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Step-Induced Frustration of Antiferromagnetic Order in Mn on Fe(001)

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Abstract

We studied the spin arrangement in ultrathin antiferromagnetic Mn films in contact with a ferromagnetic Fe(001) substrate using spin-polarized scanning tunneling microscopy. Mn shows a layerwise antiferromagnetic order on Fe(001). In regions where Mn overgrows Fe steps, a frustration of the antiferromagnetic order occurs which is similar to a 180 degrees domain wall. This topologically enforced frustration was studied as a function of Mn thickness. A linear increase of the width of the frustration region with the Mn thickness was found.

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... Indeed, surface defects of this type are 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 A c c e p t e d M a n u s c r i p t Journal XX (XXXX) XXXXXX X Chen et al 2 practically unavoidable. Their presence at magnetically active interfaces often leads to non-negligible magnetic frustration, resulting in complex spin textures [1][2][3][4][5][6][7][8][9]. This is especially true for most of the cases of practical relevance, where the exchange coupling across the AFM/FM interface is quite strong and is of primary importance for the understanding and design of various front-edge applications, such as magnetic storage, magnetic sensors, etc. [10][11][12]. ...
... This is due without a doubt to the complex geometrical and physical nature of the interfaces as well as to the great difficulty of extracting direct experimental information from the buried interfaces and their defects [17,18]. In particular, magnetic frustration was successfully observed in the Mn/Fe(001) system via spin-polarized scanning tunnelling microscopy (SP-STM) by Schlickum et al. [4][5][6]. In that system, the width of the AFM domain wall (DW) was found to increase linearly as a function of Mn thickness. ...
... Clearly, one anticipates that here the DW width is P-layer dependent. Previous experimental work established that the DW width, which we choose to denote as 2 DW W   , for the Mn/Fe(001) system is proportional to the thickness d of the P-atomic layers, that is, / 1.12 d   [4]. This is equivalent to say that the angle between DW boundary curve (red line in Fig. 1(a)) and the x-axis is about 45º. ...
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Article
We present a study of the magnetic configuration due to step-induced magnetic frustration at ferromagnetic/antiferromagnetic (FM/AFM) interfaces. At a substrate monatomic step edge, a 180° domain wall emerges. A physically appealing form for the thickness dependence of the domain wall width is obtained. It follows a universal behaviour in the whole thickness range, from ultrathin film to bulk and in both cases of an AFM domain wall on top of the FM layer and a FM domain wall on top of an AFM substrate. In the ultrathin limit of the capping layer, the domain wall grows linearly with the slope depending only on the ratio of the inter-layer and intra-layer Heisenberg exchange constants, regardless of the presence of magneto-crystalline anisotropy. These findings are in good agreement with previous experimental observations. As the thickness grows beyond the ultrathin regime, the corresponding thickness dependence departs from linearity and tends to its bulk value. The analytical insights are supported by conclusive numerical simulations of two independent varieties, namely, the Monte Carlo method which also includes the growth kinetics and the object oriented micromagnetic framework based micromagnetic simulations. While the quantitative details of the study are naturally dependent on the specific material parameters of the complex magnetic system, the global features of the spin texture in the capping layer are dictated by the topological step-edge defect. The latter in itself is quantifiable by a winding number of ± 1 2 .
... In magnetic thin film heterostructures, the interaction at the heterointerface plays a dominant role in the development of novel electronic and magnetic properties. Especially, the heterointerface between ferromagnetic and antiferromagnetic thin films is under intensive focus with respect to its promising application in the exchange coupling effect for magnetic devices as well as the fundamental mechanism underlying expected at the Mn/Fe heterointerface as observed in the reference system of Mn thin films on the bulk bcc Fe(001) substrate, [17,18] thereby allowing complementary microscopic and macroscopic approaches to Mn/Fe thin film heterostructures. We show from XAS/XMCD measurements that the Fe layer in Mn/Fe thin film heterostructures exhibits a two-step spin reorientation transition (SRT) due to the Mn overlayer with the change in the magnetic easy axis from out-of-plane to in-plane direction. ...
... T. Note that the number of Fe 3d holes, n hole , of 3.22 for the sum-rule analysis is evaluated from the comparison of the XAS area with a bulk bcc Fe reference spectrum with n hole = 3.4. [18] Taking magnetically compensated non-collinear antiferromagnetic inner 5 ML in the 7 ML fcc Fe thin films on Cu(001) into account, [20,25,26] the spin magnetic moment of the ferromagnetically coupled top two layers can be extracted as 1.8 ± 0.3 μ B per atom. This value is quite close to that obtained by X-ray resonant magnetic scattering experiments. ...
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Magnetic thin film heterostructures have been widely studied for fundamental interests in the emergence of novel phenomena associated with the heterointerface formation as well as their promising practical potential. Combining X‐ray magnetic circular dichroism with scanning tunneling microscopy, it is shown for fcc Fe thin films grown on Cu(001) with Mn overlayers (Mn/Fe thin film heterostructures) that the interfacial factors dominating the electronic and magnetic properties of the entire system dynamically change with the amount of the Mn overlayer. Element specific magnetization curves of the Fe layer exhibit a two‐step spin reorientation transition from out‐of‐plane to in‐plane direction by increasing the Mn coverage. The atomic‐scale characterizations of structural and electronic properties in combination with the first‐principles calculations successfully unravel the roles of the entangled interfacial factors and clarify the driving forces of the transition. The first step of the transition at a low Mn coverage is dominantly induced by the formation of FeMn disordered alloy at the heterointerface, and the electronic hybridization with the interfacial FeMn ordered alloy is dominant as the origin of the second step of the transition at a high Mn coverage.
... [3][4][5]8 Figure 1(a) shows a sketch of STM magnetic junctions, in which electrode (1) is the magnetic tip and electrode (2) is the magnetic sample. Ferromagnetic (FM) Fe-coated W tips have been used only for studying nanomagnets with higher anisotropies (e.g., Fe thin-films on W(110) 9 and Co bilayer islands on Cu(111) 4,10 ) or antiferromagnetic (AFM) samples (e.g., Mn(001) ultrathin films [11][12][13][14] and Cr(001) surfaces 15 ). However, if the magnetic nanostructure under study has a small magnetic anisotropy, the spin structure may be easily affected by external magnetic fields, 16 and therefore, the use of FM tips should be avoided because they produce a considerable stray field. ...
... Since the contrast in the STM spectroscopy maps may have its origin in different local density of states, different chemical species, and different spin polarizations, the bct-Mn(001) films thicker than four layers are considered to be one of the test samples to check spin contrast. [11][12][13][14] In this study, we used fifth ("5") and sixth ("6") layers of the Mn films to check the spin contrast. ...
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Antiferromagnets produce no stray field, and therefore, a tip electrode made of antiferromagnetic material has been considered to be the most suitable choice to measure such as magnetoresistance (MR) through single isolated magnetic nanoparticles, molecules, and ultrathin films. Spin polarizations (P) of antiferromagnetic 3-nm, 6-nm, and annealed 3-nm Mn films grown on W tips with a bcc(110) apex as well as bulk-NiMn tips were obtained at 300 K by measuring MR in ultrahigh vacuum by means of spin-polarized scanning tunneling microscopy using a layerwise antiferromagnetically stacking bct-Mn(001) film electrode. The Mn-coated tips with coverages of 3 and 6 nm exhibited P values of 1 ± 1% and 3 ± 2%, respectively, which tips likely contain α- or strained Mn. With a thermal assist, the crystalline quality and the magnetic stability of the film could increase. The annealed tip exhibited P = 9 ± 2%. The bulk-NiMn tips exhibit spin polarizations of 0 or 6 ± 2% probably depending on the chemical species (Mn or Ni) present at the apex of the tip. Fe-coated W tips were used to estimate the bct-Mn(001) film spin polarization.
... nm. Such small widths are commonly observed in permanent magnetic materials 16 due to their exceptionally high magnetic anisotropy such as Nd 2 Fe 14 B (3.9 nm), SmCo 5 (2.6 nm), CoPt (4.5 nm) and Mn overlayers on Fe (001) (4.55 nm) 17 . In these systems, magnetic domains are energetically stable after zero-field cooling due to long-range dipole interactions which were also checked in our study resulting in no modifications of the results. ...
