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Alterations in the ground state of thin (001) films of a BiFeO3 (BFO)-type multiferroic in a magnetic field are studied theoretically, with changes in the energy of induced anisotropy taken into account. The anisotropy vs. field phase diagrams identifying the stability regions of homogeneous antiferromagnetic states and the regions of emergence of spatially modulated antiferromagnetic states are constructed for three mutually orthogonal orientations of applied magnetic field. We show that, as the magnetic field decreases, the transformation of a homogeneous phase into a spatially modulated state occurs at the instability point of the homogeneous state via gradual emergence of the conical phase that transforms into a planar cycloid with the decreasing magnetic field. A multiferroic film grown on a (001) substrate develops considerable anisotropy of the energy of spatially modulated state, depending on the modulation orientation. Meanwhile, cycloids with different orientations undergo the transitions from incommensurate phase into the homogeneous state differently: either the conical cycloid is formed followed by its collapse into the homogeneous state or an unlimitedly growing domain of the homogeneous phase is formed within the flat cycloid. Examples of field-induced changes in magnetization, with changes in spin states taken into account, are provided. These results are of value in practical applications of multiferroic strain engineering.

The use of the spin-torque diode effect, which is generated upon the current-induced transfer of spin angular momentum in magnetic tunnel junctions, opens the prospect of considerably improving microwave sensitivity in the gigahertz frequency band over that of semiconductor Schottky diodes. In this work, the spin-diode effect of microwave signal rectification in a magnetic tunnel junction upon the resonant excitation of spin waves in the free magnetic layer as a result of the current-induced spin-transfer torque effect is analyzed theoretically. Frequency characteristics of the resonant response of a spin-torque diode to a microwave signal are calculated in the linear macrospin approximation as functions of the direction and value of the applied magnetic field and bias current. It is shown that with zero bias current, the maximum rectified voltage across a junction is obtained in the geometry of mutually perpendicular magnetizations of magnetic layers; as the resonant frequency of oscillations in a magnetic field increases, this voltage drops at the retained equilibrium orientation of spins in the layers. After the bias current is switched on, the resonance amplitude of forced oscillations of the free layer spins upon microwave excitation grows sharply at the critical point of the spin-torque diode’s loss of equilibrium state stability. The linewidth at this point is limited only by nonlinear effects. Improving spin-torque diode sensitivity is important for its use in microwave holographic vision systems.

Spintronics is a new trend in the development of nanoelectronics and, therefore, requires the development of new software tools for modeling aimed at the development and further miniaturization of spintronic devices. In this paper, we present a new software package for the technology computer aided design modeling of spintronic devices based on magnetic tunnel junctions. The theoretical models and scenarios of the software package are described. Examples of application of the software package to solving the main problems arising in the design of magnetoresistive memory elements and consistent miniaturization of these devices are considered.

Thermally assisted magnetic random access memory (TAS-MRAM) nanodevices are promising candidates for future memory applications, because they provide non-volatile data storage at fast operation and low energy consumption. In this work, we analyze operating regimes of these devices using macrospin and micromagnetic modeling. It is shown that, for the successful performance of write and read operations in a TAS-MRAM device, the intrinsic magnetic anisotropy of ferromagnetic layers must not exceed some critical value. The relationship between the device parameters and the value of critical magnetic anisotropy is explained using analytical model and proved by the results of numerical simulation. This analysis is important for the future downscaling of TAS-MRAM nanodevices.

The specific features of spin-transfer torque in vacuum tunnel structures with magnetic electrodes are investigated using the quasi-classical Sommerfeld model of electron conductivity, which takes into account the exchange splitting of the spin energy subbands of free electrons. Using the calculated voltage dependences of the transferred torques for a tunnel structure with cobalt electrodes and noncollinear magnetic moments in the electrodes, diagrams of stable spin states on the current–field parameter plane in the in-plane geometry of the initial magnetization are obtained.

