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The magnetization of a magnetoactive elastomer (MAE) with microparticles of soft magnetic carbonyl iron embedded in a highly elastic matrix has been studied. It is shown that at high temperatures its magnetization curve has the form of a specific hysteresis loop. This hysteresis is attributed to the influence of displacement of magnetized particles in the elastically soft elastomer matrix under the effect of magnetic forces, leading to the change of magnetic interaction between the particles. In this case, there is a maximum in the field dependence of the magnetic susceptibility, the occurrence of which has been associated with the competition between re-arrangement of particles, when they are displaced in a magnetic field, and saturation of particles’ magnetization. When the MAE is cooled below approximately 225 K, both the magnetic hysteresis and the maximum in the field dependence of the magnetic susceptibility disappear. When the MAE material is cooled below the solidification temperature of the elastomer matrix, the displacements of the magnetic particles during magnetization are blocked by the rigid matrix. The magnetization reversal of the MAE is reversible. This means that the shape of subsequent magnetization loops remains constant and the sample returns into the initial non-magnetized state after the magnetic field is turned off.

Polydimethylsiloxane based magnetoactive elastomers demonstrate above the melting transition range (e.g. at room temperature) an induced uniaxial magnetic anisotropy, which grows with increasing magnetic field. By freezing a material down to 150 K, displaced iron microparticles are immobilized, so that the magnetic anisotropy can be measured. Magnetic anisotropy “constant” is a consequence of particle displacements and a characteristic of the energy of internal deformations in the polymer matrix. The maximum anisotropy constant of the filling is at least one order of magnitude larger than the shear modulus of the pure elastomer (matrix). In a magnetic field, the gain in the rigidity of the composite material is attributed to the magnetomechanical coupling, which is in turn a source of anisotropy. The concept of effective magnetic field felt by the magnetization allows one to explain the magnetization curve at room temperature from low-temperature measurements. The results can be useful for developing vibration absorbers and isolators.

Magnetic properties of a magnetoactive elastomer (MAE) filled with {\mu}m-sized soft-magnetic iron particles have been experimentally studied in the temperature range between 150 K and 310 K. By changing the temperature, the elastic modulus of the elastomer matrix was modified and it was possible to obtain magnetization curves for an invariable arrangement of particles in the sample as well as in the case when the particles were able to change their position within the MAE under the influence of magnetic forces. At low (less than 220 K) temperatures, when the matrix becomes rigid, the magnetization of the MAE does not show a hysteresis behavior and it is characterized by a negative value of the Rayleigh constant. At room temperature, when the polymer matrix is compliant, a magnetic hysteresis exists and exhibits local maxima of the field dependence of the differential magnetic susceptibility. The appearance of these maxima is explained by the elastic resistance of the matrix to the displacement of particles under the action of magnetic forces., Comment: 16 pages, 6 figures

This paper reports on the results of the magnetostatic measurements for Co-Al-O nanogranular films over a wide range of concentrations of the ferromagnetic component x. It has been revealed that grains in the films are characterized by the growth-induced anisotropy with easy axes directed perpendicular to the film plane. The maximum field of the single-grain perpendicular anisotropy reaches ∼2.5 kOe for samples in the vicinity of the percolation threshold (x ≈ 61 at % Co). It has been established that the characteristic features of the superparamagnetic behavior of an ensemble of oriented Stoner-Wohlfarth particles are retained for the sample with x ≈ 61 at % Co in the presence of the demagnetization field associated with the net magnetization of the film. The influence of the demagnetization field of the film on the shape of the magnetization reversal curves, the coercivity, and the blocking temperature has been investigated and simulated. The results of the simulation are consistent with the experimental data.