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The discovery of two-dimensional (2D) magnetic van der Waals (vdW) materials has flourished an endeavor for fundamental problems as well as potential applications in computing, sensing and storage technologies. Of particular interest are antiferromagnets, which due to their intrinsic exchange coupling show several advantages in relation to ferromagnets such as robustness against external magnetic perturbations. Here we show that, despite of this cornerstone, the magnetic domains of recently discovered 2D vdW MnPS 3 antiferromagnet can be controlled via magnetic fields and electric currents. We achieve ultrafast domain-wall dynamics with velocities up to ~3000 m s ⁻¹ within a relativistic kinematic. Lorentz contraction and emission of spin-waves in the terahertz gap are observed with dependence on the edge termination of the layers. Our results indicate that the implementation of 2D antiferromagnets in real applications can be further controlled through edge engineering which sets functional characteristics for ultrathin device platforms with relativistic features.
... Therefore, the Cr(001) surface can be used as a prototypical system to study spin frustration caused by microscopic defects [11,12,15,16], which is also a particularly interesting topic for spin-polarized STM/STS studies. Few investigations focusing on the spin frustrations of the AF order, however, have been reported so far [11,12,[15][16][17][18][19]. ...
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We have studied the surface structure and morphology of epitaxial Cr films on Au(001) substrate with Cr thickness (dCr) below 3 nm. We have characterized the films prepared under various growth conditions by using scanning tunneling microscopy and spectroscopy (STM/STS), low-energy electron diffraction, and Auger electron spectroscopy. The good growth conditions of the Cr(001) films, which have atomically flat terraces without surface segregation of Au and with distinct surface states, are obtained by the following two methods. (I) dCr=3 nm film realized by two-step growth, i.e., first 1.5 nm of Cr deposited at room temperature (∼290 K) and an additional 1.5 nm Cr deposition at 570 K and (II) dCr=1.5 nm film by room temperature growth and subsequently post-annealing at 470 K. The magnetic imaging of (I) dCr=3 nm Cr(001) film was performed by means of spin-polarized STM/STS. The observed magnetic image indicates that the topological antiferromagnetic (AF) order appears in a series of adjacent terraces and two types of spin frustration caused by screw dislocations lead to modifications of the topological AF order. One type of spin frustration is accompanied by narrow domain walls between two screw dislocations. The other type is a complex spin frustration caused by a cluster of an odd number of screw dislocations. The difference of the two types of spin frustration is explained through a micromagnetic simulation. We have also identified a large spin-frustrated area, consisting of a cluster of multiple number of screw dislocations.
... The tip material has to be chosen carefully as magnetostriction produces an additional signal with the same frequency as the modulation. Domain wall widths in Mn/Fe bilayers in the range of 1 nm and step-induced frustration of antiferromagnetic order have been resolved in this manner [71]. The most convenient way of separating magnetic and topographic signals is by comparing two measurements obtained for opposite magnetization directions of tip or sample [72]. ...
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Preprint
The magnetic domain configuration of a system reveals a wealth of information about the fundamental magnetic properties of that system and can be a critical factor in the operation of magnetic devices. Not only are the details of the domain structure strongly governed by materials parameters, but in thin-films and mesoscopic elements the geometry has an often pivotal effect, providing a convenient handle to tailor desired domain states. Furthermore a full understanding of a system requires, in addition, investigation of the dynamic evolution of the spin-state, which is of particular importance for applications relying on e.g. the switching of magnetic elements. Here we review some of the main modern techniques for magnetic imaging, highlighting their respective advantages and limitations. The methods for imaging domain configurations and spin structures cover various spatial and temporal resolution scales and encompass those based on electron and x-ray microscopy as well as scanning probe techniques. Furthermore, away from the discipline of condensed-matter physics, magnetic effects are instrumental in a number of techniques for medical imaging, some key examples of which we also present.
... Scanning tunneling microscopy/spectroscopy (STM/STS) is a powerful tool to investigate structural and electronic properties of multilayer films on an atomic scale. In the vicinity of the AFM/FM interface, previous STM works could relate the surface morphology and electronic structures of AFM thin films grown on a FM single crystal to their magnetism [10][11][12][13][14]. However, the atomic-scale investigation of the growth, electronic and magnetic properties of the AFM/FM interface in the form of multilayers has rarely done so far. ...
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Mixing of atoms at the interface was studied for Mn/Fe magnetic hetero-epitaxial layers on Cu(001) by scanning tunneling microscopy/spectroscopy. The formation of a surface alloy was observed when the Mn layer was thinner than 3 atomic layers. From the fourth layer, Fe segregation is suppressed, and a pure Mn surface appears. Accordingly, spectroscopic measurements revealed the electronic difference between the surface alloy and Mn layers. The surface electronic structure of the fourth Mn layer is slightly different from that of the fifth layers, which is attributed to the hybridization of the fourth layer with the underneath Fe-Mn alloy.
... This might come from the dislocation pattern which forms as a consequence of the tetragonal distortion during the fcc to fct phase transition. In the case for Mn grown on Fe(001) whisker substrates 15) with the unidirectional magnetic anisotropy provided by the underlying iron whisker, only two intensities are detected in the spin signal representing two unique in-plane orientations of the Mn magnetization (parallel or antiparallel to the Fe direction). For Mn on Cu 3 Au(100), however, there should exist four unique in-plane orientations of the Mn local magnetic moments owing to the four-fold cubic symmetry of the Cu 3 Au(100) surface. ...
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It is shown that 21 monolayers of Manganese films grown on Cu3Au(100) at room temperature have an uncompensated layered antiferromagnetic structure that show a deviation from this configuration due to structural imperfections. In some areas of the film a p(2 × 2) magnetic super-structure was observed on the surface.
... The development of spin-polarized (SP) scanning tunneling microscopy (STM) [4,5], however, made it possible to visualize the magnetic properties in real space with unprecedented resolution. This technique, initially used to study the magnetic ground state of bulk crystal surfaces [6,7] and thin films [8], was then rapidly applied to study a large variety of different magnetic systems like nanostructures [9][10][11] and domain walls [12,13], and in more recent years, it has been pushed to its ultimate limit: studies of the magnetic state of single atoms [14]. ...
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We report a detailed study of the magnetic properties of a monoatomic layer of Mn on W(110). By comparing multiscale numerical calculations with measurements we evaluate the magnetic ground state of the system and its temperature-dependent evolution. We find that the ground state consists of a cycloidal spin spiral (CSS) that persists up to the Néel temperature with a temperature-independent wavelength. However, by continuously increasing the temperature, that CSS becomes thermally depinned. This results in a time-averaged absence of magnetic order, a process that can be viewed as the antiferromagnetic analog of superparamagnetism.
... The roughness of the interfaces between layers in multilayer magnetic structures leads to frustration exchange interaction between the spins. In [3] the spin arrangement in ultrathin antiferromagnetic Mn films in contact with a ferromagnetic Fe(001) substrate was studied using spin-polarized scanning tunneling microscopy. Mn shows a layerwise antiferromagnetic order on Fe(001). ...
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The unique electrical and magnetic properties Fe86Mn13C alloy (Hadfield's steel) are due to special modulated structure. The alloy is widely used in mechanical engineering. Complex studies of the structure and properties of the alloy both in bulk samples and thin-films are suggested to describe the formation of a deformation martensite structure in Fe86Mn13C alloy in the form of self-assembled clusters. It is shown that combinations of anti-ferromagnetic austenite and ferrimagnetic martensite of deformation create unique electric and magnetic properties of Fe86Mn13C alloy both in bulk and in a thin-film state. We seek to explain the reason for the change of the sign of thermoelectric effect depending on temperature. Our task is to investigate thin Fe86Mn13C films as a possible solution to the problems of spintronics.
... Even surfaces of antiferromagnets can be probed with these methods. Here, spin-polarized scanning tunneling microscopy was applied to study the behavior of magnetic frustrations at the surface of thin antiferromagnetic Mn films which are in direct contact to a ferromagnetic Fe(001) substrate [3]. ...