The output characteristics of a membrane-based heat-resistant calorimetric gas flow sensor are analyzed. The dependence of the output characteristics on the design parameters of the flow sensor is investigated. The change in heat distribution along the membrane depending on the flow rate is demonstrated and the effect of the distance between thermal resistors, as well as the thickness and material of the membrane, on the characteristics of the flow sensor is investigated. The simulation results are compared with the experimental data. The dependences found can be used for optimizing the design of flow sensors in order to increase their sensitivity and expand the dynamic range of a thermal mass flow meter.

The influence of an electric field on the structural changes and related transformation of the magnetic subsystem in a BiFeO3 multiferroic material has been investigated in the framework of the phenomenological model based on the Ginzburg-Landau theory. The hysteresis dependences of the electric polarization, the counter-rotation of oxygen octahedra, the magnetization, and the antiferromagnetic vector have been obtained as a function of the electric field applied along the crystallographic directions [110] and [001] of the pseudocubic structure. The model parameters are consistent with the results of the ab initio calculations of the structural and magnetic transitions in this material. It has been shown that, in the region of the existence of a spatially modulated spin state, an abrupt change in the polarization vector leads to a stepwise transformation of the antiferromagnetic structure. Within the proposed approach, the electric-field-controlled change in the spin structure of the multiferroic material is adequately described in the range of weak and strong magnetic fields.

Spin oscillations and their phase synchronization are discovered in a spin 1D waveguide (nanowire) after the local injection of a spin-polarized current in the vertical (perpendicular) geometry of magnetization. The mode composition of nonlinear spin self-oscillations is analyzed for a single nano-oscillator using the effect of spin transfer torque with regard to spin wave runaway over the 1D waveguide and synchronized spin self-oscillations at current pumping by two nano-oscillators. It is shown specifically that, along with simple (“nontopological”) oscillation modes, in which singular points in the oscillation amplitude spatial distribution are absent, the 1D waveguide may support modes with pole-type singular points inside the current pumping domain, which are characteristic of the geometry of a precessing 2π-domain boundary. A diagram for frequency- and current-detuning-synchronized spin self-oscillations that are excited by two spin nano-oscillators in the 1D spin waveguide is constructed.

The phenomena of spin transport and spin-transfer torque between layers of a magnetic hetero-structure with conducting and tunneling interlayers, which can lead to switching of micromagnetic states in the structure’s layers and microwave generation, are considered. The features of spin transport in the field-emission mode for magnetic tunnel structures and problems of the current generation of spin waves in the magnetic film during local spin pumping are discussed. The possibility of the phase locking of the system of nanogenerators on a common magnetic platform and prospects for using nanogenerators based on the effect of spin transport in microwave communication engineering are analyzed.

Possible types of spatially modulated periodic antiferromagnetic structures in a uniaxial rhombohedral multiferroic with BiFeO3 crystal symmetry are studied depending on the ratio of the parameters of uniaxial and basal anisotropy and on magnetoelectric coupling.

Possible types of spatially modulated periodic antiferromagnetic structures in a uniaxial rhombohedral multiferroic with BiFeO3 crystal symmetry have been studied depending on the ratio of the uniaxial anisotropy and magnetoelectric interaction parameters. It has been shown that, along with symmetric cycloid antiferromagnetic structures with zero transverse component of the antiferromagnetism vector, there are changes in the antiferromagnetism vector direction with both right and left nonzero components of the antiferromagnetic moment, which are branched from the high-symmetry spatially modulated distribution. These solutions degenerate into a homogeneous state at a critical value of the normalized easy-plane anisotropy parameter. The existence of the found spatially inhomogeneous antiferromagnetic states with an incommensurate period can lead to additional features in magnetoelectric properties in multiferroics of the type under consideration near magnetic phase transitions in electric and magnetic fields.