The crystallographic, magnetic, electrical, and magnetoresistive properties of samples of La0.775Sr0.225Mn1−xSnxO3 synthesized by the sol-gel method are studied. It is shown that introducing tin atoms, with their large size, into the manganese sublattice produces significant deformations of the crystal lattice and enhances the structural and magnetic inhomogeneity of the samples. It is discovered that increasing the tin content leads to a reduction in magnetization, a lowering of the Curie temperature, and an increase in the electrical resistivity. The way the crystallographic parameters change is found to correlate with the character of the changes in the magnetic parameters. It is shown that the low-temperature magnetoresistance increases with x, while the magnetoresistance around room temperature is a nonmonotonic function of the tin concentration and has a maximum at x=0.015.

The temperature (4.5–550K) and angular (in the film plane) dependences of the coercivity field for physically nonpercolated nanogranular (CoFeB)x–(SiO2)1−x films with an oriented intraplane granule anisotropy have some unusual properties compared to those owing to blocking of thermally activated reorientation of the magnetic moments of the granules. At temperatures above the blocking temperature Tb≈350–370K, the dependence of the coercivity on the measurement duration is weak compared to that below Tb. Its angular (in the film plane) dependence differs qualitatively from that expected with blocking of superparamagnetic particles. This behavior is explained by the appearance of superferromagnetic ordering of the granule magnetic moments at T>Tb owing to interactions among them. As the temperature is reduced below 100K, the coercive field Hc for magnetization in the easy direction increases significantly more than expected for a blocked superparamagnetic ensemble. Here Hc≠0 for magnetization along the hard ...

In order to study the magnetic interactions influence on the relaxation processes in superparamagnetic media, the magnetostatic measurements were carried out for the granular (CoFeB)x-(SiO2)1-x films with x values near percolation threshold. The observed temperature dependences of the coercivity for the samples magnetized along in-plane easy axis have shown two parts linear on with different inclination angles, which cross each other at the blocking temperature (Tb). The first part of the curve, which corresponds to the temperature range below Tb, depends on the measuring time and is related to the thermal activation nature of magnetization reversal process in the blocked superparamagnetic state. Second part of the curve, which corresponds to the temperature range above Tb, is practically independent on the measuring time and is related to existence of correlated state of superparamagnetic grains moments, i.e. to superferromagnetic state. The method of the magnetization reversal curves simulation based on the numerical solving of kinetic equation has been applied to explain the experimental results.

Temperature and field investigations of the magnetic properties of multilayer films, consisting of a sequence of alternating magnetic layers of iron and the rare-earth terbium separated by layers of nonmagnetic gold, are performed. The samples were obtained by electron-beam deposition. The field dependence of the magnetic moment of a film with H∥n are analyzed to determine the characteristics of the uniaxial anisotropy. It is shown that a nonuniform uniaxial anisotropy results in nonlinearity in the magnetization of the film in H∥n. The process of magnetization of a film in H⊥n, when the transition to a single-domain state occurs only as a result of the motion of domain walls, is described. It is shown on the basis of the data on the field dependences of the magnetic moment at low temperatures that the magnetic moments of the Fe and Tb layers are oriented oppositely with respect to one another. A compensation point is observed in the temperature dependence of the magnetization; this indicates the presence...

We study the formation of antiferromagnetic magnetoelastic domains in easy-plane antiferromagnets of the iron-group dihalides, in which the infinite degeneracy of the spatial orientation of the antiferromagnetic vector in the basal plane is lifted on account of the spontaneous magnetostriction. In these crystals the domains differ from each other not only by the direction of the antiferromagnetic vector L in them but also by the related directions of the principal axes of the spontaneous magneostrictive strain. The system of antiferromagnetic domains turns out to be identical to the system of elastic domains. It is shown that the processes of magnetization and the induced striction in a magnetic field in the multidomain antiferromagnetic state are interconnected. Data on the field dependence of the induced magnetostriction in the multidomain state of the easy-plane antiferromagnets CoCl2 and NiCl2 are presented and analyzed. It is shown that although the magnetostriction in the cycles of imposition and re...

