Pulsed Field-Induced Magnetization Switching in Antiferromagnetic Ferrihydrite Nanoparticles

The dynamic magnetization switching of ferrihydrite nanoparticles has been investigated by a pulsed magnetometer technique in maximum fields H_max of up to 130 kOe with pulse lengths of 4, 8, and 16 ms. Ferrihydrite exhibits antiferromagnetic ordering and defects cause the uncompensated magnetic moment in nanoparticles; therefore, the behavior typical of magnetic nanoparticles is observed. The dynamic hysteresis loops measured under the above-mentioned conditions show that the use of pulsed fields significantly broadens the temperature region of existence of the magnetic hysteresis and the coercivity can be governed by varying the maximum field and pulse length. This behavior is resulted from the relaxation effects typical of conventional ferro- and ferrimagnetic nanoparticles and the features typical of antiferromagnetic nanoparticles.     

Magnetic properties and the mechanism of formation of the uncompensated magnetic moment of antiferromagnetic ferrihydrite nanoparticles of a bacterial origin

The magnetic properties of the superparamagnetic ferrihydrite nanoparticles that form as a result of the vital activity of Klebsiella oxytoca bacteria are studied. Both an initial powder with an average number of iron atoms N _Fe ～ 2000–2500 in a particle and this powder after annealing at 140°C for 3 h in air are investigated. The following substantial modifications of the magnetic properties of the ferrihydrite nanoparticles are detected after annealing: the superparamagnetic blocking temperature increases from 23 to 49.5 K, and the average magnetic moment of a particle increases (as follows from the results of processing of magnetization curves). The particles have antiferromagnetic ordering, and the magnetic moment resulting in the superparamagnetism of the system appears due to random spin decompensation inside the particle. For this mechanism, the number of uncompensated spins is proportional to the number of magnetically active atoms raised to the one-half power, and this relation holds true for the samples under study at a good accuracy. The possible causes of the detected shift of magnetic hysteresis loops at low temperatures upon field cooling are discussed.     

Mechanism of the formation of an uncompensated magnetic moment in bacterial ferrihydrite nanoparticles

The magnetic properties of antiferromagnetic nanoparticles of FeOOH · nH₂O with sizes of 3–7 nm, which are products of vital functions of Klebsiella oxytoca bacteria, have been studied. Particles exhibit a superparamagnetic behavior. The characteristic blocking temperature is 23 K. Analysis of the magnetization curves shows that the mechanism of the formation of the uncompensated magnetic moment of particles is the random decompensation of magnetic moments of Fe³⁺ ions both on the surface and in the bulk of the antiferromagnetic particle. In this mechanism, the exchange coupling between the uncompensated magnetic moment of the particle and its antiferromagnetic “core” is implemented. It has been found that the temperature dependence of the uncompensated magnetic moment has the form 1 — constT ².     

Uniform spin wave modes in antiferromagnetic nanoparticles with uncompensated moments

In magnetic nanoparticles the uniform precession (q = 0 spin wave) mode gives the predominant contribution to the magnetic excitations. We have calculated the energy of the uniform mode in antiferromagnetic nanoparticles with uncompensated magnetic moments, using the coherent potential approximation. In the presence of uncompensated moments, an antiferromagnetic nanoparticle must be considered as a kind of a ferrimagnet. Two magnetic anisotropy terms are considered, a planar term confining the spins to the basal plane, and an axial term determining an easy axis in this plane. Excitation energies are calculated for various combinations of these two anisotropy terms, ranging from the simple uniaxial case to the planar case with a strong out-of-plane anisotropy. In the simple uniaxial case, the uncompensated moment has a large influence on the excitation energy, but in the planar case it is much less important. The calculations explain recent neutron scattering measurements on nanoparticles of antiferromagnetic α-Fe₂O₃ and NiO.     