The effect of the geometric shape of elements of a spin-dependent tunneling gate on their magnetic characteristic is studied theoretically and experimentally. The hysteresis dependences of the magnetization and magnetoresistance of rectangular, hexagonal, and ringed elements on the magnetic field are calculated for different characteristically geometric parameters of the elements. In the studies of the magneto-optical Kerr effect, magnetic hysteresis loops are observed. The calculated and experimental magnetization curves are compared, and qualitative agreement between the curves is gained.

The static and dynamic equilibrium states of spins in a thin layer of a conducting nanosized column containing two magnetic layers are analyzed theoretically. The magnetization of one of the layers is assumed to be fixed. The analysis is performed in terms of a macrospin model with allowance for the Slonczewski-Berger torque transfer. Bifurcation diagrams are constructed describing the change of spin states in the current-field plane. The relation of the specific features of varying magnetization and the spin precession frequency to bifurcations in the dynamic system under study is discussed. It is shown that the soft creation of cycles with a zero amplitude is accompanied by precession at a finite frequency and that the precession frequency becomes zero when a cycle with a finite amplitude disappears or arises in a jump. Comparative analysis is performed for two orientations of a magnetic field (parallel and perpendicular to the easy magnetization axis in the layer plane) in the presence of a current with a given spin orientation.

Reduced equations are derived for the slow evolution of the phase and amplitude of spin self-oscillations arising in a free magnetic layer of a nanosized microwave oscillator under the action of an over-threshold spin-polarized current. These equations are used to calculate the spectral intensity of the fundamental-harmonic signal of a spin self-oscillator allowing for thermal noise in the spin subsystem. It is shown that the line width as a function of the current is determined by a bifurcation-type variation in the spin state near the self-oscillation threshold. The self-oscillation nonisochronism makes the spectral line profile asymmetric. The line-profile asymmetry can change as the cycle-appearance point is approached.

Transmission of Yersinia pestis to the long-tailed suslik (Citellus undulatus) by fleas (Citellophilus tesquorum) in the Tuva natural plague focus in different seasons (spring, summer, and autumn) was studied experimentally. Between feeding periods, insects were kept in an artificial nest under temperature and humidity closely corresponding to seasonal ones. The character of the agent transmission was estimated according to the fraction of fleas with the agent in the aggregated state (bacterial lumps, partial blocks of proventriculus), the fraction of blocked individuals, and the fraction of infected susliks and of those with the generalized form of infection. Seasonal dynamics of epizootic process of the Y. pestis transmission corresponded to the results obtained in the epizootic examination of the Tuva natural plague focus and reflected the dynamics of the epizootic process (increase-peak-decline). The activity of the formation of a proventriculus block in C. t. altaicus, the infection ability of the fleas, and the sensitivity of long-tailed Siberian susliks to Y. pestis were the highest in mid-summer (July-first ten days of August), during the period of epizooty activation in the focus. The maximal number of C. t. altaicus with the plague agent at the aggregated state was observed in the cold period, before wintering of insects and after their hibernation.

A simplified model of the Jahn-Teller band effect in a conducting ferromagnet with two degenerate subbands with the peak density of states of itinerant electrons is considered. It is found that the martensite transition temperature in a narrow-band conductor as a function of the position of the Fermi level near the peak of the energy density of states varies nonmonotonically in the narrow spin electron subband. The magnetic field dependence of the martensite-austenite structural phase transition temperature in the ferromagnet is analyzed. The developed theory and calculated data for the electron density of states in Ni{sub 2}MnGa are used as the basis for estimating the variation of the martensite transition temperature with the magnetic field ({partial_derivative}T{sub m}/{partial_derivative}H), which demonstrates a satisfactory agreement with experimental data for the Heusler alloy Ni{sub 2+x}Mn{sub 1-x}Fe{sub y}Ga{sub 1-y}.

