[Show abstract][Hide abstract] ABSTRACT: The pursuit of higher densities in binary storage media is facing serious operating limitations. In order to overcome these constraints, several multistate techniques have been investigated as alternatives. Here, we report on an approach to define multistate switching memory units based on magnetic nanostructures exhibiting exchange bias. Writing and reading conditions were studied in patterned antiferromagnetic/ferromagnetic thin films. We establish the necessary and sufficient requirements for this multidigit memory concept that might open up new possibilities for the exploration and design of suitable room temperature spintronic devices.
[Show abstract][Hide abstract] ABSTRACT: In situ transmission electron microscopy (TEM) during annealing of metal-insulating granular thin films reveals significant microstructural changes: nucleation of ultra-small particles and growth of those already existing. Radio frequency impedance spectroscopy shows that by subsequent annealing, tunnelling conductance becomes more important at the expense of the decrease of capacitive contributions, giving place to an almost frequency-independent conduction, although being very far from physical percolation. These results are simulated assuming a random resistor–capacitor network, which represents the competing conduction channels between particles through thermally assisted tunnelling and capacitive conductance. The physical parameters derived from this simple model agree well with the microstructural changes observed by TEM through annealing, and evidence the increase of tunnelling conduction paths mediated by segregated ultra-small glue particles in between bigger ones. Those glue particles grow with further annealing and account for the correlation of the microstructural changes to the ac transport properties. (Some figures may appear in colour only in the online journal) Since the works of Abeles and co-workers [1, 2], metal-insulating nanocomposite granular films have been widely studied, as they provide promising applications and theoretical challenges . When the metallic nanoparticles are magnetic, those materials exhibit tunnel-type giant magnetoresistance (TMR), and are thus suitable for magnetic recording media and high-sensitivity magnetic sensors [4, 5]. In such practical applications, post-annealing the as-deposited samples has been shown to improve their electrical and magnetic behaviour for the required capabilities. Some recent studies have related changes in dc and ac electrical transport to microstructural changes during annealing, since those thermal treatments can yield an increase in particle size and affect grain boundaries and local defect density [6–9]. On the other hand, there is 1 Present address: ICMAB-CSIC, 08193-Bellaterra, Catalonia, Spain.
Journal of Physics D Applied Physics 11/2013; 46:495304-5. · 2.52 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present radio frequency transverse susceptibility (TS) measurements on oleic acid-coated and SiO2-coated Fe3-xO4 magnetite nanoparticles. The effects of the type of coating on the interparticle interactions and magnetic anisotropy are evaluated for two different particle sizes in powder samples. On the one hand, SiO2 coating reduces the interparticle interactions as compared to oleic acid coating, the reduction being more effective for 5 nm than for 14 nm diameter particles. On the other hand, the magnetic anisotropy field at low temperature is lower than 1 kOe in all cases and independent of the coating used. Our results are relevant concerning applications in biomedicine, since the SiO2 coating renders 5 and 14 nm hydrophilic particles with very limited agglomeration, low anisotropy, and superparamagnetic behavior at room temperature. The TS technique also allows us to discriminate the influence on the anisotropy field of interparticle interactions from that of the thermal fluctuations.
[Show abstract][Hide abstract] ABSTRACT: Magnetization reversal mechanisms are studied in Ni/FeF2 heterostructures with the coexistence of positive and negative exchanged bias (PEB/NEB), showing single and double hysteresis loops (DHL) in magnetoresistance measurements. Micromagnetic simulations show that PEB and NEB domains of a minimum critical size must be introduced in order to reproduce the occurrence of DHLs. The simulations reveal that different magnetic configurations and, hence, different magnetization reversal processes take place in a ferromagnet (FM) on top of minority PEB domains that are either greater or smaller than the critical size. In particular, for the case of DHLs, core reversal of a depthwise domain wall is observed over minority PEB domains when the magnetic field is decreased from positive saturation. As the field is further decreased, a complex domain-wall evolution takes place in the FM, including the dependences of the domain-wall width and domain size on the magnetic field and distance from the antiferromagnet (AF). These effects should be taken into account when the domain size is estimated from data measured by depth-dependent techniques since they average the distribution of domain sizes in the FM for different distances from the AF.