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In many magnetic devices, antiferromagnets in direct contact to ferromagnets play an es-sential role. Such combined systems allow the creation of artificial structures in which the magnetic properties can be tuned to technolog-ical needs. Fundamental properties concern-ing the interplay between ferromagnets and antiferromagnets are, however, not fully un-derstood. In the last years, spin-polarized scanning tunneling spectroscopy [1] and spin-polarized scanning tunneling microscopy [2] became powerful tools for investigating mag-netic structures on the nanometer scale. Even surfaces of antiferromagnets can be probed with these methods. Here, spin-polarized scanning tunneling microscopy was applied to study the behavior of magnetic frustrations at the surface of thin antiferromagnetic Mn films which are in direct contact to a ferromagnetic Fe(001) substrate [3]. To investigate the in-plane component of the spin polarization at the sample surface, we use ferromagnetic rings as electrodes [4]. A photo of a typical ring is shown in Fig. 1. The arrow indicates the approximate position at which tunneling occurs. A Ta wire is used to fix the ring to the scanner. A small alternating current is applied to the coil wound around the ring to Fig. 1: Ring electrode as used for in-plane spin-polarized scanning tunneling microscopy measure-ments (outer diameter: 2 mm, inner diameter: 0.7 mm). Beside the ring, a small coil around the ring and a Ta wire are visible. switch the magnetization of the ring. A fast switching of the ring magnetization (switching frequency is much higher than the cut off fre-quency of the feedback loop) allows the sepa-ration of the topographic and magnetic infor-mation and the simultaneous imaging of both. Mn on Fe(001) is a topological antifer-romagnet as schematically shown in Fig. 2. Thus, adjacent Mn layers exposing at the sam-ple surface are oppositely magnetized. Where Mn overgrows a monatomic Fe substrate step edge, the thickness of the Mn film on both sides of the step edge differs by one atomic layer. Buried Fe step edges appear as steps of sub atomic height at the surface of the Mn film because the out-of-plane lattice constant of Mn is slightly higher than the one of Fe(001), as indicated in Fig. 2.
... [1][2][3] Interesting phenomena, such as exchange bias [1][2][3] and enhanced coercivity 4 in FM/AFM systems, can be attributed to the additional magnetic anisotropy caused by the broken rotational symmetry that the AFM layer induces in the FM layer through exchange coupling. Several models, such as collinear coupling at the spin-uncompensated interface, 2 and non-collinear (spin-flop) coupling 5 originating from spin frustration 6 have been proposed. Although the angle between FM and AFM spins has been shown to significantly affect the exchange coupling, 7 it has remained a fundamental issue to identify how the FM spins and AFM spins are coupled at the FM/AFM interface, because it is difficult to investigate the AFM spins at the interface and inside the bulk separately. ...
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The effect of CoO spin orientation on exchange coupling was investigated in single-crystalline Fe/CoO/MnO/MgO(001) systems. An antiferromagnetic CoO spin reorientation transition from the in-plane direction to the out-of-plane direction was found to be associated with the in-plane strain transition in CoO film from compression to expansion. The induced uniaxial anisotropies by exchange coupling at the Fe/CoO interface are significantly stronger for the in-plane CoO spin orientation than for the out-of-plane CoO spin orientation. Our study provides a way to modify the exchange coupling in the ferromagnetic (FM)/antiferromagnetic (AFM) bilayer by modulating the strain in the AFM film.
... Authors of Ref. [27] have shown for the rectangular steps that new type of domains with different orientations of the magnetization in FM layer could arise in the case when the collinear co-directional orientation of FM and AFM spins at the boundary corresponds to the interface energy minimum. Similar mechanisms associated with the FM-AFM boundary surface roughness was considered in Ref. [28] to explain the domain structure observed experimentally in the Mn on Fe (0 0 1) structure. ...
Article
Magnetic and magneto-optical properties of layered Ge–Ni–Ge–Ni–Ge films have been investigated with the morphology of the film surface being studied also. It is shown that the films reproduce roughness of the glass-substrate surface which is similar to the grains 2–4 nm high and 100–400 nm long in any direction within the film plane. Thermal treatment results in a decrease of the grains height and their broadening. Non-monotonous change of the Faraday rotation (FR) value due to thermal treatment has been revealed. The inverse linear FR value dependence on the intermediate Ge layer thickness has been obtained. The exchange bias effect is observed at temperatures lower than approximately 50 K which was ascribed to the formation of the complex composition interface due to the Ge and Ni mutual diffusion.
... Recent neutron diffraction and scattering work 3 has shown that the AF domain size is little affected by the type of FM layer the AF layer is coupled to, and the AF domains are smaller than the AF layer thickness (laterally constrained), which indicates that the FM/AF interface is what dominates exchange bias magnetism. At the AF/FM interface, magnetic frustration arises, caused typically by the complex exchange interactions at the interface and the unavoidable atomic-scale interface roughness, 4 and this interface frustration is linked to both the exchange bias loop shift and an enhanced coercivity. 1,2 A link between the exchange bias loop shift and pinned interfacial orbital moments in an uncompensated AF layer has been observed recently, 5 indicating a connection between spin-orbit coupling at the AF/FM interface and the requisite unidirectional anisotropy. ...
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The effects of interfacial coupling at the boundary of ferromagnetic and antiferromagnetic components in a nanoscale columnar-structured thin film of Ni80Fe20/CoO have been examined. Field-cooling the film results in very different temperature dependences of the enhanced coercivity and exchange-bias shift of the hysteresis loop. The exchange-bias temperature dependence is well described by thermal fluctuations of the interfacial spins while the coercivity temperature dependence indicates that single-domain-like columns are being coherently rotated by the thermal fluctuations of the interface spins. Furthermore, only a portion of the spins in the antiferromagnetic layer seem to be associated with the spin coupling that results in exchange bias. X-ray magnetic resonant scattering measurements show clearly the presence of canted Co interfacial moments that provide a local field which enables exchange bias at a significantly higher temperature than the onset of an enhanced coercivity.
... 3(a) and 3(b), not only the LAF-spin structures but also the striking phenomenon of spin frustration across the same third Mn layer have been resolved. Similar to the e-bct-Mn films grown on the Fe(001) system, 26,27 the LAF-spin configuration can construct a prototypical model system to study topologically induced spin frustrations regarding the competition between domain-wall energy in AFM-and exchange-coupling energy in the interface of antiferromagnetism and ferromagnetism. As the region framed by the white-dashed square in Fig. 3(b), we show the averaged spin-resolved conductanceline profile in Fig. 3(c). ...
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Spin structures of an exchange-coupled-bilayer system of expanded-face-centered-tetragonal (e-fct) Mn(001) ultrathin films grown on Co/Cu(001) were resolved by means of spin-polarized scanning-tunneling microscopy. With an in-plane spin-sensitive probe, a layered antiferromagnetic-spin ordering of Mn overlayers was evidenced directly. In addition, the spin frustration across the same Mn layer creating a narrow domain wall down to nanometer scale was also observed along the buried step of Co underlayers. According to the micromagnetic simulation, the step-induced domain-wall width is in agreement with the experimental results. Such in-plane layered antiferromagnetic-spin structures of e-fct Mn(001) provide uncompensated spins at the interface with Co underlayers and elucidate the mechanism of the corresponding exchange-bias field observed in the previous studies.
... Direct imaging of the AFM domains by XMLD has been applied to AFM/FM systems to determine the magnetic correlation across the interface, while the sp-STM was utilized to explore the step-induced frustration in uncompensated AFM films. [7][8][9] In an effort to minimize the interfacial roughness, single crystalline FeMn/ Co/ Cu͑001͒ thin films have become one of the model systems for the study of interfacial interactions in a fully compensated AFM/FM system. [10][11][12] The epitaxial growth of FeMn and Co on Cu͑001͒ simplifies the spin configuration at the interface to permit a systematic study of the effect of steps or roughness. ...
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FeMn∕Co ultrathin films were epitaxially grown on vicinal Cu(001) and investigated by using photoemission electron microscopy and surface magneto-optical Kerr effect. The FeMn antiferromagnetic order increases the Co coercivity and switches the Co magnetization easy axis from the in-plane [110] axis to the in-plane [100] axis. To separate the effects of atomic steps along different orientations, we modify the [110] and [100] step densities by growing the FeMn∕Co film on vicinal Cu(001) with steps parallel to [110] and [100] axes. We find that the [110] steps weaken the Co coercivity enhancement, while the [100] steps enhance the Co coercivity enhancement. This result supports the mechanism that the Co in-plane magnetization switching is due to the spin pinning of the FeMn at the [100]-type atomic steps.