The forced magnetostriction and magnetization are measured in the easy-plane-type two-sublattice NiCl2 antiferromagnet (AFM) in the case where this AFM passes from the multidomain to a single-domain state. It is shown that, in accordance with the magnetoelastic nature of the multidomain state, the field dependences of the forced magnetostriction and magnetization are interrelated and affected by the transition from the multidomain to the single-domain state. The character of these dependences corresponds to the case where the magnetization and striction are proportional to the number of domains with an energetically favored orientation with respect to the external magnetic field.

An analysis is made of the experimental data on the magnetic-field dependence of the magnetostriction and magnetization of the NiCl2 crystal at its transition from the multidomain antiferromagnetic state to a uniform state. It is shown that their field dependence is determined by the mean orientation of the domains, which is characterized by a domain coalignment parameter. That parameter is used to give a phenomenological description of the multidomain state of the antiferromagnet NiCl2.

Experimental data from measurements of the induced magnetostriction of the easy-plane two-sublattice layered antiferromagnet NiCl2 in the homogeneous and multidomain states at T=4.2 K are analyzed. It is shown that the intersublattice magnetoelastic interactions are dominant in NiCl2. The induced magnetostriction of the crystal in the single-domain state is due to canting of the spins of the sublattices toward the magnetic field direction. The induced magnetostriction in the multidomain state is due to a rearrangement of the domain structure and is independent of the small in-plane anisotropy. In both cases the values of the induced magnetostriction are directly proportional to the square of the value of the external magnetic field.

The features of the transformation of the multidomain state of an easy-plane antiferromagnet in an external magnetic field are investigated in the case of a magnetoelastic origin of the multidomain state. Experimental data are presented on the field dependence of the induced magnetostriction of the easy-plane two-sublattice antiferromagnet CoCl2 at different temperatures. In the discussion of the experiments and in the theoretical description we use the approximation of a continuous distribution of domains in the easy plane. It is shown that the orientational distribution of the domains upon the introduction of a magnetic field depends only on the magnitude and direction of the field and is independent of temperature. These features of the distribution of domains can be explained by a matching of their equilibrium magnetostriction with the elasticity of defects wherein the elasticity of the domains compensates the elasticity of the defects.

A structure is proposed for the magnetoelastic antiferromagnetic domains in easy-plane antiferromagnets of the iron-group dihalides. The proposed structure is made up of a triad of regions with 120° rotation of the antiferromagnetic vector on passing between adjacent regions. This domain structure may be stabilized by defects which are isotropic in the easy plane and which preserve the orientational degeneracy of the directions of the antiferromagnetic vector in the easy plane.

An analysis is made of the experimental magnetic-field curves of the anisotropic (in the easy-plane) magnetostriction of the antiferromagnet CoCl2 at different temperatures in the ordered phase. It is shown that the field dependence of the magnetostriction in a single-domain antiferromagnetic state remains proportional to the square of the applied field over the entire temperature range; this behavior corresponds well to the classic (Neel) scheme of collapse of the sublattice spins (spin flip). The temperature dependences of the intrasublattice and intersublattice contributions to the magnetoelasticity are proportional to each other. The temperature dependence of the spin-flip field obtained from the magnetostriction data agrees with that obtained previously from observations of the antiferromagnetic resonance (AFMR). The temperature dependence of the spontaneous anisotropy of the magnetostriction, obtained by extrapolating the magnetostriction in the single-domain state to zero field, is close to M∼T4.

The magnetic ordering model is considered for a single-lattice spin subsystem with S=1 and a large biquadratic exchange whose energy has the same order of magnitude as the bilinear exchange interaction energy. The expression for free energy is obtained in the microscopic approximation, and the phase states are defined and analyzed. The Landau potential is represented as a power expansion in the model microscopic free energy. This expression is used to describe the peculiarities of the temperature dependences of the magnetic susceptibility and magnetoelastic deformation.