[Show abstract][Hide abstract] ABSTRACT: Thin films of the giant magnetoresistive granular CoFe-AgCu system prepared by rf sputtering displayed a great variety of
domain-like microstructures with a net out-of-plane component of the magnetization for ferromagnetic volume concentrations
above about 0.25. Therefore, magnetic percolation takes place at ferromagnetic concentrations much lower than the physical
percolation threshold. The out-of-plane structure of the as-deposited samples in magnetic virgin state consisted of a distribution
of both quasi-circular domains and short stripes depending on the ferromagnetic content. Furthermore, these samples present
high metastability and a variety of remanent in-plane and out-of-plane microstructures can be achieved as a function of the
magnetic history. Besides, the evolution of the magnetic microstructure yields strong training effects on magnetotransport
properties, due to the extra contribution of the electron scattering at the domain walls. All in all, the observed behavior
is the result of a subtle correlation between perpendicular anisotropy produced by residual stresses, exchange interparticle
interactions due to CoFe alloyed in the matrix, and dipolar interactions. Thus, as high structural evolution occurs through
annealing, the features of randomly distributed ferromagnetic particles are recovered and, the out-of-plane domain structures
and the training effects disappear.
PACS. 75.70.Kw Domain structure (including magnetic bubbles) - 75.70.Pa Giant magnetoresistance - 75.60.Lr Magnetic aftereffects
Physics of Condensed Matter 04/2012; 17(1):43-50. · 1.46 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The properties of magnetic nanoparticles tend to be depressed by the unavoidable presence of a magnetically inactive surface layer. However, outstanding magnetic properties with a room-temperature magnetization near the bulk value can be produced by high-temperature synthesis methods involving capping with organic acid. The capping molecules are not magnetic, so the origin of the enhanced magnetization remains elusive. In this work, we present a real-space characterization on the subnanometer scale of the magnetic, chemical, and structural properties of iron-oxide nanoparticles via aberration-corrected scanning transmission electron microscopy. For the first time, electron magnetic chiral dichroism is used to map the magnetization of nanoparticles in real space with subnanometer spatial resolution. We find that the surface of the nanoparticles is magnetically ordered. Combining the results with density functional calculations, we establish how magnetization is restored in the surface layer. The bonding with the acid's O atoms results in O-Fe atomic configuration and distances close to bulk values. We conclude that the nature and number of molecules in the capping layer is an essential ingredient in the fabrication of nanoparticles with optimal magnetic properties.
[Show abstract][Hide abstract] ABSTRACT: Magnetically-disordered layers at the surface of nanoparticles and low
dimensionality magnetic oxides significantly reduce the magnetization
density with respect to bulk values. However, high crystal quality
Fe3O4 nanoparticles capped with non-magnetic
organic acid molecules display a surprisingly high magnetization, of
unknown origin. Here, we present a real space structural, chemical and
magnetic characterization of oleic-acid-caped
Fe3O4 nanoparticles with nanometer resolution,
demonstrating the presence of a strong magnetic surface layer. In
combination with theoretical calculations, we establish the key role of
the nanoparticle/organic-acid bond. Magnetization is restored in the
surface layer because the bonding with the acid's O atoms partially
lifts the surface reconstruction, resulting in surface O-Fe atomic
configuration and distances close to the bulk values. Our findings have
implications for the optimization of magnetic properties of
nanoparticles and thin films.
[Show abstract][Hide abstract] ABSTRACT: The process of formation of magnetite nanoparticles has been investigated by liquid chromatography and mass spectroscopy in the liquid phase decomposition of either Fe(III) acetylacetonate with decanoic acid or Fe(III) decanoate. In both cases, the dissociation into radicals of the iron carboxylate bonds provides the reduction of the Fe(III) cations and the oxygen atoms required for the formation of the mixed-valence inverse spinel magnetite structure. A reaction mechanism is proposed. It is also shown that the reaction of free decanoic acid with the Fe(III) cations in solution promotes the growth of faceted particles at the reflux temperature of the solvent (ca. 280 °C), while, under the same conditions, the stepwise decomposition of the Fe(III) decanoate generates smaller and pseudo-spherical particles. The latter also yields faceted particles when the temperature is increased above that of the total decomposition of the salt. Magnetic measurements make evident that the reaction starting from Fe(III) acetylacetonate yields nanoparticles with higher magnetization and lower spin disorder, due to the improved regularity of the surface crystal structure. The starting conditions for the decarboxylation process thus affect the morphology and magnetic properties of the resulting nanoparticles.
