It is shown that in external magnetic fields, a uniaxial magnetic anisotropy comes into being in a magnetoactive elastomer (MAE). The magnitude of the induced uniaxial anisotropy grows with the increasing external magnetic field. The filler particles are immobilized in the matrix if the MAE sample is cooled below 220 K, where the anisotropy can be read out. The cooling of the sample is considered as an alternative methodological approach to the experimental investigation of the magnetized state of MAEs. The appearance of magnetic anisotropy in MAE is associated with restructuring of the filler during magnetization, which leads to an additional effective field felt by the magnetization. It is found that the magnitude of the effective magnetic anisotropy constant of the MAE is approximately two times larger than its effective shear modulus in the absence of magnetic field. It is proposed that the experimentally observed large (about 40) ratio of the magnetic anisotropy constant of the filler to the shear modulus of the matrix deserves attention for the explanation of magnetic and magnetoelastic properties of MAEs. It may lead to additional rigidity of the elastic subsystem increasing the shear modulus of the composite material through the magnetomechanical coupling.

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.

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 rearrangement 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.

The 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 and 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 where the dependence of the differential magnetic susceptibility on the magnetic field exhibits local maxima. 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. Published by AIP Publishing.

Films of Co2Fe-Ge Heusler alloy with variable Ge concentration deposited on monocrystalline MgO (100) substrates by magnetron co-sputtering are investigated using microstructural, morphological, magnetometric, and magnetic resonance methods. The films were found to grow epitaxially, with island-like or continuous-layer morphology depending the Ge-content. The ferromagnetic resonance data versus out-of-plane and in-plane angle indicate the presence of easy plane and 4-fold in-plane anisotropy. The magnetometry data indicate additional weak 2-fold in-plane anisotropy and pronounced at low fields rotatable anisotropy. The observed magnetic anisotropy properties discussed in correlation with the microstructure and morphology of the films.

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.

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)Ge1-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 magnitude larger than the averaged granule size. The temperature evolution of the spontaneous magnetization inside the stripe domains has been measured by the MFM up to the temperature of superferromagnetic ordering (420 K). The formerly developed mean-field-based model of the coercivity of an interacted ensemble has been tested and found to yield adequate quantitative predictions.

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 < H < Ha. The positive magnetoresistance effect is isotropic in the film plane and reduces as the field direction is rotated out of the film plane up to the complete quenching for the field perpendicular to the film plane. The effect is not related to an anisotropic magnetoresistance manifestation and is temperature-dependent. We propose a model, which explains the phenomenon and bases on an accounting of thermal fluctuations of uniaxial particle magnetic moments in the fields perpendicular to the particle easy anisotropy axis in the presence of the ferromagnetic interaction between the particles.

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.

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.

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<100 K the in-plane angular dependence of coercive field acquires two maxima corresponding to easy and hard (in a film plane) directions. This signifies the emergence of coercivity for hard direction in a film plane. The experimental results are explained in terms of a random field model, which describes the onset of spin glass-like correlations in the ensemble of oriented weakly interacting Stoner–Wohlfarth particles.

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.

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 direction in the plane. The angular (in-plane) dependence of Hc acquires two maxima, along the easy and hard (in-plane) directions. This phenomenon is explained by the appearance of superspin-glass correlations in the mutual orientations of the granule magnetic moments of the ensemble.

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.

Magnetostatic properties of a La0.7Sr0.3MnO3 single crystal have been studied in the vicinity of its critical temperature Tc. A nonlinear temperature dependence of the inverse magnetic susceptibility which is characteristic of the Griffiths phase, has been found in the minimal measuring magnetic field at temperatures slightly above the temperature of ferromagnetic (FM) ordering. A conclusion was made that such a nonlinearity arises owing to the formation of magnetic polarons. The applicability of the Belov-Arrott plots to the systems with the Griffiths phase has been analyzed, and their characteristic features have been revealed. These features have been demonstrated to present in the Belov-Arrott plots for the experimental data on La0.7Sr0.3MnO3. It has been shown that the critical properties of the La0.7Sr0.3MnO3 crystal are described by somewhat different sets of critical indices in the FM, T< T c, and paramagnetic, T> T c, phases.

Magnetostatic properties of a La0.7Sr0.3MnO3 single crystal have been studied in the vicinity of its critical temperature T-c. A nonlinear temperature dependence of the inverse magnetic susceptibility which is characteristic of the Griffiths phase, has been found in the minimal measuring magnetic field at temperatures slightly above the temperature of ferromagnetic (FM) ordering. A conclusion was made that such a nonlinearity arises owing to the formation of magnetic polarons. The applicability of the Belov- Arrott plots to the systems with the Griffiths phase has been analyzed, and their characteristic features have been revealed. These features have been demonstrated to present in the Belov- Arrott plots for the experimental data on La0.7Sr0.3MnO3. It has been shown that the critical properties of the La0.7Sr0.3MnO3 crystal are described by somewhat different sets of critical indices in the FM, T < Tc, and paramagnetic, T > T c, phases.