The mechanisms and properties of the equilibrium magnetization process for nanogranular films with perpendicular anisotropy placed in a tilted magnetic field are considered. The contributions of the effects of canting and flipping of the granules' magnetic moments to the process of film magnetization are studied. A critical behavior of the film magnetization at the transition, induced by a tilted magnetic field, from a state with non-uniform orientation of the granules' magnetic moments to one with a similar orientation is revealed. The results obtained within the two-level model of the orientation of the particles' magnetic moments are in good agreement with the experimental data for Co-Al(2)O(3) (61 at.% Co) granular film. The perpendicular anisotropy of the granules in this film originates mainly from their elongated shape. It is shown that in the non-uniform state the magnetostatic energy of a granular film with similarly oriented elongated granules can be described by the sum of contributions of two types: quasi-single-granular and quasi-film. The effective constant of the single-particle anisotropy of the granules in this case turns out to be dependent on the factor of volume filling of the film by granules, but not on its magnetization.

It is shown that the spin Hamiltonian of the spin–spin interactions, written with the use of effective spinons defined on the two lowest doublets of magnetic ions with a partially frozen orbital moment, has a non-Heisenberg form. The Heisenberg contribution is represented by spin–spin terms containing three powers of ion spin projections and is anisotropic and comparable to the bilinear spin–spin interaction.

We consider the influence of interparticle interaction on the magnetization reversal in the oriented Stoner-Wohlfarth nanoparticles ensemble. To do so, we solve a kinetic equation for the relaxation of the overall ensemble magnetization to its equilibrium value in some effective mean field. Latter field consists of external magnetic field and interaction mean field proportional to the instantaneous value of above magnetization. We show that the interparticle interaction influences the temperature dependence of a coercive field. This influence manifests itself in the noticeable coercivity at $T>T_{b}$ ($T_{b}$ is so-called blocking temperature). The above interaction can also lead to a formation of the "superferromagnetic" state with correlated directions of particle magnetic moments at $T>T_{b}$. This state possesses coercivity if the overall magnetization has a component directed along the easy axis of each particle. We have shown that the coercive field in the "superferromagnetic" state does not depend on measuring time. This time influences both $T_{b}$ and the temperature dependence of coercive field at $T

Magnetic properties of nanogranular ferromagnetic Co/Al2O3 films with 74.5 at% Co, which is above the percolation limit, are investigated. It is established that the films have perpendicular magnetic anisotropy and a weaker in-plane anisotropy. The magnetization curves show that the film consists of two magnetic components: a dominating contribution from magneto-anisotropic isolated grains with the anisotropy axis perpendicular to the film plane and a weaker contribution from the percolated part of the film. This two-component magnetic composition of the films, with the dominating contribution from the nanograins, is confirmed by transmission electron microscopy as well as by ferromagnetic resonance spectroscopy. It is further established that the coercive field of the film is almost entirely determined by the percolated part of the film. In this, the angular dependence of the coercive force, H-c(theta(H)), is essentially proportional to sin(-1)theta(H), where theta(H) is the angle between the applied field and the film's normal. However, for theta(H) -> 0, H-c(theta(H)) there is a narrow minimum with H-c approaching zero. Such non-linear dependence agrees well with our modelling results for a two-component magnetic system of the film, where the non-percolated nanograins have a distinct perpendicular anisotropy. The reported results should be important for in-depth characterization and understanding the magnetism and anisotropy in inhomogeneous systems as well as for applications, specifically in perpendicular magnetic recording.

Polycrystalline (Co2Fe)(x)Ge(1-x) Heusler alloy films are fabricated by sputtering on amorphous substrates and shown to possess three types of magnetic anisotropy. The nearly stoichiometric composition of x = 50 m.f.% shows a rectangular hysteresis loop and isotropic coercive and ferromagnetic resonance fields when the film is field-magnetized along any in-plane direction, thus predominantly possessing rotatable in-plane magnetic anisotropy. Higher-x compositions show evidence of two- and fourfold in-plane anisotropy superposed on the rotatable one. A qualitative model of the observed anisotropic magnetic properties is proposed. The model explains the rotatable anisotropy by taking into account dry friction for the in-plane rotation of the magnetization direction in a fine-grained polycrystalline film with the magnetic grain size smaller than the correlation length of the inter-grain exchange interaction. The observed two- and fourfold magnetic anisotropy contributions are attributed to partial texturing of the fine-grained films, even though the films are grown on amorphous SiO2 substrates. These results should be valuable for understanding and controlling the magnetic behaviour of highly spin-polarized Heusler alloy films used in various magnetic nanodevices.