Physical Chemistry Chemical Physics 09/2011; 13(43):19485-9. · 4.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Magnetic correlations appearing in polycrystalline Gd5Ge4 are studied. On the one hand, ac susceptibility measurements as functions of temperature at several dc fields and frequencies show the existence of ferromagnetic (FM) and antiferromagnetic (AFM) correlations in the paramagnetic (PM) region, where a Griffiths-like phase appears below ~225 K. The value for the effective magnetic moment within the Griffiths-like phase is 10.1 muB. FM correlations also extended all the way into the AFM phase below ~127 K. The onset of the Griffiths-like phase is associated with an effective critical slowing down. On the other hand, high field magnetization measurements reveal the presence of FM and AFM correlations in the three magnetic phases (PM, FM, and AFM), giving rise to a variety of mixed magnetic states. In particular, at high fields the magnetostructural transition takes place in several stages that extend along a wide temperature range. A three-dimensional (T,H,M) phase diagram is proposed, including the new experimental findings.
[Show abstract][Hide abstract] ABSTRACT: The magnetic behavior of Fe3−xO4 nanoparticles synthesized by either high-temperature decomposition of an organic iron precursor or low-temperature coprecipitation in aqueous conditions is compared. Transmission electron microscopy, x-ray absorption spectroscopy, x-ray magnetic circular dichroism, and magnetization measurements show that nanoparticles synthesized by thermal decomposition display high crystal quality and bulklike magnetic and electronic properties, while nanoparticles synthesized by coprecipitation show much poorer crystallinity and particlelike phenomenology, including reduced magnetization, high closure fields, and shifted hysteresis loops. The key role of the crystal quality is thus suggested, because particlelike behavior for particles larger than about 5 nm is observed only when the particles are structurally defective. These conclusions are supported by Monte Carlo simulations. It is also shown that thermal decomposition is capable of producing nanoparticles that, after further stabilization in physiological conditions, are suitable for biomedical applications such as magnetic resonance imaging or biodistribution studies.
Journal of Applied Physics 04/2011; 109(7):07B524-07B524-6. · 2.19 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Magnetic iron oxide nanoparticles are good candidates for biomedical applications due to their low toxicity and easy functionalization. We synthesized magnetite (Fe3O4) nanoparticles by a high temperature decomposition method. They present some very desirable properties for applications: very high saturation magnetization, and excellent degree of crystallinity. Transmission electron microscopy images, and electron energy loss spectroscopy with atomic resolution allow a composition map that shows small variations in relative composition between the core and the surface, and subtle changes in the absorption spectra. Our density functional (DFT) calculations address different factors contributing to the magnetic properties. Changes in the electronic structure correlate with different features in the experimental absorption spectra, yielding a better understanding of the magnetic order. We study the role of structural defects, the organic surfactant, stoichiometry and the nominal oxidation state of iron, and their effect in determining the equilibrium magnetic state. This work is supported by DOE Materials Sciences and Engineering Division and the European Research Council Starting Investigator Award.
[Show abstract][Hide abstract] ABSTRACT: Positively and negatively exchange biased (PEB and NEB) magnetoresistance (MR) loops in Ni/FeF2 ferromagnetic/antiferromagnetic (AF) heterostructures proceed through the same reversal mechanisms. The MR curves exhibit mirror symmetry: the increasing (decreasing) field branch of the PEB (NEB) loop is identical to the decreasing (increasing) branch of the NEB (PEB) loop, suggesting that the interfacial areal density of pinned uncompensated AF spins responsible for PEB and NEB is similar. Micromagnetic simulations are in agreement with experimental results and imply the coexistence of EB domains of opposite sign for all cooling fields, which results in a reversal mechanism not previously reported.
[Show abstract][Hide abstract] ABSTRACT: The influence of a variety of parameters on the synthesis of iron oxide nanoparticles (magnetite/maghemite Fe3O4/γ-Fe2O3) by thermal decomposition of a metal−organic iron precursor in an organic medium is reported. We study the role of both the surfactant and the reducing agent on the shape, the size distribution, and the magnetic properties. We aim at synthesizing magnetic nanoparticles with high crystal quality and good magnetic response. A narrow size distribution of pseudospherical and faceted particles (4−20 nm) with a high saturation magnetization (Ms ≈ 80−85 emu/g at 5 K) is obtained when using oleic acid as a surfactant. In contrast, decanoic acid yields much larger pseudocubic particles (45 nm) with a wider size distribution and a larger saturation magnetization (Ms = 92 emu/g at 5 K), close to the expected value for bulk magnetite. Besides, the use of a variety of reducing agents monitors the magnetic behavior. In the case of 1,2-hexadecanediol, magnetic characterization suggests that the nanoparticles have uniform oxidation. However, those particles prepared without the use of any reducing agent also show uniform oxidation just with a slightly smaller value of the saturation magnetization (Ms = 76 emu/g at 5 K). In contrast, hydrazine seems to promote a nonuniform oxidation that results in the appearance of the exchange bias phenomenon and in a smaller saturation magnetization (Ms = 67 emu/g at 5 K). New ways to tune the shape, the size, and the magnetic properties are discussed.