... For an AFM/FM bilayer with a fully uncompensated AFM order and a smooth interface, the collinear (either parallel or antiparallel) type of coupling could be the stable state, which may force the FM spins in line with the AFM spins when the coupling is established. 1,4 For the bilayer with the AFM film having intrinsically compensated spin order at each layer 5 or the rough interface 6 which causes spin frustration at the surface of the antiferromagnet, the noncollinear (spin-flop) coupling could be established to minimize the interface exchange energy. [7][8][9] It has been suggested that the noncollinear type of exchange coupling at an interface could also induce a flipping of the FM spins in the FM/AFM bilayer, in which the direction depends on the sum of spin vectors at the AFM surface. ...
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We present a direct observation of a flipping magnetization of a uniform Fe film/wedged-Mn bilayer induced by a biquadratic-type exchange coupling established at the interface. The element-resolved magnetic imaging shows that the Fe film exhibited a flipping of magnetization between the [1̅ 00] and [010] directions with a periodicity of one monolayer Mn thickness as below a critical temperature. The enhancement of the variation angle with the increase of Mn thickness follows the tendency of the finite-size effect of antiferromagnetism on interface exchange coupling. The flipping magnetization emerging coincidentally with the uncompensated-compensated transition of Mn magnetic surface indicates a frustration-induced biquadratic-type interface exchange coupling, and suggests a layered-like uncompensated AFM ordering for the Mn layer.
... [1][2][3] Many new magnetic phenomena have been discovered, thereafter, in the FM/AFM systems, such as coercivity enhancement, 4 training effect, 5 rotational hysteresis, 6 nonlinear exchange coupling, 7,8 etc. Different from FM/FM systems, magnetic interaction energy at the FM/AFM interface cannot be minimized for all nearest-neighbor spin pairs, therefore it is inevitable to have the so-called spin frustration at the FM/AFM interface. 9 While being a complicated phenomenon, spin frustration plays a key role in generating all the fascinating magnetic phenomena mentioned above. Therefore the study of FM/AFM interfacial interactions has remained as one of the most active subjects in nanomagnetism research. ...
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Single crystalline FeMn/Co bilayers were grown epitaxially on Cu(001) and investigated by magneto-optic Kerr effect (MOKE). By doing the MOKE measurement within a rotating magnetic field, we were able to retrieve quantitatively the anisotropy constant of the ferromagnetic Co layer. We show unambiguously that as the FeMn layer changes from paramagnetic (PM) to antiferromagnetic (AFM) states, it enhances the interfacial magnetic anisotropy at the FeMn/Co interface by an order of magnitude. A thickness dependent study of the magnetic anisotropy constant revealed that this induced magnetic anisotropy may originate from the FeMn/Co interfacial spin frustration.
... The seventh layer has larger current than the sixth layer over the whole voltage range. rent asymmetry of 5% measured from Fig. 2͑c͒ is comparable to the previous result in Ref. 23. ...
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We report a method of magnetic probe fabrication using ring-shaped and iron-coated tungsten wire for spin-polarized scanning tunneling microscopy. Magneto-optic Kerr effect measurement on the probe front end shows that by controlling the saturating field direction, we can fix the probe magnetization in the specific in-plane direction. The ring is applied to the scanning tunneling microscopy and spectroscopy experiment on 6.8 ML Mn/Fe(001), and spin contrast in the in-plane direction is demonstrated.
... Recent neutron diffraction and scattering work 3 has shown that the AF domain size is little affected by the type of FM layer the AF layer is coupled to, and the AF domains are smaller than the AF layer thickness (laterally constrained), which indicates that the FM/AF interface is what dominates exchange bias magnetism. At the AF/FM interface, magnetic frustration arises, caused typically by the complex exchange interactions at the interface and the unavoidable atomic-scale interface roughness, 4 and this interface frustration is linked to both the exchange bias loop shift and an enhanced coercivity. 1,2 A link between the exchange bias loop shift and pinned interfacial orbital moments in an uncompensated AF layer has been observed recently, 5 indicating a connection between spin-orbit coupling at the AF/FM interface and the requisite unidirectional anisotropy. ...
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The overall and element specific magnetism in an exchange biased [Ni80Fe20/Mn]3 film system, with film layers 3.5 nm thick, has been studied to examine the magnetism between interfacial Mn, Ni, and Fe spins. Field-cooling the film results in large exchange bias loop shifts at temperatures from 2 to 30 K, and an enhanced coercivity remains until 75 K. The elemental magnetism ascertained from x-ray magnetic circular dichroism measurements shows that Mn appears to be a fully compensated antiferromagnet down to 20 K, and there is clear evidence of an enhanced orbital moment for both Fe and Ni from 20 to 300 K. This magnetism is likely due to overlapping d orbitals between Fe-Mn and Ni-Mn, with this coupling increasing the local anisotropy resulting in the enhanced coercivity and enabling exchange bias.
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Over the past few decades, spin detection and manipulation at the atomic scale using scanning tunneling microcopy has matured, which has opened the possibility of realizing spin-based functional devices with single atoms and molecules. This article reviews the principle of spin polarized scanning tunneling microscopy and inelastic tunneling spectroscopy, which are used to measure the static spin structure and dynamic spin excitation, respectively. Recent progress will be presented, including complex spin structure, magnetization of single atoms and molecules, as well as spin excitation of single atoms, clusters, and molecules. Finally, progress in the use of spin polarized tunneling current to manipulate an atomic magnet is discussed.
Chapter
Introduction Magnetism in Low Dimensions Experiments Probing Magnetic Moment (and Magnetic Hyperfine Field) at Surfaces and in Ultrathin Films Magnetic Moments at Surfaces and in Ultrathin Films of Fe Magnetic Excitations Magnetic Anisotropy and Magnetization Distribution References
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One of substantial achievements in the development of information technologies is the increase in the capacity of digital media and the decrease in their size. A paper heard at an RAS Presidium meeting reports about the distinguishing features of multilayered magnetic structures and their characteristic fundamental effects, the discovery of which allowed for this technological breakthrough, as well as about modern studies aimed at further enhancement of information reading and recording technologies. The authors focus on the creation of magnetoresistive memory with electric field-assisted writing.
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We report an experimental investigation of the magnetoelastic effects of ultrathin antiferromagnets, performed by comparing the characteristic behavior of the induced spin-reorientation transition (SRT) of Co films in two types of epitaxially grown bilayers: face-centered-cubic (fcc)\char21{}like Mn/Co (fcc-Mn: $a=3.75\phantom{\rule{0.16em}{0ex}}\AA{},\phantom{\rule{0.16em}{0ex}}c=3.76\phantom{\rule{0.16em}{0ex}}\AA{})$ and face-centered-tetragonal (fct)\char21{}Mn $(a=3.61\phantom{\rule{0.16em}{0ex}}\AA{},c=3.78\phantom{\rule{0.16em}{0ex}}\AA{})$. Magnetic hysteresis loops and magnetic domain images indicate that both fcc-Mn and fct-Mn films can produce a $$\langle${}110$\rangle${}$ to $$\langle${}100$\rangle${}$ SRT in adjacent Co films when the thickness of the Mn layer is greater than a temperature-dependent critical value. Detailed analysis of the critical thickness of Mn films and the evolution of the Co domain structure upon SRT indicate that the fct-Mn film had a higher antiferromagnetic ordering temperature and stronger lateral Mn-Mn exchange coupling compared with the fcc-Mn film. The enhanced long-range antiferromagnetic ordering emerging concurrently with the in-plane lattice variation of the fcc-like Mn film in Mn/Co bilayers clearly showed the magnetoelastic effect of ultrathin antiferromagnets.
Article
Antiferromagnetic spin ordering has been revealed by room-temperature spin-polarized scanning tunneling microscopy (SP-STM) in thin epitaxial films of c-FeSi on Si(111). Spin polarization of tunneling current for unoccupied states is found to be unusually large I↑↑/I↓↑=3.8. Atomically sharp spin-frustration domain walls, developing on the surfaces of nanoscale islands, have been observed on SP-STM images. Our results suggest that antiferromagnetism in c-FeSi is driven by Mott-Hubbard transition, and the atomically narrow domain walls are caused by local insulator-to-metal breakdown.