Temperature behavior of the antiferromagnetic susceptibility of nanoferrihydrite from the measurements of the magnetization curves in fields of up to 250 kOe

The cross-breeding problem of the temperature dependence of the antiferromagnetic susceptibility of ferrihydrite nanoparticles is considered. Iron ions Fe3+ in ferrihydrite are ordered antiferromagnetically; however, the existence of defects on the surface and in the bulk of nanoparticles induces a noncompensated magnetic moment that leads to a typical superparamagnetic behavior of ensemble of the nanoparticles with a characteristic blocking temperature. In an unblocked state, magnetization curves of such objects are described as a superposition of the Langevin function and the linear-in-field contribution of the antiferromagnetic “core” of the nanoparticles. According to many studies of the magnetization curves performed on ferrihydrite (and related ferritin) nanoparticles in fields to 60 kOe, dependence χ_AF(T) decreases as temperature increases, which was related before to the superantiferromagnetism effect. As the magnetic field range increases to 250 kOe, the values of χ_AF obtained from an analysis of the magnetization curves become lower in magnitude; however, the character of the temperature evolution of χ_AF is changed: now, dependence χ_AF(T) is an increasing function. The latter is typical for a system of AF particles with random orientation of the crystallographic axes. To correctly determine the antiferromagnetic susceptibility of AF nanoparticles (at least, ferrihydrite) and to search for effects related to the superantiferromagnetism effect, it is necessary to use in experiments the range of magnetic field significantly higher than that the standard value 60 kOe used in most experiments. The study of the temperature evolution of the magnetization curves shows that the observed crossover is due to the existence of small magnetic moments in the samples.     

Magnetic properties of biomineral particles produced by bacteria <Emphasis Type="Italic">Klebsiella oxytoca</Emphasis>

Ferrihydrite nanoparticles (2–5 nm in size) produced by bacteria Klebsiella oxytoca in the course of biomineralization of iron salt solutions from a natural medium exhibit unique magnetic properties: they are characterized by both the antiferromagnetic order inherent in a bulk ferrihydrite and the spontaneous magnetic moment due to the decompensation of spins in sublattices of a nanoparticle. The magnetic susceptibility enhanced by the superantiferromagnetism effect and the magnetic moment independent of the magnetic field provide the possibility of magnetically controlling these natural objects. This has opened up the possibilities for their use in nanomedicine and bioengineering. The results obtained from measurements of the magnetic properties of the ferrihydrite produced by Klebsiella oxytoca in its two main crystalline modifications are reported, and the data obtained are analyzed theoretically. This has made it possible to determine numerical values of the magnetic parameters of real biomineral nanoparticles.     

Non-Langevin behaviour of the uncompensated magnetization in nanoparticles of artificial ferritin

The magnetic behaviour of nanoparticles of antiferromagnetic artificial ferritin has been investigated by ⁵⁷Fe M?ssbauer absorption spectroscopy and magnetization measurements, in the temperature range 2.5-250 K and with magnetic fields up to 7 T. Samples containing nanoparticles with an average number of ⁵⁷Fe atoms ranging from 400 to 2 500 were studied. By analysing the magnetic susceptibility and zero field M?ssbauer data, the anisotropy energy per unit volume is found, in agreement with some of the earlier studies, to have a value typical for ferric oxides, i.e. a few 10⁵ ergs/cm³. By comparing the results of the two experimental methods at higher fields, we show that, contrary to what is currently assumed, the uncompensated magnetization of the ferritin cores in the superparamagnetic regime does not follow a Langevin law. For magnetic fields below the spin-flop field, we propose an approximate law for the field and temperature variation of the uncompensated magnetization, which was early evoked by Néel but has so far never been applied to real antiferromagnetic systems. More generally, this approach should apply to randomly oriented antiferromagnetic nanoparticles systems with weak uncompensated moments. Received 20 January 2000     

Change in the magnetic properties of nanoferrihydrite with an increase in the volume of nanoparticles during low-temperature annealing

The results of the investigation into the effect of low-temperature annealing of a powder of nanoparticles of bacterial ferrihydrite on its magnetic properties have been presented. It has been found that an increase in the time (up to 240 h) and temperature (in the range from 150 to 200°C) of annealing leads to a monotonic increase in the superparamagnetic blocking temperature, the coercive force, and the threshold field of the opening of the magnetic hysteresis loop (at liquid-helium temperatures), as well as to an increase in the magnetic resonance line width at low temperatures and in the magnetic susceptibility at room temperature. At the same time, according to the results of the analysis of the Mössbauer spectra, the annealing of ferrihydrite does not lead to the formation of new iron oxide phases. Most of these features are well consistent with the fact that the low-temperature annealing of ferrihydrite causes an increase in the size of nanoparticles, which is confirmed by the results of transmission electron microscopy studies.     

Rotation of the pinning direction in the exchange bias training effect in polycrystalline NiFe/FeMn bilayers

For polycrystalline NiFe/FeMn bilayers, we have observed and quantified the rotation of the pinning direction in the exchange bias training and recovery effects. During consecutive hysteresis loops, the rotation of the pinning direction strongly depends on the magnetization reversal mechanism of the ferromagnet layer. The interfacial uncompensated magnetic moment of antiferromagnetic grains may be irreversibly switched and rotated when the magnetization reversal process of the ferromagnet layer is accompanied by domain wall motion and domain rotation, respectively.     