The subject of discussion is calibration of the tip of a magnetic force microscope using the field of a ring-shaped current loop. To calculate the calibration parameters, the magnetic contribution from the extended tip of the probe in the field of the current loop to the rigidity of the cantilever is approximated by the contribution from a point magnetic dipole and magnetic “charge” in terms of the theoretical model adopted. Three simplified models of the conic tip (with a sharpened, blunted, and rounded top) are considered. The calculated dependences of the effective calibration parameters on the radius of the current loop are compared with experimental data. It is found that the model of a uniformly magnetized tip in the form of a blunted cone provides the best fit to the experiment. The calculation results may be helpful in simulating images obtained with a magnetic force microscope and numerically testing magnetic objects.

The magnetostriction of KDy(WO4)2 single crystals is measured in an external magnetic field at temperatures below the temperature of a structural phase transition of the Jahn-Teller type. A steplike irreversible variation in the elastic strain is observed to occur with an increase in the magnetic field applied along the a or b axis of the monoclinic cell of the crystal. The residual change in the strain is retained after changing the sign of the magnetic field. The return to the initial state characterized by field-induced jumps in the strain is possible only after thermal cycling well above the structural phase transition temperature. The theory of this phenomenon is developed using a phenomenologically derived thermodynamic potential of the elastic sub-system that takes into account the crystal symmetry and the field renormalization of the elastic moduli. The jumplike transitions are interpreted as being due to the magnetic softening of the elastic moduli in the vicinity of the structural phase transition temperature.

The paper presents a theoretical analysis of macroscopic quantum tunneling phenomena in small particles of a cubic ferromagnet of the HoxY3−x Fe5O12 type with strongly anisotropic (Ising) impurity ions present in a low concentration x ≪ 1 in the region of strong magnetic fields, at which many orientation phase transitions related to the competition of external and exchange field actions on the spin subsystem are observed. The theory of path integrals for the magnetic subsystem was used to calculate the instanton contributions to interphase tunneling amplitudes in the vicinity of first-order transitions for three principal orientations of an external magnetic field in a cubic crystal. It was shown that low-energy barriers separating angular phases could result in anomalously large mesoscopic volumes at which macroscopic spin tunneling effects could appear in the energy spectra of particles. The special features of spectral splitting caused by the mixing of azimuthally degenerate angular phases and phases with different polar angles of magnetization orientation were revealed.

Micromagnetic properties of submicron ferromagnetic elements ellipsoidal in shape are studied theoretically and experimentally. By numerically solving the equations of magnetodynamics, it is found that different remanent magnetization states can be obtained, depending on the manner of magnetization reversal in such elements: one-vortex states, two-vortex states, and vortex-free states with skew-symmetric spin pinning. The magnetization configurations predicted in the calculations have been observed experimentally using magnetic force microscopy in regular lattices of microstructures formed in thin-film samples of Fe-Cr alloys under irradiation by interfering laser beams.

The effect of thermal fluctuations on the structure of a domain wall (DW) in a fine magnetic wire is analyzed. It is shown that the polarization of a DW in a magnetic nanowire is changed spontaneously as a result of thermal and quantum fluctuations. There is a critical diameter of the wire below which a transformation analogous to the superparamagnetic transition occurs, with the result that the Neel wall is transformed into a Ginzburg-Bulaevski $$\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\smile}$}}{l} $$ wall.

A numerical solution to the Landau-Lifshitz and magnetostatics equations is used to study the residual magnetic configurations as functions of magnetic and geometrical parameters in the region of a magnetic nanocontact between oppositely magnetized ferromagnetic electrodes in the form of bulk bars. The conditions for the emergence of antiparallel orientations of adjacent spins and the domain wall in the conducting channel are determined.