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FeMn∕Co∕Cu(001) films were epitaxially grown and investigated using photoemission electron microscopy and the surface magneto-optic Kerr effect. The FeMn antiferromagnetic order increases the Co film coercivity, switches the Co magnetization easy axis from the [110] direction to the [100] direction, and increases the Co film critical thickness at which the ferromagnetic transition occurs. The Néel temperature of the FeMn film increases with increasing FeMn film thickness. At room temperature, we constructed a magnetic phase diagram in the FeMnCo thickness plane to describe the FeMn and Co magnetic phases.
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A bulk ring probe made of pure iron wire with diameter of 0.125 mm was prepared for spin-polarized scanning tunneling microscopy at room temperature in ultrahigh vacuum. The layerwise antiferromagnetic spin contrast of 2.8 atomic monolayers (ML) Mn/4.5 ML Co/Cu(001) observed with such a probe revealed a spin asymmetry of 14% and a signal-to-noise ratio of 1.8. Areas of reversed spin contrast on the same atomic layer of Mn were observed and attributed to the influence from underlying Co steps and islands. This demonstrates the simplicity of preparation and capability of such bulk Fe ring probes.
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The phenomenon of switching of the domain walls generated by frustrations in a two-layer ferromagnet-antiferromagnet nanostructure has been studied theoretically taking into account the energy of the uniaxial anisotropy beyond the exchange approximation. This phenomenon manifests itself in the fact that, as the magnetic field increases, the ferromagnetic layer divided into nanodomains by domain walls perpendicular to the layer plane becomes single-domain, and the antiferromagnetic layer that is uniform in weak fields is divided into 180° domains by the domain walls perpendicular to the layer. The phase diagram of the two-layer nanostructure has been constructed in the variables “the magnetic field-the characteristic distance between atomic step edges at the interface between the layers.”
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The phenomenon of “switching” of the domain walls generated by frustrations in a two-layer ferromagnet-antiferromagnet nanostructure has been studied using numerical simulation methods. This phenomenon manifests itself in the fact that, as the magnetic field increases, the ferromagnetic layer divided into nanodomains by domain walls perpendicular to the layer plane becomes single-domain, and the antiferromagnetic layer that is uniform in weak fields is divided into 180° domains by the domain walls perpendicular to the layer. The structures of these domain walls have been calculated in various magnetic fields.
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The surface morphology and local structure of layers in the Ni-Ge and Ge-Ni-Ge-Ni-Ge films have been investigated. It has been shown that the surface of the films follows the roughnesses of the substrate surface, which have characteristic dimensions of 2–4 nm in height and ∼100 nm in plane. It has been found that an interface with the depth ranging from 9 to 18 nm is formed at the boundaries between the Ni and Ge layers. The data obtained have been used to explain the specific features of the magnetic properties of the studied films, such as the asymmetry of hysteresis loops at low temperatures and the difference between the temperature dependences of the magnetization of the samples for two cooling modes: in a magnetic field and without a magnetic field.
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The frustrations of exchange interaction between ferromagnetic and antiferromagnetic layers, which arise at the uncompensated interface between the layers due to the interface roughness, have been described. The distribution of magnetic order parameters in the vicinity of the interface between the layers has been investigated, and the “layer thickness-roughness” magnetic phase diagram has been obtained in the case of the two-layer ferromagnet-antiferromagnet system and the ferromagnet-antiferromagnet-ferromagnet spin-valve system. An analysis has been performed taking into account the single-ion anisotropy energy, i.e., beyond the scope of the exchange approximation. It has been demonstrated that the number of easy axes in the layer plane, in many respects, determines the existence of an exchange shift of the hysteresis loop of the ferromagnet due to its interaction with the antiferromagnetic substrate.
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The direct observation of spin structures with atomic-scale resolution, a long-time dream in condensed matter research, recently became a reality based on the development of spin-sensitive scanning probe methods, such as spin-polarized scanning-tunneling microscopy (SP-STM) and magnetic exchange force microscopy (MExFM). This article reviews the basic principles and methods of SP-STM and MExFM and describes recently achieved milestones in the application of these techniques to metallic and electrically insulating magnetic nanostructures. Discoveries of novel types of magnetic order at the nanoscale are presented as well as challenges for the future, including studies of local spin excitations based on spin-resolved inelastic tunneling spectroscopy and measurements of damping forces in MExFM experiments.
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The structural and magnetic properties of room-temperature (RT: 300K )-grown and low-temperature (LT: 100K )-grown Mn/Cu3Au(100) thin films were investigated. Mn films deposited at RT and LT demonstrate very different behaviors in the crystalline structure, morphology, and magnetism. RT-Mn films reveal apparent layer-by-layer growth for 0-2ML (monolayer) followed by reduced oscillations. Although the medium-energy electron diffraction (MEED) oscillation is reduced, the intensity of specular spot increases monotonically after 6-7ML , inferring the tendency of smooth morphology. The study of scanning tunneling microscopy also shows that even in 19ML Mn/Cu3Au(100) , the surface morphology is composed of large terraces with the size up to hundreds of nanometers. The LT-Mn films reveal apparent layer-by-layer growth for 0-5ML followed by the reduced oscillations, and then the MEED intensity remains at low intensity, inferring the rough surface. The RT- and LT-Mn films exhibit a thickness-dependent structural transition from a face-centered cubic to a face-centered tetragonal structure at different critical thicknesses, ˜12-14 and ˜8ML , respectively. Significant exchange bias is observed in Fe/RT-Mn bilayers. It increases monotonously with Mn thickness. The exchange bias coupling in Fe/LT-Mn is much weaker than Fe/RT-Mn and drastically varies with Mn film thickness. The presence of exchange bias in the Fe/Mn bilayers also indicates the antiferromagnetism of gamma -phase Mn/Cu3Au(100) .
Article
The thickness–roughness phase diagram of a thin ferromagnetic film on an antiferromagnetic substrate is studied in the case where the roughness of the interface between the layers causes frustration of the exchange interaction between them. It is shown that the account of single-ion anisotropy makes the phase diagram significantly more complicated in comparison with that calculated within the exchange approximation. The evolution of a new type of domain walls caused by frustrations is traced with an increase in the film thickness and the width of the atomic steps on the film–substrate interface.
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For a magnetized one-dimensional Luttinger liquid (LL) we study the influence of spin-dependent scattering from a localized magnetization inhomogeneity caused, e.g. by a local external magnetic field. Two cases are considered: (i) a uniformly magnetized LL close to an external magnetic tip that scatters the electrons and (ii) a LL with two domains, one paramagnetic and the other ferromagnetic. A renormalization group treatment of electron interactions yields scaling equations for the transmission and reflection amplitudes. For repulsive interactions we find two possible zero temperature insulator fixed points: one in which the carriers are reflected in the same spin channel and another where the electron spin is reversed upon reflection, leading to a finite spin current and vanishing charge current under a small voltage bias. In case (ii) we find the “spin–flip-insulating” regime to be stabilized by a pure potential scattering as caused, e.g. by impurities.
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The magnetic properties of the tetragonally distorted γ and δ phases of Mn stabilized by epitaxial growth on metallic surfaces are still subject of a lively debate, but so far no consistent and generally accepted picture has emerged. We have performed detailed and comprehensive investigations of the geometric and magnetic properties of tetragonal Mn in the bulk, at a (100) surface, in ultrathin Mn∕Fe(100) films, and in Mn∕Fe multilayers using ab initio spin-density functional techniques. The cubic structures of both γ (face-centered cubic) and δ (body-centered cubic) Mn are unstable against tetragonal distortion. Whereas for δ-Mn a structure contracted along the c axis and with c(2×2) in-plane (100) antiferromagnetism (AFM) is the unique ground state, for γ-Mn a contracted tetragonal (c∕a=0.945) phase with a layered (100) AFM, and an expanded (c∕a=1.048) phase with in-plane (100) AFM are energetically almost degenerate. In addition we find that the antiferromagnetic phases of both δ and γ Mn are susceptible to long-period helical modulations. At the (100) surface, the Mn moments are strongly enhanced, and the strong antiferromagnetic coupling between the high surface moments favors in-plane AFM in the surface layer even on top of the tetragonally compressed near-fcc phase stabilizing layered antiferromagnetism in the bulk. A similar result is found for ultrathin Mn∕Fe(100) films with up to six monolayers. A strong ferromagnetic Mn∕Fe coupling at the interface favors layered antiferromagnetism in the deeper layers, but the in-plane antiferromagnetic structure in the top layer is stable in any case. For the thinnest Mn films we have also examined noncollinear magnetic structures and found evidence for a perpendicular coupling between the Mn surface layer and the deeper layers of film and substrate. The strong ferromagnetic Mn∕Fe interface coupling also determines the properties of Fe∕Mn multilayers. The ferromagnetic interface coupling is not perturbed by Fe∕Mn intermixing and stabilizes a layered antiferromagnetism in the Mn spacer. We discuss our results in the light of the available experimental data and of previous theoretical calculations.