Anisotropic pinned/biased magnetization in SrRuO3/SrMnO3 superlattices

The exchange coupling at the interfaces of magnetic superlattices consisting of ferromagnetic SrRuO3 and antiferromagnetic SrMnO3 grown on (001) oriented SrTiO3 is studied with in-plane and out-of-plane orientations of the cooling magnetic field, with respect to the substrate plane. The magnetization of the in-plane, field cooled hysteresis loop is lower than the corresponding in-plane zero-field-cooled hysteresis loop. The out-of-plane field cooled hysteresis loop is shifted, from the origin, along the graphical magnetization axis. We attribute this irreversible rotation of the moment to the pinning/biasing of spin in the SrRuO3 layer in the vicinity of interfaces by the antiferromagnetic SrMnO3 layer.     

Frustrated multilayer ferromagnet-antiferromagnet structures: Beyond the scope of the exchange approximation (a review)

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.     

Iron nanoparticles prepared from natural ferrihydrite precursors: kinetics and properties

Natural ferrihydrite was used as the precursors for the preparation of the nanocrystalline alpha-Fe powder using heat treatment in hydrogen atmosphere. Transformation temperatures were estimated from the temperature dependences of magnetic moment. Structure and phase composition of the precursors and the final powders were characterized by X-ray diffraction and Mössbauer spectroscopy. Magnetic properties were characterized according to hysteresis loop parameters. The kinetics of the transformation was described using Avrami relation applied on the data of the isothermal time dependence of magnetic moment measurements and subsequently the activation enthalpy of transformation was estimated using Arrhenius plots of their parameters. Comparison of the changes of activation enthalpies derived from a different temperature ranges indicates effects of different processes on formation of alpha-Fe nanoparticles.     

Unified nonequilibrium dynamical theory for exchange bias and training effects

We have investigated the exchange bias and training effect in the ferromagnetic/antiferromagnetic (FM/AF) heterostructures using a unified Monte Carlo dynamical approach. The magnetization of the uncompensated AF layer is still open after the first field cycling is finished. Our simulated results show obvious shift of hysteresis loops (exchange bias) and cycling dependence of exchange bias (training effect) when the temperature is below 45~K. The exchange bias field decreases with decreasing cooling rate or increasing temperature and the number of the field cycling. Essentially, these two effects can be explained on the basis of the microscopical coexistence of both reversible and irreversible moment reversals of the AF domains. Our simulations are useful to understand the real magnetization dynamics of such magnetic heterostructures.     

Microscopic nature of ferro- and antiferromagnetic interface coupling of uncompensated magnetic moments in exchange bias systems

Exchange bias in layered CoO/Fe structures is investigated by x-ray resonant magnetic reflectivity (XRMR) measurements. Element-specific hysteresis loops are obtained from x-ray magnetic circular dichroism effects in the XRMR spectra. Evidence is provided for the existence of different types of uncompensated moments in the antiferromagnetic material. Explanations are given for the microscopic nature of these moments and the complex exchange interactions that determine the magnetization reversal in exchange bias systems.     

On the interpretation of magnetization data for antiferromagnetic nanoparticles

We have investigated the influence of anisotropy on the magnetization curves of antiferromagnetic nanoparticles. We show that if such curves are analyzed in a conventional way, i.e. using a Langevin function in combination with a linear term, this usually results in good quality fits, but with an apparent temperature dependence of parameters such as the magnetic moment per particle and the antiferromagnetic susceptibility. In order to avoid the problems associated with anisotropy as well as volume/moment distributions we propose that the initial susceptibility is used when analyzing the temperature dependence of the magnetic moment.     

Temperature-dependent magnetic anomalies of CuO nanoparticles

Copper oxide (CuO) nanoparticles with an average size of 25 nm were prepared by a sol-gel method. A detailed study was made of the magnetization of CuO nanoparticles using a maximum field of 60 kOe for temperatures between 8 and 300 K. Antiferromagnetic CuO nanoparticles exhibit anomalous magnetic properties, such as enhanced coercivity and magnetic moments. Significantly, the magnitude of the hysteresis component tends to weaken upon increase in temperature (>8 K). In addition, a hysteresis loop shift and coercivity enhancement are observed at 8 K in the field-cooled (FC, at 50 kOe) case. It is thought that the change in hysteresis behavior is due to the uncompensated surface spins of the CuO nanoparticles. The susceptibility (χ) plot showed that χ varied substantially at temperatures below 12 K, and this transition is due to the exchange interactions between the neighboring atoms at the nanoscale.     

