The influence of magnetostriction on static fluctuations of the magnetic moment in ferromagnetic polycrystals has been theoretically studied. Conditions have been found under which magnetoelastic interaction leads to stabilization of long-range magnetic order in these systems.

The results of experimental studies and micromagnetic modeling of magnetic states in a one-dimensional array are presented. The array has the form of a chain of ferromagnetic disks coupled with a ferromagnetic nanowire made of the same material. The disks are located on opposite sides of the nanowire, which makes it possible to obtain distributions when the chiralities of the magnetic vortex shells in neighboring disks alternate, which can find application in vortex spin nanooscillators. By applying a magnetic field of an excited objective lens in situ and using Lorentz transmission electron microscopy, it is shown that in this system the chiralities of the shells of magnetic vortices can be controlled by magnetization in the sample plane along various azimuthal directions. When magnetized along the nanowire in disks located on opposite sides of it, vortex states with opposite chiralities are realized. An antivortex is formed in the nanowire itself at the boundary with the disk, since the local direction of magnetization in the wire and in the disk are anticollinear. When magnetized perpendicular to the nanowire, states with the same chirality are realized in all disks. In this case, two perpendicular domain walls are formed between the disks in the nanowire and the vortex in the disk is shifted to one of the edges along the nanowire.

This paper reports on the formation and properties of a kind of nanostructured magnetic material, which is ferromagnetic (FM) inclusions in a paramagnetic (PM) but well magnetized matrix. It has been argued that such FM/PM nanostructures are formed due to local chemical disordering along ion tracks in thin-film Fe0.6Al0.4 alloys irradiated with xenon ions at an energy of 160 MeV. The nonirradiated matrix, obtained by thermal annealing of the as-prepared alloy, has a PM-like behavior (with no hysteresis and remanence) at room temperature (RT). Interestingly, the irradiated samples exhibit a sharp peak in the temperature dependence of the magnetic entropy change Delta S at T similar to 320 K. The emergence of this maximum is attributed to the interfacial exchange interaction in the formed FM/PM-like nanostructures, which affects the matrix magnetization near the Curie temperature T-C of the PM-like matrix. The peak value of Delta S obtained has been compared to that theoretically predicted for plane-layered FM/PM structures upon the basis of the Landau theory for the second-order phase transitions. A discrepancy observed between the theory and experiment can be explained by occurrence of quite big superparamagnetic (SPM) clusters in a partially ordered (similar to 0.6) Fe0.6Al0.4 alloy. These entities essentially contribute the matrix magnetization at RT. The study reported here provides a better understanding of the structural, magnetic, and magnetocaloric properties of the heterogeneous FexAl1-x system, which can be viewed as particular type of nanocomposite.

We have constructed a theory of the Hall effect appearing during the passage of current in a magnetic tunnel junction due to the spin–orbit interaction in an insulator barrier in the approximation of a delta-shaped barrier potential. Both the normal Hall current flowing in metal banks as a result of asymmetric scattering in the tunneling barrier and the anomalous current existing only in the tunneling barrier due to the presence of the spin–orbit interaction in it are taken into account. We have considered the Rashba interaction that can be of intrinsic origin (noncentrosymmetric form of the barrier) or can be induced by an extraneous electric field emerging as a result of application of a potential difference to the barrier. Such a field can reach a value on the order of 10 9 W/m, which is close to intrinsic atomic fields. The Hall current has both linear and quadratic components in the voltage applied to the tunnel junction. The existence of the nonlinear Hall voltage corresponding to it has been illustrated experimentally in a CoFeB/MgO/Pt tunnel junction, in which the transverse (Hall) voltage has been measured in the Pt layer.

The general form of chiral terms associated with deformation of a ferromagnet is determined. The possibility of a transition from the helical state to the vortex state is demonstrated for a ferromagnetic rod subjected to elastic torsional strain. In the case of a single screw dislocation, the formation of the skyrmion state due to elastic strains and the helical distribution of magnetization induced by strains in the dislocation core is indicated. The conditions in which a transition from one chiral magnetization distribution to another distribution in an ensemble of identical dislocations are determined.