The Journal of Physical Chemistry C 12/2010; 115(2). · 4.84 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Iron oxide nanoparticles with uniform sizes between 13 nm and 180 nm can be selectively prepared through the "heating up" thermal decomposition method by using decanoic acid and carefully tuning the heating rate.
Chemical Communications 09/2010; 46(33):6108-10. · 6.38 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We compare magnetic reversal of nanostructured circular magnetic dots of different sizes. This comparison is based on superconducting quantum interference device SQUID magnetometry, neutron scattering, Monte Carlo simulation, and analytical calculations and is quantified using a parameter which characterizes the variation in the hysteresis curve width. Below a critical dot diameter, the magnetic reversal occurs by coherent rotation and above that diameter, the reversal occurs by formation of a magnetic vortex. The vortex-core diameter is controlled by competing magnetic energy contributions. For 20-nm-thick Fe dots, the values of the critical diameter 58–60 nm and the vortex core 16–19 nm are in very good agreement between the different experimental and theoretical methods: neutron scattering, SQUID magnetometry, Monte Carlo simulations, and analytical calculations.
Physical Review B 05/2010; 81(18-20):184417. · 3.66 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Uptake, cytotoxicity and interaction of improved superparamagnetic iron oxide nanoparticles were studied in cells, tissues and organs after single and multiple exposures.
We prepared dimercaptosuccinic acid-coated iron oxide nanoparticles by thermal decomposition in organic medium, resulting in aqueous suspensions with a small hydrodynamic size (< 100 nm), high saturation magnetization and susceptibility, high nuclear magnetic resonance contrast and low cytotoxicity.
In vitro and in vivo behavior showed that these nanoparticles are efficient carriers for drug delivery to the liver and brain that can be combined with MRI detection.
[Show abstract][Hide abstract] ABSTRACT: The fabrication of ordered arrays of exchange biased Ni/FeF(2) nanostructures by focused ion beam lithography is reported. High quality nano-elements, with controlled removal depth and no significant re-deposition, were carved using small ion beam currents (30 pA), moderate dwell times (1 micros) and repeated passages over the same area. Two types of nanostructures were fabricated: square arrays of circular dots with diameters from 125 +/- 8 to 500 +/- 12 nm and periodicities ranging from 200 +/- 8 to 1000 +/- 12 nm, and square arrays of square antidots (207 +/- 8 nm in edge length) with periodicities ranging from 300 +/- 8 to 1200 +/- 12 nm. The arrays were characterized using scanning ion and electron microscopy, and atomic force microscopy. The effect of the patterning on the exchange bias field (i.e., the shift in the hysteresis loop of ferromagnetic Ni due to proximity to antiferromagnetic FeF(2)) was studied using magneto-transport measurements. These high quality nanostructures offer a unique method to address some of the open questions regarding the microscopic origin of exchange bias. This is not only of major relevance in the fabrication and miniaturization of magnetic devices but it is also one of the important proximity phenomena in nanoscience and materials science.
[Show abstract][Hide abstract] ABSTRACT: We report on the effect of using decanoic acid as capping ligand on the synthesis of iron oxide nanoparticles by thermal decomposition of an organic iron precursor in organic medium. This procedure allowed us to control the particle size within 5 nm and about 30 nm by modifying the precursor-to-capping ligand ratio in a systematic fashion and to further expand the particle size range up to about 50 nm by adjusting the final synthesis temperature. The nanoparticles also showed high saturation magnetization of about 80-83 emu/g at low temperature, almost size-independent and close to the value for the bulk counterpart. Decanoic acid-coated nanoparticles were transferred to water by using tetramethylammonium hydroxide, which allowed further coating with silica in a tetraethyl orthosilicate solution. Consequently, these iron oxide nanoparticles are tunable in size and highly magnetic, and they could become suitable candidates for various biomedical applications such as contrast agents for magnetic resonance imaging and magnetic carriers for drug delivery.