurations dictate that each lamella has two contact layers magnetically in phase with each other, and out of phase with the magnetic moment of an odd non-self-canceling Fe3+-rich layer in the hematite host. The two contact layers and the odd hematite layer form a magnetic substructure with opposite but unequal magnetic moments: a lamellar "ferrimagnetism" made possible by the exsolution. Because it is confined to magnetic interaction involving the moments of just three ionic layers associated with each individual exsolution lamella, lamellar magnetism is unique and quite distinct from conventional ferrimagnetism. Simulation cells indicate that the magnetic moments of contact layers are locked to the magnetic moments of adjacent AF hematite layers and are parallel to the basal plane (001). Thus, lamellar magnetism is created at the temperature of chemical exsolution, and is a chemical remanent, rather than thermal remanent, magnetization. However, in thermal demagnetization experiments, too short for lamellar resorption, demagnetization temperatures are those of the CAF hematite, considerably higher than temperatures of original lamellae formation. Internal crystal structure cannot dictate that the contact layers of different lamellae will form magnetically in phase with each other to give the highest net magnetic moment, but magnetic moments of lamellae can be made to form in phase by the external force of the magnetizing field at the time of exsolution. A thesis of this paper is that an external magnetic field can dictate the magnetic moments and hence the chemical location of ilmenite lamellae in a hematite host, and that once in place, neither the location nor the magnetic moment will be easily disturbed. In an ilmenite host, the external magnetic field cannot control the chemical location of a hematite lamella, which is dictated by the enclosing ilmenite, but once lamellae have formed, the field can dictate their magnetic moments. These moments, however, are not locked chemically to the host, resulting in lower coercivity. The effectiveness of the external force in single crystals is dictated by their orientation with respect to the magnetizing field. In grains with (001) oriented parallel to the field, it would be effective in producing in-phase magnetic moments and very strong remanence. In grains with (001 normal to the field, the field would be less effective in producing in-phase magnetic moments, hence producing weak remanence. The most intense lamellar magnetism per formula unit occurs with in-phase magnetization, high lamellar yields, and the largest number of lamellae per unit volume (i.e., smallest lamellar size). Compared to the magnetic moment per formula unit (Mpfu) and magnetic moment per unit volume (Mv) of end-member magnetite (Mpfu = 4 μB, Mv = 480 kA/m) and hematite (Mpfu = 0.0115 μB, Mv = 2.1 kA/m), results for some atomic models reasonably tied to natural conditions are Mpfu = 0.46-1.36 μB and Mv = 84-250 kA/m.
Article
We review the phenomenology of exchange bias and related effects, with emphasis on layered antiferromagnetic (AFM)–ferromagnetic (FM) structures. A compilation of materials exhibiting exchange bias and some of the techniques used to study them is given. Some of the applications of exchange bias are discussed. The leading theoretical models are summarized. Finally some of the factors controlling exchange bias as well as some of the unsolved issues associated with exchange bias are discussed.
Article
Mid‐Proterozoic granulites in SW Sweden, having opaque minerals hematiteilmenite with minor magnetite, and occurring in an area with negative aeromagnetic anomalies, have strong and stable reversed natural remanent magnetization ∼9.2 A/m, with 100% remaining after demagnetization to 100 mT. Samples were characterized by optical microscopy, electron microprobe (EMP), transmission electron microscopy (TEM), and rock‐magnetic measurements. Earliest oxide equilibrium was between grains of titanohematite and ferri‐ilmenite at 650°–600°C. Initial contacts were modified by many exsolution cycles. Hematite and ilmenite (Ilm) hosts and lamellae by EMP are Ilm 24–25, ILm 88–93, like titanohematite, and ilmenite above 520°C on Burton's diagram [1991]. Finer hosts and lamellae by TEM are Ilm16 ±3 and Ilm 88±4, like coexisting antiferromagnetically ordered (AF) hematite and ilmenite below 520°C on Burton's diagram. This may be the first example of analytical identification, in one sample, of former hematite, now finely exsolved, and AF hematite. TEM microstructures consist of gently curving semicoherent ilmenite lamellae within hematite, flanked by precipitate‐free zones and abundant ilmenite disks down to unit cell scale (1–2 nm). Strain contrast of disks suggests full coherence with the host, and probable formation at the reaction titanohematite ‐‐‐> AF hematite + ilmenite at 520°C. Magnetic properties are a consequence of chemical and magnetic evolution of hematite and ilmenite with bulk compositions ilmenite‐richer than Ilm 28, that apparently exsolved without becoming magnetized, down to 520°C where hematite broke down to AF hematite plus ilmenite, producing abundant AF hematite below its Néel temperature. Intensity of magnetization is greater than possible with hematite alone, and TEM work suggests that ultrafine ilmenite disks in AF hematite are associated with a ferrimagnetic moment due to local imbalance of up and down spins at coherent interfaces.
