arXiv subjects

S. M. Ryabchenko

Publications and source records attributed to S. M. Ryabchenko.

9 recordsLinked to original sources

arXiv2026

Equilibrium kink-like torsion deformation of a magnetoactive elastomer under a magnetic field

A novel effect involving the formation of a stable kink-like torsion deformation in a magnetoactive elastomer (MAE) beam subjected to a uniform magnetic field is theoretically predicted and experimentally confirmed. The phenomenon was demonstrated using an elastomer beam containing soft magnetic carbonyl iron microparticles within a silicone matrix. The torsion kink acts as a transition boundary between two undeformed homogeneous states of the beam. We show that the elastic moment is compensated by a magnetoelastic moment in the kink region, where the local magnetization of the beam is non-collinear to the applied magnetic field due to shape anisotropy. It is established that within the kink region, the MAE beam exists in a non-uniformly elastically deformed, low-symmetry magnetic state. Outside the kink, the beam's magnetization is collinear with the magnetic field, corresponding to an undeformed, high-symmetry homogeneous magnetic state.

cond-mat.mtrl-sci↗

arXiv2021

Molecular field approximation in theory of ferromagnetic phase transition in diluted magnetic semiconductors

In this pedagogical paper, the comparative analysis of two common approaches describing the ferromagnetic phase transition in diluted magnetic semiconductors (DMS) is expounded in terms of Weiss field approximation. Assuming a finite spin polarization of the magnetic ions, the treatment of carrier-ion exchange interaction in first order evokes the homogeneous Weiss molecular field that polarizes the spins of free carriers. In turn, this spin polarization of the free carriers exerts the effective field that may stabilize DMS spin polarization below a critical temperature Tc. The treatment of such self-consistent spontaneous DMS magnetization can be done in terms of spin-spin interaction independent on inter-ion distance and infinitesimal in thermodynamic limit. On the other hand, the taking additionally into account the second order effects of carrier-ion exchange interaction treat a Weiss field in term of the RKKY indirect spin-spin interaction, which oscillates and not disappears at finite inter-ion distance in case of finite carriers concentration. These both approaches result in same Curie temperature Tc provided non-correlated homogeneous random distribution of the localized spin moments over sample volume. We discuss the origin of such coincidence and show when this is not a case in other more realistic models of the conducting DMSs.

cond-mat.mtrl-sci↗

arXiv2015

In-plane anisotropy effect on critical transition field in nanogranular films with perpendicular anisotropy

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.

cond-mat.mes-hall↗

arXiv2009

The superferromagnetic state in the ensemble of oriented Stoner-Wohlfarth particles: a coercivity due to phase stability

It is observed experimentally that the coercive field has an anomalous angular dependence at temperatures above the blocking temperature in physically nonpercolated granular films CoFeB-SiO$_{2}$ with anisotropic granules oriented in the same direction. It is shown that the anomaly is determined by the singularity of an angular dependence of the critical field causing the absolute loss of phase stability of the superferromagnetic state of an ensemble of interacting superparamagnetic granules.

cond-mat.mes-hall↗

arXiv2008

The influence of intergranular interaction on the magnetization of the ensemble of oriented Stoner-Wohlfarth nanoparticles

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 $T<T_{b}$. We corroborate our theoretical results by measurements on nanogranular films (CoFeB)$_{x}$-(SiO$_{2}$)$_{1-x}$ with concentration of ferromagnetic particles close, but below percolation threshold.

cond-mat.mes-hall↗

arXiv2008

Anomalous low-temperature magnetoelastic properties of nanogranular (CoFeB)$_{x}$-(SiO$_{2}$)$_{1-x}$

We report magnetostatic measurements for granulated films (CoFeB)$_{x}$-(SiO$_{2}$)$_{1-x}$ with fabrication induced intraplanar anisotropy. The measurements have been performed in the film plane in the wide temperature interval 4.5$÷$300 K. They demonstrate that above films have low-temperature anomaly below the percolation threshold for conductivity. The essence of the above peculiarity is that below 100 K the temperature dependence of coercive field for magnetization along easy direction deviates strongly from Neel-Brown law. At temperature lowering, the sharp increase of coercivity is observed, accompanied by the appearance of coercive field for magnetization along hard direction in the film plane. We establish that observed effect is related to the properties of individual ferromagnetic granules. The effect weakens as granules merge into conglomerates at $x$ higher then percolation threshold and disappears completely at $x>1$. We explain the above effect as a consequence of the difference in thermal expansion coefficients of granule and cover material. At temperature lowering this difference weakens the envelopment of an individual granule by the cover matrix material, thus permitting to realize the spontaneous magnetostriction of a granule. The latter induces an additional anisotropy with new easy axis of a granule magnetization along the external magnetic field direction. Our explanation is tested and corroborated by the ferromagnetic resonance measurements in the films at $T$ = 300 K and $T$ = 77 K.

cond-mat.mes-hall↗

arXiv2008

Dependence of the Magnetization of an Ensemble of Single-Domain Particles on the Measurement Time within Various Experimental and Computational Methods

The effect of a measurement time duration on the parameters of magnetization curves for an ensemble of identical noninteracting single-domain particles with equally oriented axes under the uniaxial anisotropy has been specifed for different experiment modes, in particular for the cases of relaxation measurements and the continuous sweep of a static magnetic field. The relation between a blocking temperature and experiment characteristics has been found for these modes. A recursion method to calculate the magnetization reversal curves for such an ensemble of particles is proposed. By comparing the results of calculations of the magnetic properties by the recursion and Monte-Carlo methods, an algorithm to establish the relation between the equivalent measurement time and such parameters of the Monte-Carlo method as the number of steps and the value of aperture is suggested.

cond-mat.mes-hall↗

arXiv2003

Anisotropy of heavy hole spin splitting and interference effects of optical polarization in semiconductor quantum wells subjected to an in-plane magnetic field

Strong effects of optical polarization anisotropy observed previously in the quantum wells subjected to the in-plane magnetic field arrive at complete description within microscopic approach. Theory we develop involves two sources of optical polarization. First source is due to correlations between electron and heavy hole (HH) phases of $ψ$-functions arising due to electron Zeeman spin splitting and joint manifestation of low-symmetry and Zeeman interactions of HH in an in-plane magnetic field. In this case, four possible phase-controlled electron-HH transitions constitute the polarization effect, which can reach its maximal amount ($\pm $1) at low temperatures when only one transition survives. Other polarization source stems from the admixture of excited light-holes (LH) states to HH by low-symmetry interactions. The contribution of this mechanism to total polarization is relatively small but it can be independent of temperature and magnetic field. Analysis of different mechanisms of HH splitting exhibits their strong polarization anisotropy. Joint action of these mechanisms can result in new peculiarities, which should be taken into account for explanation of different experimental situations.

cond-mat.mtrl-sci↗

arXiv1999

Mechanisms of Carrier-Induced Ferromagnetism in Diluted Magnetic Semiconductors

Two different approaches to the problem of carrier-induced ferromagnetism in the system of the disordered magnetic ions, one bases on self-consistent procedure for the exchange mean fields, other one bases on the RKKY interaction, used in present literature as the alternative approximations is analyzed. Our calculations in the framework of exactly solvable model show that two different contributions to the magnetic characteristics of the system represent these approaches. One stems from the diagonal part of carrier-ion exchange interaction that corresponds to mean field approximation. Other one stems from the off-diagonal part that describes the interaction between ion spins via free carriers. These two contributions can be responsible for the different magnetic properties, so aforementioned approaches are complementary, not alternative. A general approach is proposed and compared with different approximations to the problem under consideration.

cond-mat.stat-mech↗