Tuning the coercive field of Ni and CuNi thin films with the embedding of Co nanoparticles: An element-specific study

Cobalt nanoparticles have been embedded either within magnetic thin films of CuNi or Ni, or at their interface with an Al capping layer. Soft X-ray magnetic circular dichroism (XMCD) was used to measure element-specific hysteresis loops, allowing discrimination between the nanoparticles and the host film. The hysteresis shows coupling between the nanoparticles and the films and a reduction in coercivity is observed when positioning the nanoparticles within the film, as opposed to at its interface with the cap. Sum rule analysis of the XMCD spectra makes it possible to determine the orbital to spin moment ratios for Co and Ni in the samples. The difference in the calculated ratios is consistent with the differences in the coercivities, the films appearing more homogenous due to hybridization of the 3d bands of Co and Ni when the nanoparticles are embedded deeper within the films, creating less pinning sites and hence a lower coercivity.     

Non-equilibrium cation distribution and enhanced spin disorder in hollow CoFe2O4 nanoparticles

We present magnetic properties of hollow and solid CoFe(2)O(4) nanoparticles that were obtained by annealing of Co(33)Fe(67)/CoFe(2)O(4) (core/shell) nanoparticles. Hollow nanoparticles were polycrystalline whereas the solid nanoparticles were mostly single crystal. Electronic structure studies were performed by photoemission which revealed that particles with hollow morphology have a higher degree of inversion compared to solid nanoparticles and the bulk counterpart. Electronic structure and the magnetic measurements show that particles have uncompensated spins. Quantitative comparison of saturation magnetization (M(S )), assuming bulk Néel type spin structure with cationic distribution, calculated from quantitative XPS analysis, is presented. The thickness of uncompensated spins is calculated to be significantly large for particles with hollow morphology compared to solid nanoparticles. Both morphologies show a lack of saturation up to 7 T. Moreover magnetic irreversibility exists up to 7 T of cooling fields for the entire temperature range (10-300 K). These effects are due to the large bulk anisotropy constant of CoFe(2)O(4) which is the highest among the cubic spinel ferrites. The effect of the uncompensated spins for hollow nanoparticles was investigated by cooling the sample in large fields of up to 9 T. The magnitude of horizontal shift resulting from the unidirectional anisotropy was more than three times larger than that of solid nanoparticles. As an indication signature of uncompensated spin structure, 11% vertical shift for hollow nanoparticles is observed, whereas solid nanoparticles do not show a similar shift. Deconvolution of the hysteresis response recorded at 300 K reveals the presence of a significant paramagnetic component for particles with hollow morphology which further confirms enhanced spin disorder.     

Evidence of a core–shell structure in the antiferromagnetic La0.2Ce0.8CrO3 nanoparticles by neutron scattering

We report the evidence of a core?Cshell structure in the antiferromagnetic La_0.2Ce_0.8CrO₃ nanoparticles by using a combination of neutron diffraction, polarized neutron small angle scattering (SANSPOL), and dc magnetization techniques. The neutron diffraction study establishes that the present nanoparticles are antiferromagnetic in nature. The magnetic scattering in the SANSPOL study arises from the shell part of the nanoparticles due to the disordered surface spins. The analysis of the SANSPOL data shows that these nanoparticles have a mean core diameter of 12.3±1.1?nm, and a shell thickness of 2.8±0.4?nm, giving a core?Cshell structure with an antiferromagnetic core, and a shell with a net magnetic moment under an applied magnetic field.     

Direct observation of exchange bias related uncompensated spins at the CoO/Cu interface

Magnetization-induced optical second harmonic generation (MSHG) from the exchange-biased CoO/Cu-(X)/Fe multilayer shows the presence of pinned uncompensated spins at the CoO/Cu interface. For increasing Cu spacer thickness, the exchange bias measured via the hysteresis loop shift diminishes and disappears at X = 3.5 nm, while the MSHG signal still shows a strong magnetic contribution from the CoO interface. This indicates that the magnetic interaction between Fe and CoO layers is sufficiently strong to induce order in the antiferromagnetic layer even at a spacer thickness for which there is no observable hysteresis loop shift.