Article
Co nanoparticle films were prepared by plasma-gas-condensation-type particle beam deposition system. High-resolution transmission electron microscopy images show that the Co nanoparticles have a very narrow size distribution with an average diameter of ~20nm, and each of the Co nanoparticles is covered with an ~3nm layer of CoO. Hysteresis loops of the films after field-cooling in a 5T magnetic field are greatly shifted, which can be attributed to the exchange bias effect caused by the interfacial exchange coupling between the CoO shell and the Co core. The zero field cooled films show several prominent properties, such as a quite large coercive field, a small remanence and their abnormal dependences on temperature. All these observations can be attributed to the existence of an exchange bias effect within each single Co nanoparticle even without a field-cooling process.
Article
Massive, nearly ‘pure’, haemo-ilmenite layers from historic ore deposits in Rogaland, Norway contain very few silicates or other oxides and typically produce remanence-dominated magnetic anomalies. These rocks are ideal for evaluating the magnetic properties of fine exsolution intergrowths and the larger titanohaematite lamellae in the host ilmenite grains. A typical bulk composition, Ilm 84, exsolved at high temperature to produce host ilmenite Ilm 94 and micron-sized haematite lamellae Ilm 23 as measured by electron microprobe (EMP). Subsequent undercooling of the ilmenite and the micron-scale haematite lamellae led to metastable nucleation of nanoscale lamellae down to unit-cell scale, leaving depleted hosts between lamellae with compositions of Ilm 98 and Ilm 15–13 as measured by TEM–EDX. Samples have high coercivities, and average NRM values of 25 A m−1, which typically show ∼2 per cent saturation in the NRM state. The amount of magnetization in these samples is too high to be solely accounted for by a spin-canted AF moment in the haematite. Based on Monte Carlo simulations of haematite–ilmenite interfaces at the atomic scale and on measured rock-magnetic properties, we predict that the magnetization is carried by a ferrimagnetic substructure produced at the contacts of the very fine-scale titanohaematite and ilmenite exsolution lamellae.
Article
From studies of about 200 samples of metamorphic and igneous rocks from the Adirondacks it has been possible to relate the magnetic oxide assemblage to the lithology of the rocks and to their magnetic properties so that the magnetic anomaly produced by a particular rock type can be estimated. The intermediate members of the FeaOa-FeO.TiChiTiOa) system are found almost uniformly to have reverse remanent magnetism and as much as several percent of disseminated magnetic oxides of this system may give rise to intense negative aeromagnetic anomalies. Where both magnetite and magnetic oxides of the Fe20s-FeO.Ti02-(TiOs) system occur together in the same rock the resultant magnetic properties are such as to indicate that the magnetite grains have normal or positive remanent magnetization and the grains of the magnetic oxides of the FeaOn-FeO.Ti02-(Ti02) system have reverse or negative remanent magnetization. The value of the reverse magnetism of titanhematite, ilmeno-hematite, and rutiloilmenohematite is such that it can neutralize the total positive magnetism (remanent magnetism plus induced magnetism) of an equivalent or greater amount of magnetite. In the metamorphism of a pyroxene gabbro, giving a positive magnetic anomaly, to a hornblende-plagioclase amphibolite, the primary magnetite may be taken up by the hornblende, leaving ferrian ilmenite as the only oxide and the amphibolite may then give a neutral or negative anomaly. Various investigators have suggested that the reverse magnetization of rocks is produced by a reversal of the earth's magnetic field at the time they cooled through the Curie point. This hypothesis does not seem to fit the geological, chemical, and magnetic data for the Adirondack rocks, for the reverse magnetization here appears to be related to the content of the intermediate members of the FezOa-FeO.TiChTiC) system. These mineral mixtures have the property of "self-reversal," that is, the ability to become magnetized in a direction opposite to that of the existing magnetic field. Until the existence of "self-reversal" can be disproved, one cannot state categorically that the reversed magnetization of a rock has been produced by a reversed field of the earth.
Article
The anisotropism of magnetic susceptibility of nearly pure hemo-ilmenite ore from deposits in the Allard Lake area consists of a distinct plane of maximum susceptibility (defined by maximum and intermediate axes along which the susceptibility is of similar magnitude) with a minmum susceptbility axis at right angles. This plane coincides with a preferred crystallographic grain-orientation indicated by the parallelism of titanhematite lamellae which have exsolved on the basal plane of the ferrianilmenite host grains. Measurement of the remanent magnetism (RM) shows a striking tendency for the RM vectors to lie in or near this preferred plane. On the assumption that hematite (if not ilmenite as well) is potentially ferromagnetic in the basal plane only (paramagnetic parallel to the c-axis), it was inferred that the RM vectors could represent the resolved component of the magnetizing field which tended to lie in the plane of maximum susceptibility. Under this hypothesis, it is possible to resolve the broad spread shown by the RM vectors of samples from one deposit in terms of a single magnetizing field direction. The variation of the intensity of RM of otherwise similar samples can also be explained on this basis. With one exception, the RM of 47 measured samples of hemo-ilmenite ore was reversely polarized (N-seeking pole up). Additional ‘natural history’ evidence bearing on the problem of self-reversal of thermo-remanent magnetism (TRM) is provided by this study.