It is shown that the energy of a ferromagnetic film deposited onto a paramagnetic or superconducting substrate acquires a contribution in the form of the Dzyaloshinskii–Moriya interaction. This contribution appears as a result of the magnetostatic interaction of the magnetization of the ferromagnetic film with the magnetization induced by it in a paramagnet or by the supercurrent in the superconductor and leads to the removal of the chiral degeneracy, nonreciprocity of spin waves, and the formation of chiral states such as magnetic skyrmions. Our estimates indicate the possibility of experimental observation of predicted effects.

The gyrotropic motion of vortex magnetization distributions in two coupled ferromagnetic disks has been experimentally studied and numerically simulated. The dependence of the resonant frequency of the collective gyrotropic oscillation mode of vortices on the distance between the centers of disks has been studied by magnetic resonance force spectroscopy. The energy of the interaction of magnetic vortices as a function of the distance between disks has been estimated from this dependence using solutions of the Thiele equation.

The low-frequency (gyrotropic) self-oscillations of the magnetic vortices in interacting ferromagnetic disks, which are caused by a spin-polarized current, are studied by numerical simulation. Various magnetization oscillation modes depending on the configuration of the magnetic state of the system are considered. The influence of the pumping current nonuniformity on the phase difference of the vortex gyration in neighboring disks is investigated. The overlap of the disks is shown to increase the interaction between the vortices and, hence, to decrease the dephasing of the vortex core oscillations. The prospects of using overlapping disks to ensure phase synchronization of arrays of spin-transfer vortex oscillators are discussed.

The dynamics of the magnetization induced by an electric current flowing in a multilayer nanoparticle is studied theoretically. A region of the parameters where the coherent rotation of a magnetic helix, which is formed in this system due to the magnetostatic interaction of ferromagnetic layers, has been determined analytically. Estimates indicate that the predicted nonlinear oscillation mode of the magnetization can be observed experimentally.

It is shown that a term in the form of Dzyaloshinskii-Moriya interaction (DMI) contributes to the free energy of a ferromagnetic (FM) film on a paramagnetic (PM) (an FM above the critical temperature, Tc) or superconducting (SC) substrate occurring in the London limit. This contribution results from magnetostatic interaction between the film and substrate under which the substrate affects FM magnetization back via its magnetic field produced by magnetization inhomogeneity in the film. Strikingly, in the FM/PM system this effective DMI stabilizes chiral magnetic textures, e.g., magnetic skyrmions (MSk's) of the Neel-type, which is in contrast to that in the FM/SC one. A strong temperature sensitivity of the effective DMI allows for tuning the coupling between the FM film and PM or SC substrate and thus controlling the MSk radius in FM/PM.

The Hall effect that occurs when current flows through a CoFeB/MgO/Pt tunnel junction is investigated. It is shown that the transverse voltage in Pt electrode is nonlinear on a DC voltage applied to the tunnel junction. It has both linear (odd) and quadratic (even) parts. The linear part contains well-known contributions of the anomalous Hall effect in the ferromagnetic electrode, inverse spin-hall effect in platinum and others. The quadratic part is a phenomenon caused by the spin-orbit scattering of electrons in an external electric field induced by a voltage applied to the barrier. This field reaches values of $10^9$ V/m which is close to internal atomic fields. The magnitude of both effects decreases as thickness of Pt electrode is increased due to shunting effects.

The results of micromagnetic modeling of the gyrotropic mode of magnetization auto-oscillations in overlapping ferromagnetic disks under the action of a spin-polarized current are presented. It is shown that the exchange interaction between disks significantly increases the binding energy of magnetic vortices and, as a consequence, reduces the dephasing of the vortex core gyration in neighboring disks. Keywords: vortex nanooscillator, auto-oscillations, synchronization, exchange coupling.

Spin-wave (SW) spectra have theoretically been studied in a thin film of a ferromagnet (FM) on a substrate from a paramagnet (PM) (an FM above the critical temperature) or from superconductor (SC). A spin-wave propagating in the FM induces the dynamic magnetization and superconducting current in the underlying PM and SC, respectively, which affect the SW propagation by their magnetic fields. As a result of this interaction, the SW spectrum becomes nonreciprocal to depend on the sign of the SW wave-vector q. We show that the nonreciprocal contribution to the SW spectra in FM/PM and FM/SC systems is given by the frequency shift of Delta omega(q) (math) omega(q)-omega(-q) = a(T)(tau.q) with tau = (n x M) being the toroidal magnetic moment, M the FM magnetization, n the unit vector normal to the FM/PM(SC) interface, and a(T) the temperature-dependent constant of a dipole nature, whose sign depends on the substrate type. As the Delta omega(T) dependence is strong at temperatures T close to the critical temperature T-c for the FM-PM or normal metal-SC transition, one gets a possibility to control the frequency SW nonreciprocity with temperature variation near T-c. The dipolar mechanism we propose for SW frequency nonreciprocity is promising for introducing this property of SW propagation into functional devices.