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The magnetic properties of antiferromagnetic NiO thin films grown on curved Ag(001) substrate were investigated using x-ray magnetic linear dichroism. We show that atomic steps on a vicinal Ag(001) surface induce an in-plane uniaxial magnetic anisotropy, which favors the Ni spins to be perpendicular to the steps, which are parallel to a [110] axis and parallel to the steps for steps that are along a [100] axis. Temperature- and thickness-dependent measurements show that [110] and [100] steps produce different in-plane crystal field, which may be responsible for the different NiO in-plane magnetic anisotropy.
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Mn films show improved layer-by-layer growth on the Fe(001)-p(1×1)O surface compared to the clean Fe(001) surface. From Auger electron spectroscopy the surfactant role of the oxygen was confirmed. A layerwise antiferromagnetic order in the Mn films is preserved as seen from the magnetic contrast between adjacent Mn layers in spin-polarized scanning tunneling microscopy while the surface spin polarization is enhanced by a factor 2 compared to Mn films grown on the clean Fe(001) surface. Further, topologically induced magnetic frustrations of the Mn layer above buried Fe steps appear wider than without oxygen.
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The magnetic properties of Mn nanostructures on the Fe(001) surface have been studied using the noncollinear first-principles real space–linear muffin-tin orbital–atomic sphere approximation method within density-functional theory. We have considered a variety of nanostructures such as adsorbed wires, pyramids, and flat and intermixed clusters of sizes varying from two to nine atoms. Our calculations of interatomic exchange interactions reveal the long-range nature of exchange interactions between Mn-Mn and Mn-Fe atoms. We have found that the strong dependence of these interactions on the local environment, the magnetic frustration, and the effect of spin-orbit coupling lead to the possibility of realizing complex noncollinear magnetic structures such as helical spin spiral and half-skyrmion.
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FeMn/Ni/Cu(001) bilayer films are grown epitaxially and investigated by photoemission electron microscopy and magneto-optic Kerr effect. We find that as the FeMn overlayer changes from paramagnetic to antiferromagnetic state, it switches the ferromagnetic Ni spin direction from the out-of-plane to an in-plane direction of the film. This phenomenon reveals the mechanism of creating magnetic anisotropy by the out-of-plane spin frustration at the FeMn-Ni interface.
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Fe-Cr alloy layers on Fe(001) substrates with different atomic environments are theoretically investigated through a self-consistent real-space tight-binding method parametrized to density functional theory and without collinear restriction for the spin directions. We show that the intermixing at the Cr∕Fe interface, experimentally observed at the first stages of the Cr growth on Fe, originates noncollinear magnetic arrangements within the system and even induces magnetic walls in the Fe layer with a magnetic domain close to the interface. A different behavior is shown if steps are present at the interface; i.e., noncollinear magnetism becomes more localized and the induced magnetic wall in Fe vanishes.
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We describe the design and development of a scanning tunneling micoscope (STM) working at very low temperatures in ultra-high vacuum (UHV) and at high magnetic fields. The STM is mounted to the 3He pot of an entirely UHV compatible 3He refrigerator inside a tube which can be baked out to achieve UHV conditions even at room temperature. A base temperature of 315 mK with a hold time of 30 h without any recondensing or refilling of cryogenics is achieved. The STM can be moved from the cryostat into a lower UHV-chamber system where STM-tips and -samples can be exchanged without breaking UHV. The chambers contain standard surface science tools for preparation and characterization of tips and samples in particular for spin-resolved scanning tunneling spectroscopy (STS). Test measurements using either superconducting tips or samples show that the system is adequate for performing STS with both high spatial and high energy resolution. The vertical stability of the tunnel junction is shown to be 5 pmpp and the energy resolution is about 100 μeV.
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The magnetic ordering in ultrathin Mn films grown on Fe(100) substrates is studied using spin-polarized scanning tunneling microscopy/scanning tunneling spectroscopy. Enhancement of spin contrast is observed due to a tip modification. Detailed analysis carried out using normalized dI/dV spectra indicates the appearance of resonant tunneling behavior. This is attributed to the attachment of a magnetic cluster at the apex of the magnetic thin film tip. Our results compare well with a recent theoretical prediction of a high vacuum spin-polarization of an Fe tip with an antiferromagnetically coupled Mn adatom [ Ferriani et al., Phys. Rev. B 82, 054411 (2010)] .
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We present an overview of the technique of spin-polarized scanning tunneling microscopy (Sp-STM) and its application to high-resolution magnetic imaging. In STM, the electron density near the sample surface is imaged. Additionally, Sp-STM allows a mapping of the spin polarization of the electronic density, which is related to the magnetic configuration of the sample. Two primary imaging modes of Sp-STM are currently in use: the spectroscopic mode and the differential magnetic mode. The principles of the two modes are explained in the framework of imaging ferromagnetic nanostructures and antiferromagnetic surfaces. The advantages and drawbacks of the two approaches are discussed, and the strength of Sp-STM to map even complex spin structures on the nanometer scale is illustrated.
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Information on the spin resolved band structure of ferromagnetic materials can directly be obtained from spin resolving photoelectron spectroscopy. Using polarized radiation spin integrating photoemission techniques already enable to have access to magnetic properties. An enhancement of the surface sensitivity can be achieved using neutral excited spin polarized atoms which move towards the sample and are de-excited by tunneling electrons from the surface with a subsequent emission of electrons.
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The atomic and electronic structures at the apex of W tips were studied by means of field ion microscopy and field emission microscopy, before and after the thermal deposition of a 5 nm Fe film. Two geometries of W tip, a conventional hemi-spherical type and a chisel (flat needle) type, were prepared. The hemispherical and the chisel W tips had a 110 direction parallel and perpendicular to the tip axis, respectively. The coated Fe films were found to be most likely in a non-crystalline phase, and to have a lower work function leading to a drastic change in electron emission from the apexes. The spin-polarization vectors of field-emitted electrons from these Fe/W tips were investigated with a Mott detector with a rotatable mechanism of tips. A similar absolute value of the spin-polarization vector was obtained for each Fe/W, while the direction of the spin-polarization vector was dependent on the shape of the apex. The angle from the tip axis was θ=45° for the hemispherical apex and θ=66° for the chisel apex. A spin-polarized scanning tunneling microscopy setup with a rotation mechanism of such Fe/W tips made it possible to detect both the in-plane and the out-of-plane spin component of a sample magnetization.
Article
The magnetoresistance of a hydrogen-phthalocyanine molecule placed on an antiferromagnetic Mn(001) surface and contacted by a ferromagnetic Fe electrode is investigated using density functional theory based transport calculations and low-temperature scanning tunneling microscopy. A large and negative magnetoresistance ratio of ∼50% is observed in combination with a high conductance. The effect originates from a lowest unoccupied molecular orbital (LUMO) doublet placed almost in resonance with the Fermi energy. As a consequence, irrespective of the mutual alignment of magnetizations, electron transport is always dominated by resonant transmission of Mn-majority charge carries going through LUMO levels.
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Magnetization-induced optical second harmonic generation MSHG from exchange-biased Mn/ Co thin films shows monolayer period oscillations at the Mn/ Co interface as a function of Co thickness. Similar oscillations are found in the exchange bias H E and the coercivity H C in both the interface sensitive MSHG and the bulk sensitive magneto-optical Kerr effect, indicating that magnetic reversal in the Co bulk and at the Mn/ Co interface is collinear. Assuming a linear relationship between the MSHG asymmetry and the magnetic moment, our results suggest that there is an enhancement of the interface net magnetic moment at the full monolayer regions.