Article
The phenomenology of exchange bias and related effects in nanostructures is reviewed. The types of systems discussed include: lithographically fabricated ferromagnetic (FM)—antiferromagnetic (AFM) nanostructures, chemically surface modified FM particles, FM particles embedded in an AFM matrix, controlled core–shell particles, nanoparticles with surface effects and coupled AFM–AFM systems. The main applications of exchange biased nanostructures are summarized. Finally, the implications of the nanometer dimensions on some of the existing exchange bias theories are briefly discussed.
Article
The interpretation of both palaeomagnetic and geophysical prospecting data requires a better understanding of the magnetic properties of the iron oxide minerals found in rocks. The magnetic properties of ilmenite-haematite solid solutions have been investigated, the unusually large and pure haemo-ilmenite crystals from the Allard Lake region of Quebec being used. The magnetic component of these crystals is an ilmen-haematite phase, having a composition of about 10 mole % of ilmenite in haematite, and present in the form of exsolution lamellae that are roughly 5 μ\mu long, 1 μ\mu wide and 0.2 μ\mu thick. The crystals have a very strong anisotropy causing magnetization in the basal plane and a weak anisotropy which produces an easy direction of magnetization within the basal plane. An improved ilmenitehaematite solvus curve has been produced by X-ray and Curie-point analysis of heat-treated crystals. Spontaneous reversal of magnetic polarity takes place with change in temperature in ilmen-haematite having between about 25 and 15% of ilmenite in haematite. This new reversing range of composition is quite different from that found to reverse by Uyeda. The reversal is due to a new antiparallel moment which grows as temperature falls. This has been interpreted as being due to an ordering of Fe2+^{2+} ions on alternate cation layers by an electron transfer mechanism between trivalent and divalent iron atoms.
Article
To investigate the acquisition mechanism of high and stable natural remanent magnetization (NRM) in rocks of the Russell Belt, Adirondack Mountains, New York, we examined the exsolution microstructures and compositions of magnetic minerals using three samples with different magnetic properties. The samples contain titanohematite with ilmenite lamellae, end-member hematite without lamellae and rare magnetite as potential carriers for the NRM. Transmission electron microscopy (TEM) observations and element mapping by energy-filtered TEM (EFTEM) of the titanohematite indicated that very fine ilmenite lamellae with a minimum thickness ∼2 nm are abundant between larger ilmenite lamellae a few hundreds of nanometers thick. The ilmenite lamellae and titanohematite hosts, with the compositions of Ilm90–100Hem10–0 and Ilm7–16Hem93–84, respectively, share (001) planes, and the abundant fine ilmenite lamellae have coherent, sharp structural and compositional interfaces with their titanohematite hosts. Comparison between samples shows that the magnetization is correlated with the amount of fine exsolution lamellae. These results are consistent with the lamellar magnetism hypothesis, suggesting that the acquisition of high and stable NRM is related to the interfacial area between fine ilmenite lamellae and their host titanohematite. End-member hematite with a multi-domain magnetic structure only contributes a minor amount to the NRM in these samples.
Article
Exchange bias (>1 T at 10 K) has been observed in natural sample of Fe2O3 containing abundant nanoscale exsolution lamellae of FeTiO3. Exchange bias is first observed below the Néel temperature of FeTiO3 (55 K). Possible interface magnetic structures are explored within the framework of a classical Heisenberg model using Monte Carlo simulations. The simulations predict a threshold value of the Fe2O3 anisotropy constant, below which Fe3+ spins become tilted out of the basal plane in the vicinity of the interfaces. This tilting creates a c-axis component of magnetization in the Fe2O3 host that couples to the c-axis magnetization of the FeTiO3 lamellae. Exchange interactions across the interfaces are frustrated when the FeTiO3 lamellae contain an even number of Fe2+ layers, resulting in zero net exchange bias. Lamellae containing an odd number of Fe2+ layers, however, are negatively exchange coupled to the Fe2O3 host across both (001) bounding surfaces, and are the dominant source of exchange bias. Exchange bias is observed whenever there is a significant c-axis component to both the Fe2O3 magnetization and the applied field. An exchange bias of 0.9 T was obtained with an anisotropy constant of 0.1 K.