The magnetocaloric properties of a thin spacer of gadolinium (Gd) between layers of "strong" ferromagnets (relatively high Curie temperatures) are studied experimentally. It is found that, at room temperatures, the magnetocaloric efficiency Delta S/Delta H (Delta S is the isothermal magnetic entropy change and Delta H is the range of applied magnetic fields) of Gd spacer of thickness of 3 nm is up to two orders in magnitude higher than this value in an individual thicker (30 nm) Gd layer. This opens up opportunities for using the magnetocaloric effect in micro(nano)electronics and biomedicine using relatively weak magnetic fields H<1 kOe. The observed increase in the magnetocaloric efficiency is explained by the influence of direct exchange coupling between Gd spacer and its surroundings, which changes the distribution of magnetization in the spacer and, ultimately, its magnetocaloric potential. Keywords: magnetocaloric effect, magnetic heterostructures, exchange coupling at interfaces, Curie temperature.

The results of micromagnetic modeling of the gyrotropic mode of magnetization auto-oscillations in overlapping ferromagnetic disks under the action of a spin-polarized current are presented. It is shown that the exchange interaction between disks significantly increases the binding energy of magnetic vortices and, as a consequence, reduces the dephasing of the vortex core gyration in neighboring disks.

We study the influence of magnetic resonance force microscope (MRFM) probe on the low frequency magnetization oscillations in a single permalloy disk connected with gyrotropic motion of magnetic vortex core. It is shown that the resonant frequency of gyrotropic mode can be tuned over a wide range by changing the distance between the probe and the sample. The possibility of switching the vortex core polarity under the action of the field of MRFM probe with large magnetic moment is demonstrated. The experimental data are analyzed using micromagnetic simulations and simple analytical models.

The Hall effect that occurs when current flows through a CoFeB/MgO/NM (NM = Pt,Ta) tunnel junction is investigated. It is shown that the transverse voltage in NM electrodes is nonlinear on a DC voltage applied to the tunnel junction. It has both linear (odd) and quadratic (even) parts with respect to electric field. The linear part contains well-known contributions of the anomalous Hall effect in the ferromagnetic electrode, inverse spin-hall effect in NM, and others. The quadratic part is a phenomenon caused by the spin-orbit scattering of electrons in an external electric field induced by a voltage applied to the barrier. This field reaches values of 10(9) V/m, which is close to internal atomic fields. The magnitude of both effects decreases as the thickness of the NM electrode is increased due to shunting effects.

The conditions of formation of chiral magnetization distributions in the systems ferromagnet/superconductor and ferromagnet/paramagnet are theoretically determined. The formation of chiral states is caused by the magnetostatic interaction in inhomogeneous magnetic systems. The estimates performed demonstrate that the predicted effects can be experimentally observed.

Предложен новый подход к проблеме магнитного охлаждения, основанный на эффекте близости ферромагнетиков с различными температурами Кюри. Проводится аналогия между транспортным и магнитокалорическим эффектами обменной природы в магнитных многослойных структурах. Теоретически показано, что эффективность магнитного охлаждения в этих системах может существенно превосходить значения, характерные для однородных магнетиков. Обсуждаются результаты экспериментального изучения магнитокалорического эффекта в наноструктурах и перспективы их использования для магнитного охлаждения.

The magnetocaloric effect in nanosystems based on exchange-coupled ferromagnets with different Curie temperatures is calculated within the mean-field theory. Good agreement between the results of the mean-field theory and the Landau theory, valid near the critical phase transition temperature, is demonstrated for a flat-layered Fe/Gd/Fe structure. We show that a high magnetic cooling efficiency in this system is attainable in principle and prove the validity of the Maxwell relation, enabling an experimental verification of the predictions made. The theory developed for flat-layered structures is generalized to a granular medium.