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In this paper the magnetic properties of Mn ultrathin films grown on (001) Fe are shown to depend on the oxygen contamination. For very small amounts of oxygen, the Mn absorption spectra clearly indicate the metallic character of the bonding between Mn atoms, and the Mn/Fe coupling is ferromagnetic. In oxidized Mn films, localization of the 3d electrons due to Mn-O bonding is observed and the Mn/Fe coupling is antiferromagnetic. The Mn dichroic signal is explained quantitatively as soon as the amount of oxygen in the Mn layer is known, leading to a Mn atomic moment of 3μB in unoxidized films.
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This work is dedicated to the study of structural and magnetic properties of Mn films epitaxially grown on (001) bcc Fe. At room temperature, the Mn growth mode on (001) Fe is observed to be layer by layer, without any interdiffusion, as shown by reflection high-energy electron-diffraction and Auger spectroscopy. Moreover, Mn is observed to segregate on top of the Fe surface. Two-dimensional pure Mn films are thus grown and the structural and magnetic properties of these Mn films are investigated by in situ and ex situ x-ray absorption (EXAFS) and magnetic circular dichroism. For uncapped Mn films, a structural and magnetic transition is observed between 2 and 3 Mn monolayers. Up to 2 Mn atomic planes, the EXAFS oscillations are consistent with calculated EXAFS spectra assuming a bct structure close to the Fe bcc structure. A ferromagnetic behavior is observed in these films. From 3 to 10 atomic planes, the Mn film structure changes and the ferromagnetic behavior disappears. Moreover, EXAFS analysis shows that this structural transition occurs at higher thicknesses for Fe capped Mn films. Nevertheless, the ferromagnetic behavior observed on 0–2-ML-thick uncapped Mn films is no more stable in thick bct Mn films capped by Fe.
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The spin polarization of secondary electrons from a Cr film on Fe(100), measured with scanning electron microscopy with polarization analysis (SEMPA), oscillates as a function of Cr thickness with a period near two atomic layers, consistent with incommensurate spin-density-wave antiferromagnetism in the Cr. The position of a phase slip due to incommensurability varies reversibly by 14 layers over the temperature range of 310 to 550 K. The Cr surface magnetic moment persists well above the Néel temperature of bulk Cr.
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DOI:https://doi.org/10.1103/PhysRev.76.1256.2
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The growth of ultrathin Mn films on an Fe(001) whisker at 370 K is studied by scanning tunneling microscopy (STM) and scanning tunneling spectroscopy at room temperature in ultrahigh vacuum. Atomically and chemically resolved STM images show that the Mn film grows with the same in-plane lattice constant as Fe(001) and that Fe atoms intermix with the first (14%), the second (4%), and the third Mn layer (2%), while a negligible amount of Fe atoms is found above the third layer. The growth mode changes from layer-by-layer to layer-plus-island at a coverage of 3 ML Mn. dI/dV curves which are normalized by voltage-dependent tunneling probability functions show clear peaks on each Mn layer. These peaks are tentatively ascribed to surface states. On the first Mn layer, peaks are found at +0.35 eV on pure Mn areas and at +0.28 eV on mixed MnFe areas. The second and the third Mn layer show peaks at +0.20 and +0.8 eV, respectively. Mn films thicker than three layers reveal besides a strong peak at +0.8 eV a weaker peak at −0.6 eV. Our apparent step height measurements show that the Mn film relaxes at the third layer: the interlayer spacing is ∼0.16 nm for the first two layers, and it increases to ∼0.18 nm at the third layer. Starting from the fourth layer the interlayer spacings are geometrically equivalent (∼0.165 nm).
Article
Spin-polarized tunneling is demonstrated on Gd(0001) thin films using ferromagnetic probe tips in a low-temperature scanning tunneling microscope. The magnetic field dependent asymmetries in the differential tunneling conductivity are found at bias voltages which correspond to the energies of the spin components of the exchange-split Gd(0001) surface state. Maps of the spatial variation of the asymmetry reveal the magnetic structure of Gd(0001) thin films with a lateral resolution better than 20 nm. It is found that magnetic tip coatings thicker than 100 monolayers Fe may modify the sample domain structure due to the stray field of the tip.
Article
The phase diagram for ferromagnetic/antiferromagnetic bilayers with imperfect interfaces is calculated, using a Ising spin-1/2 model which is solved numerically in the mean field approximation for finite temperatures. We identify 3 stable phases: domains in the ferromagnet, domains in the antiferromagnet, and domain walls near the interfaces with homogeneous order within the layers. Phase transitions between these phases occur as a function of temperature, relative film thicknesses, and step density.
Article
We investigate the magnetic properties of Mn adsorbates on Fe(100) in the regime up to a few monolayers. Magnetic circular dichroism in absorption shows long-range ferromagnetic order for the Mn adsorbate, with antiferromagnetic alignment with respect to the Fe substrate. Element-specific magnetic domain imaging and hysteresis measurements show that the macroscopic magnetic behavior of the Mn adlayer is fully determined by the Fe substrate. For coverages below 0.5 ML the Mn absorption spectra show rich structures that are typical for localized d states. From this the Mn ground state is identified as a mixture of atomiclike d5 and d6 states, with a local spin moment of 4.5μB. However, the circular dichroism is 2.4 times smaller than expected for this ground state, suggesting disorder within the Mn adsorbate with an ordered moment of 1.9μB at 120 K. The magnetic signal vanishes near 1 ML coverage, consistent with the theoretically predicted c(2×2) antiferromagnetic ground state of the monolayer.
Article
We present a spin-polarized scanning tunneling microscope (Sp–STM) for imaging the magnetic in-plane component of magnetic surfaces. Magnetic in-plane sensitivity is obtained by using a ferromagnetic ring as a Sp–STM tip. By periodically switching the magnetization of the ring, the spin-dependent tunneling current between the ring and a spin-polarized sample is measured. The topography and the spin polarization can be imaged at the same time. We resolved the 180° domain wall of Fe whiskers and antiferromagnetic coupled Mn layers on Fe(001). © 2003 American Institute of Physics.
Article
A straightforward approach to spin-polarized scanning tunneling microscopy based on the magnetotunnel effect between a ferromagnetic tip and a ferromagnetic sample is demonstrated. By periodically changing the magnetization of the tip in combination with a lock-in technique, topographic and spin-dependent parts of the tunnel current are separated and the topography and the magnetic structure of the sample are recorded simultaneously. Results are given for polycrystalline Ni and single crystalline Co(0001) surfaces, revealing a high spin contrast, low data acquisition times, and a resolution down to 10 nm. Potentials and limitations of this technique are discussed. © 1999 American Institute of Physics.
Article
NiO/NiFe bilayer thin films were prepared by rf reactive and dc magnetron sputtering, respectively. The exchange coupling strength between NiO and NiFe as a function of NiO texture and interface roughness was investigated by using different sputtering pressures, Au, and Cu buffer layers. The experimental results show that the exchange coupling field strongly depends on the NiO/NiFe interface roughness. In addition, we found the exchange coupling is largest for the (200) texture compared to the (111)‐texture films. This is surprising since the bulk spin structure of NiO predicts the (200) plane to be compensated while the (111) plane is to be uncompensated. © 1996 American Institute of Physics.
Article
Mn is deposited on the (0001) surface of Ru and the (100) surfaces of Fe and Ni in a PHI 400 molecular‐beam epitaxy system. The first two layers of Mn on Ru (0001) maintain the close‐packed spacing of the Ru substrate. By comparison with other forms of Mn, this is an expanded structure. Beyond two layers, Mn maintains epitaxy, forming a denser phase which is most likely the hexagonal Zn 2 Mg structure, but possibly the fcc Cu 2 Mg structure, or a stacking fault variant of these two quite similar structures. The basic unit of both the Zn 2 Mg and Cu 2 Mg structures is the hexatetrahedron which is found also in the more complex α‐Mn structure. In each of these cases Mn appears to be an intermetallic compound with itself. Mn on Fe(100) forms epitaxially in the bcc phase. Mn on Ni(100) forms in a complex phase which has yet to be deciphered. The atomic magnetic moments of the Mn atoms in these phases are deduced from measurements of the exchange splitting of the 3s peak in the x‐ray photoelectron spectra. The splitting of the 3s peak is surprisingly insensitive to the Mn structure, but the intensity of the split‐off peak reflects the dependence of magnetic moment to atomic volume.