Magnetic properties of nano-granular Co0.55–(Al2O3)0.45 films have been studied by vibrating sample magnetometry, ferromagnetic resonance, and magnetic force microscopy (MFM). The films possess a growth-induced perpendicular anisotropy related to the shape anisotropy of the granules. Being unpercolated physically, the films demonstrated a clear magnetic percolation behavior in the temperature dependence of the coercivity and MFM scans. The temperature dependence of the coercivity measured along the easy magnetization axis of the granules indicates the presence of a collective long-range magnetic state sustained by a ferromagnetic interparticle interaction. This interaction is supposed to form a homogeneously magnetized superferromagnetic state, however, the perpendicular anisotropy and the competition between the dipole-dipole and ferromagnetic interactions lead to the emergence of an inhomogeneous labyrinth-like magnetic stripe-domain structure with the averaged domain width being about two orders of mag...

Magnetic and magneto-transport properties of granular ferromagnetic Cox-(Al2O3)1-x films with a large range of cobalt concentrations x are investigated. The films with x below the percolation threshold xp demonstrate the presence of oriented uniaxial anisotropy of the granules with the anisotropy field Ha ∼ 2 kOe and easy anisotropy axis perpendicular to the film plane. It is shown that for the values of x close but below xp, field dependencies of the magnetoresistance in the magnetic fields applied in the film plane, possess an anomalous field dependence with a positive magnetoresistance and a maximum in the range of field magnitudes 0

An anomalous angular dependence of the coercive field of physically nonpercolated granular CoFeB–SiO2 films with oriented anisotropy of the granules, where the presence of the superferromagnetic phase has been established earlier, is observed experimentally at temperatures above the blocking temperature. The angular dependence of the coercive field is similar to that typically observed in the case of magnetization reversal through an inhomogeneous magnetic state of single granules in the ensemble. As the temperature becomes lower than the blocking temperature, the angular dependence changes to that typical of the magnetization reversal through a coherent rotation of granules’ magnetic moments. It is shown theoretically that the anomaly is determined by a singularity of the angular dependence of the critical field causing the absolute loss of the superferromagnetic phase stability in the ensemble of weakly interacting ferromagnetic nanoparticles.

We reveal the low-temperature anomaly in the temperature and angular dependencies of the coercivity in granular (CoFeB)x–(SiO2)1−x films with oriented in-plane anisotropy. Namely, at T

Structural, electric, and magnetic properties of the bulk polycrystalline samples with nominal composition La0.7Ca0.3−xNaxMnO3 (x=0−0.10) have been investigated in the work. As follows from the results of chemical and x-ray structural analyses, the increase in x brings about the rise in the number of structural vacancies in both lanthanum and oxygen sublattices. The influence of the variation of the vacancy number on peculiar features of the transition from ferromagnetic metallic state to paramagnetic state with activated conductivity has been studied in detail. The conclusion is made that the rise in the number of structural vacancies gives rise not only to the enhancement of magnetic inhomogeneity and broadening of the magnetic transition, but also to the increase in the temperature of magnetic transition. It is demonstrated that the changes in magnetic parameters also result in the increase of the temperatures of resistivity and magnetoresistance peaks as well as in the change of the character of the t...

The studies of Fe∕Au∕Tb multilayers (mls) prepared by e-beam evaporation are presented. We found that for mls obtained by seven reiterations of the layer sequence: ⋯∕24AFe∕17.5AAu∕36ATb∕17.5AAu∕⋯ only the Fe layers are ferromagnetic (FM) at T>100K, while Tb layers are paramagnetic. At T

A thermodynamic analysis of the influence of non-Heisenberg interionic spin-spin interactions on the temperature dependence of the NMR frequency of a NiCl2 crystal is carried out.