Article
Modern applications for thin film magnets involve unique requirements for the control and design of specific magnetic properties. The exchange bias effect in ferromagnet/antiferromagnet bilayers appears to be a useful feature for controlling one of the most important characteristics of a ferromagnet: coercivity. Prospects for control and enhancement of desirable effects depend upon a clear understanding of mechanisms governing exchange bias. The processes underlying the existence and properties of exchange bias are reviewed, with particular emphasis on the roles of interface structure and temperature. Results from numerical simulations are used to illustrate how exchange bias is modified by geometric structures at the interface and randomly placed defects. A general theoretical formulation of the bias problem is proposed, and an expression for the interface energy is derived. A key result is the existence of higher-order coupling terms when more than one sublattice of the antiferromagnet is present at the interface. Results from calculations of finite temperature effects on bias and coercivity are described, and the concept of viscosity in the antiferromagnet is discussed. A brief discussion is also included of how a dynamic linear response, such as ferromagnetic resonance or light scattering, can be used to determine relevant anisotropy and exchange parameters.
Article
Long-wavelength and satellite magnetic anomalies require that some regions of the Earth's crust contain minerals that are magnetic at lower crustal conditions. Studies of high-grade metamorphic rocks emphased multi-domain magnetite as a likely source. Members of the hematite-ilmenite series were not considered, because hematite commonly has a relatively weak magnetization and, though ilmenite compositions in the range Ilm 50-70 can be strong ferrimagnets, their low Curie T<260 °C makes them an unlikely source. However, recent investigations have described hematite-ilmenite magnetism and coercivity in terms of a new ferrimagnetic substructure related to interfaces between ilmenite and hematite in exsolution lamellae down to unit-cell thickness. In this “lamellar magnetism”, ferrimagnetism is coupled to principal AF moments of hematite and retains many properties of titanohematite, including high coercivity and thermal stability.
Article
Remanent magnetization (RM) of rocks with hematite–ilmenite solid solution (HISS) minerals, at all crustal levels, may be an important contribution to magnetic anomalies measured by ground and satellite altitude surveys. The possibility that lower thermal gradient relatively deep in the crust can result in exsolution of HISS compositions with strong remanent magnetizations (RM) was studied for two bulk compositions within the HISS system. Samples from granulite-terrane around Wilson Lake, Labrador, Canada contains titanohematite with exsolved ferrian ilmenite lamellae. Other samples from the anorthosite-terrane of Allard Lake, Quebec, Canada contain ferrian ilmenite with exsolved titanohematite lamellae. In both cases, the final exsolved titanohematite has similar Ti content and carries dominant magnetic remanence with REM (=NRM/SIRM, where NRM is the natural remanent magnetization and SIRM is the saturation isothermal remanent magnetization) that is comparable to the Ti-free end member. The RM was acquired prior to exsolution and the ilmeno-hematite-rich rock possesses thermal remanent magnetization (TRM), whereas rocks with hemo-ilmenite possess chemical remanent magnetization (CRM). In both cases, we found fairly large homogeneous grains with low demagnetizing energy that acquired intense RM. The magnetism of the ilmeno-hematite solid solution phases is not significantly perturbed by the continuous reaction: ilmeno-hematite≧titanohematite solid solution. Hence, the occurrence of HISS in rocks that cooled slowly in a low intensity magnetic field will have an intense magnetic signature characterized by a large REM.
Article
Large coercivities of some ilmenohematite minerals are probably due to magnetostrictive effects associated with exsolution. © 1968, Society of Geomagnetism and Earth, Planetary and Space Sciences. All rights reserved.
Article
Magnetic anomalies associated with slowly cooled igneous and metamorphic rocks are commonly attributed to the presence of the mineral magnetite. Although the intermediate members of the ilmenite-haematite mineral series can also carry a strong ferrimagnetic remanence, it is preserved only in rapidly cooled volcanic rocks, where formation of intergrowths of weakly magnetic haematite and paramagnetic ilmenite is suppressed. But the occurrence of unusually large and stable magnetic remanence in rocks containing such intergrowths has been known for decades, and has recently been the subject of intense investigation. These unmixed oxide phases have been shown to contain pervasive exsolution lamellae with thickness from 100 microm down to about 1 nm (one unit cell). These rocks, many of which contain only a few per cent of such oxides, show natural remanent magnetizations up to 30 A m(-1) --too strong to be explained even by pure haematite in an unsaturated state. Here we propose a new ferrimagnetic substructure created by ferrous-ferric 'contact layers' that reduce charge imbalance along lamellar contacts between antiferromagnetic haematite and paramagnetic ilmenite. We estimate that such a lamellar magnetic material can have a saturation magnetization up to 55 kA m(-1) --22 times stronger than pure haematite-- while retaining the high coercivity and thermal properties of single-domain haematite.
What is magnetic in the lower crust? Earth Planet
  • McEnroe