Laser-induced local modifications of magnetic order in thin Fe‐Cr layers were investigated. Local modification in the layers were induced by interfering laser beams. The results of the study give evidence for the formation of submicron-sized anisotropically shaped ferromagnetic regions with a well-defined direction of the easy magnetic axis in the interference maxima at the modification threshold. It was also found that the magnetic anisotropy of a medium is drastically reduced with changing the shapes of these local regions and distances between them. This may be due to the strengthening of the interaction between the regions through the paramagnetic matrix. © 2001 MAIK “Nauka/Interperiodica”. PACS numbers: 75.70.Kw; 75.60.Jk; 75.50.Bb; 75.50.Cc

The paper reports the first observation of a magnetic relaxation of metastable states in an antiferromagnet (dysprosium orthoaluminate DyAlO3) at low temperatures in zero magnetic field made by magnetostriction measurements. The metastable states were excited with the heat shock at the transition through the λ point of liquid helium in the course of thermal cycling. A possible mesoscopic mechanism of thermally activated and quantum magnetic relaxation is discussed.

The conductivity of a magnetic nanocontact with a spin inhomogeneity in the form of a Ginzburg-Bulaevski\(\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\smile}$}}{l}\) domain wall at the center between oppositely magnetized edges is considered. The conductivity of the nanocontact is calculated in the ballistic approximation for Fe, Co, and Ni in the low-temperature limit, with allowance made for the spatial major-minor change in the electron spin energy due to exchange splitting in the edges. The calculations are carried out for the limiting cases of long and short nanocontacts. It is shown that the domain wall formed in a nanocontact causes considerable quantitative and qualitative changes in the dependence of the conductivity on the channel width.

We report on the development and investigation of radio frequency (RF) switches (including complete relay pairs) that employ single-crystal microelectromechanical systems (MEMSs) with mobile elements based on high-ohmic amorphous diamondlike carbon films. The use of these films in RF-MEMS switches eliminates problems related to electrode sticking, significantly decreases signal losses (to 100 dB), and ensures short switching times (∼10 ns) for commutated signals at ultrahigh frequencies (up to 2 GHz for films on silicon substrates and above 10 GHz for films on gallium arsenide substrates).

Theoretical and experimental studies are made of the spectra of spin wave excitations localized at a Bloch domain wall in a ferromagnet with combined cubic and biaxial anisotropy in [100]-or [110]-oriented bismuth-containing iron garnet films. This analysis of the spectra is used as the basis to calculate diagrams of stable states of a homogeneous Bloch domain wall as a function of the magnetic parameters in these materials. Results of measurements of the oscillation spectrum of an isolated domain wall in the mega-and gigahertz frequency ranges are presented. A description is given of an effect where the spin-wave excitations of the Goldstone translational branch and the high-frequency Gilinskii unidirectional mode are hybridized.

The influence of thermal fluctuations on magnetization-reversal processes in ultrathin magnetic particles is investigated on the basis of a numerical solution of the Landau-Lifshitz-Gilbert equations taking account the thermal-activation fluctuation field. It is shown that for nanoparticles there exists a region of magnetic and geometric parameters where a strong jump-like instability of the critical field for magnetization reversal arises. This instability is due to the thermally activated transformation of magnetization configurations far from the switching threshold. The thermal-instability mechanism described is important for particles of much larger sizes than for the single-mode Neel-Brown instability.

Effects of the resonant Bragg scattering of magnetostatic backward volume waves on the periodic structure of a conductive meander pattern with an alternating current are analyzed theoretically and compared with experiment. It is shown that unlike a static grating, a dynamic grating causes a frequency shift of the scattered wave. It is proposed that this phenomenon be utilized for effective control of the intermodal conversion of magnetostatic waves.

Calculations are made for the dynamic susceptibility of a weakly ferromagnetic mesoparticle in the macroscopic quantum coherence region and the probability of conservation of the initial polarized state in a pulsed magnetic field.

The frequency of orientational quantum oscillations of the magnetization near impurity-ion clusters with Ising properties in a saturated magnetic crystal is calculated. It is noted that in compounds of the type HoxY3−x Fe5O12, where magnetic phase transitions are observed, additional magnetization reversal and magnetic resonance features due to mesoscopic oscillations of the magnetization can be observed at low concentrations x