The influence of the in-plane anisotropy on the magnetization of a nanogranular film with perpendicular anisotropy has been studied. It is shown that if a magnetic field is tilted with respect to the film normal, a critical transition from the inhomogeneous magnetic state of granules with noncollinear directions of their moments to the homogeneous one with parallel orientation of granular magnetic moments takes place. The in-plane anisotropy is found to affect the angular dependence of the critical field. The ensemble of oriented biaxial particles is theoretically described in the double-well potential approximation. Despite the biaxial magnetic anisotropy of particles, their ensemble, if in the inhomogeneous state, is divided into two subensembles, with the magnetic moments of particles being collinear in each of them. In the critical field, a transition from the inhomogeneous state with two subensembles into the homogeneous one takes place. The results of theoretical calculations are compared with experimental data for a nanogranular Co/Al2On film with perpendicular anisotropy containing 74.5 at.% Co, which exceeds the percolation threshold. The magnetic moment of this film is a sum of two contributions: from nanogranules with biaxial anisotropy and a phase forming the percolation cluster. The magnetic properties of nanogranules, whose contribution is separated from the total film magnetization, agree well with the calculation data., Comment: 12 pages, 6 figures; www.ujp.bitp.kiev.ua

We analyzed the field dependences of forced magnetostriction in the multidomain state of the easy-plane antiferromagnet CoCl2 obtained in the following cycles: the introduction-removal of a magnetic field lying in the easy plane, the introduction-removal of a magnetic field lying in the easy plane and directed normally to that introduced earlier, etc. The magnetostriction of the crystal in the multidomain state was shown to contain two components. First, the component reversible in the cycle magnetic field introduction-removal, which makes the major contribution in the crystal under consideration, and, second, a comparatively small irreversible component, that is, the contribution retained after magnetic field removal. In low fields, the reversible magnetostriction component was proportional to the square of the applied magnetic field. Field-induced rearrangement of the multidomain antiferromagnetic state was found to be responsible for singularities of the field dependence of crystal magnetization. In particular, in a near-zero field that lay in the easy plane, the transverse susceptibility decreased twofold compared with its value in fields in which the crystal is already in the monodomain state. At the same time, close to the “monodomainization” field, transverse magnetic susceptibility was maximum. Defects were shown to favor the formation of the reversible multidomain state. Determining factors in this process were elastic and magnetoelastic interactions. The multidomain state of antiferromagnets was described using the domain distribution function over the orientations of domain antiferromagnetic vectors with respect to the magnetic field direction and the magnetic field dependence of this function. The results of our analysis were in close agreement with the experimental data on CoCl2.

The peculiarities of the magnetization of a nanopowder (particle size 12nm) of the manganite La0.7Sr0.3MnO3 are investigated. It is shown that for temperatures below the Curie point the magnetization of the powder is of the ferromagnetic type with a relatively small value of the remanent magnetization. The minimum of the derivative of the spontaneous magnetization with respect to temperature lies 20K below the Curie temperature determined by extrapolation of the temperature dependence of the coercivity. These features of the magnetization are a consequence of a large variance of the anisotropy fields of the particles, the distribution function for which was determined from incremental hysteresis loops. It is also found that the Curie temperature of the powder has a strong variance, with an rms deviation of at least 20K. The mean value of the Curie temperature of the particles is 325K. The spontaneous magnetization of the powder particles does not have (even in the low-temperature region) a saturation region. This temperature dependence of the spontaneous magnetization influences the trend of the temperature dependence of the coercive field, leading to a deviation from the Neel–Brown law. It is also shown that for the given ensemble of particles the blocking temperature corresponds to the temperature of the maximum of the ratio of the magnetization measured after cooling in the zero-field regime to the spontaneous magnetization.