Article
The magnetic ordering and the interlayer exchange coupling in Mn and Fe/Mn wedge structures grown epitaxially on Fe(0 0 1) whisker substrates were investigated using scanning electron microscopy with polarization analysis (SEMPA). In bare Mn/Fe(0 0 1) samples, the magnetization of the top Mn layer is collinear with the Fe magnetization, and oscillates between ferromagnetic and antiferromagnetic alignment as the Mn thickness increases. The period of the oscillation is two layers of Mn, consistent with the growth of an antiferromagnetic Mn wedge. The bare Mn behaves very much like antiferromagnetic Cr, however the magnetic coupling in the Fe/Mn/Fe(0 0 1) sandwich structures is very different. For Mn thicknesses greater than four layers, the coupling between the top Fe layer and the Fe whisker substrate is not collinear. Between 4 to 8 layers of Mn, the direction of the top Fe in-plane magnetization lies at an angle of 60–80° relative to the magnetization of the Fe substrate. Beginning at the 9th Mn layer, the direction of the coupling oscillates, with a two-layer period, between 90°−φ and 90°+φ, where φ is sample dependent. Values of φ between 10 and 30° were observed.
Article
Growth of ultra-thin films of Mn on Fe(001) has been studied by grazing ion-surface scattering and Auger electron spectroscopy. We find that growth is epitaxial and pseudomorph and starts in a layer-by-layer mode. Favorable growth temperature is about 570 K; lower temperatures lead to kinetic roughening. After four layers, the growth mode changes from layer (two-dimensional) to island (three-dimensional) growth for temperatures above about 420 K. Below this temperature, a (metastable) layer-by-layer growth is observed. A quantitative analysis of the data yields the nucleation length in two-dimensional growth and the three-dimensional island density for the different growth temperatures.
Article
FeGeCo junctions conductance G(V) is studied when mean magnetizations of the two ferromagnetic film are parrallel or antiparallel. Conductance measurement, in these two cases, is related to the spin polarizations of the conduction electrons.
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In this paper we present a theoretical study of the magnetic properties of n Cr monolayers (ML) deposited on a Fe(001) substrate. We use the real space recursion method in a tight binding framework to determine the electronic structure of non-periodic systems involving a large number of inequivalent atoms. The aim of this work is to investigate the possibility to obtain a magnetic defect in the Cr layer by frustrating the plane to plane antiferromagnetic (AF) order. First, we study the magnetism of n perfect Cr monolayers (1 ≤ n ≤ 10) deposited on a perfect (001) Fe substrate and we show that a magnetic defect can be obtained for n ≥ 6 ML but it is clearly less stable than the solution without defect. We show also that, after the deposition of 3 Fe monolayers onto the Cr, the magnetic defect is obtained for much smaller Cr thicknesses (n ≥ 2). Then, we consider a Fe substrate presenting a mono-atomic step. We study the magnetic moments distributions during the growth process of Cr monolayers assuming that the growth starts at the step. We show that (i) the perturbation due to the step is limited to the vicinity of the step and has a small extension in the plane of the Cr layers, and (ii) a wall is generated in the Cr layer by the step and splits clearly the Cr layer into two domains. Finally, we determine the structure of collinear walls in chromium and we discuss their role on the magnetic properties.
Article
The local magnetic moments and magnetic order are calculated for a Fe (Cr) monolayer adsorbed on semi-infinite Cr (Fe) substrates with high-index surfaces. These stepped interfaces are present in the wedge-shaped configurations recently analyzed by different experimental groups in order to investigate the interlayer magnetic coupling in Fe/Cr/Fe systems. The spin-polarized electronic distribution is obtained by solving self-consistently a d-band model Hamiltonian in the mean-field approximation within the framework of the tight-binding real-space method. Several magnetic configurations have been found for all the systems investigated. In the most stable configuration of the Cr monolayer on Fe vicinal surfaces, the local moments of most of the Cr atoms are antiferromagnetically coupled with the Fe moments, an exception being the Cr atoms at the edge of the step. Cr atoms at nearest-neighbor positions (located at the kink and edge of the steps) are always antiferromagnetically coupled. For the Fe monolayer on Cr vicinal surfaces, the average magnetization at the surface results is zero for the two most stable solutions. However, other metastable solutions with net average magnetization on the Fe overlayer are found. The Fe atoms at the edge and kink of the steps (first neighbors) are, in all cases, ferromagnetically coupled. These results are in qualitative agreement with recent experimental observations.
Article
Ultrathin films of Mn grow epitaxially and pseudomorphically on Fe\{001\}. Low-energy electron diffraction (LEED) experiments show that with increasing thickness the films contain defects and disorder, but a substantial portion remains well crystallized. A quantitative LEED analysis of a 23-Å-thick film finds that the film has a body-centered-tetragonal structure with lattice parameters: a=2.866 Å and c=3.228 Å, hence with an axial ratio c/a=1.13 and atomic volume 13.3 A&#7783/atom. Strain analysis using both the LEED results and the tetragonal structure from dilute Mn alloys determines the Poisson ratio of the film to be near 0.5, hence the film is elastically soft. Both the face-centered- and the body-centered-cubic structures are consistent with the data as underlying phases of the grown film. LEED intensity data show that the distance between Mn layers is small (about 1.3 Å) when the epitaxial film is only two- or three-layers thick, and then increases to become 1.614 Å in the bulk of 14-layer thick films.
Article
Oscillations with a period corresponding to two Mn monolayers (ML's) were measured in the antiferromagnetic exchange coupling between ultrathin Fe layers and bulk Fe single-crystal whiskers separated by epitaxial Mn layers. The films were deposited by molecular-beam epitaxy. Electron diffraction showed that the Mn grew monolayer by monolayer in a body-centered-tetragonal structure (a=2.87 Å, c=3.27 Å) up to 15-25 ML's. The coupling was determined using the magneto-optic Kerr effect. It was antiferromagnetic for all thicknesses above 4-7 ML's Mn, with five strong peaks in the 0-0.2 mJ/m2 range.
Article
The magnetic state of epitaxial overlayers of Mn grown on Fe(100) is studied using spin-polarized electron energy loss spectroscopy. Nonzero exchange asymmetries are found, demonstrating that the surface layer of the Mn overlayers has a net magnetic moment. The exchange asymmetry oscillates with a period of about two atomic layers as the Mn overlayer thickness is varied, proving that the Mn forms ferromagnetic (100) sheets and that the sheets align antiferromagnetically. The average Mn exchange splitting is found to be 2.9 eV, indicating a magnetic moment of the order 3muB.
Article
The magnetization behavior of thin (1-5 nm) epitaxial Fe films on Cr(100) is investigated by the magneto-optical Kerr effect. Hysteresis loops show a strong change of the coercive field in a very narrow temperature range around T=130 K, independent of the film thickness. In addition, films below 3 nm thickness exhibit a strongly reduced remanent magnetization in an intermediate temperature range (T~=140-300 K). An explanation for both effects is based on the interface exchange coupling which connects the intrinsic antiferromagnetic properties of the Cr substrate to the Fe overlayer and therefore modifies its magnetic properties.
Article
The magnetic structure of the Cr(001) surface was investigated by spin-polarized scanning tunneling spectroscopy by making use of the spin-polarized surface state located close to the Fermi level. Periodic alternations of the intensity of the surface state peak in local tunneling spectra measured above different ferromagnetic terraces separated by monatomic steps confirm the topological antiferromagnetic order of the Cr(001) surface. Screw dislocations cause topology-induced spin frustration, leading to the formation of domain walls with a width of about 120 nm.
Article
We report the observation of a magnetic contrast of up to 20% in the scanning tunneling spectroscopy dI/dV maps obtained with Fe-coated tips on Mn(001) layers grown on an Fe(001) whisker at 370 K. These nanometer resolution microscopy results show that the layers couple antiferromagnetically. By normalizing the dI/dV curves by tunneling probability functions, we found a spin-dependent peak on the body-centered-tetragonal (bct) Mn(001) surface at +0.8 V, whose high spin polarization gives rise to the dI/dV map contrast. Band structure calculations allow one to identify the +0.8 V peak as due to two spin-polarized d(z(2)) surface states.
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