Anatolii K. Zvezdin

Effect of spatial spin modulation on the relaxation and NMR frequencies of 57Fe nuclei in a ferroelectric antiferromagnet BiFeO3

The 57Fe NMR spectra of the 57Fe-enriched (95.43%) ceramics of a ferroelectric antiferromagnet BiFeO3 with a spatially modulated magnetic structure have been studied. It is established that a cycloidal spin modulation in BiFeO3 causes a spatial modulation of the spin-spin relaxation rate along the magnetic cycloid period and results in inhomogeneous broadening of the local NMR lineshape. It is also found that the local magnetic moments of Fe ions in various parts of the cycloid depend differently on the temperature, which is indicative of a difference in the spin wave excitation. The observed phenomena can be explained in terms of the Shul-Nakamura indirect nuclear interaction which becomes effective at high concentrations of the magnetoactive nuclei and low temperatures. Similarity of the obtained experimental results to the regularities of NMR in the Bloch walls is discussed.

Nonlinear magneto-optical Kerr effects

The polarization state of a second-harmonic wave after reflection from a semi-infinite, optically isotropic magnetic medium is considered for the three characteristic uniformmagnetization directions corresponding to the linear magneto-optical Kerr effects. Expressions for the complex amplitudes of the wave field which specify the nonlinear Kerr effects, viz., the polar, meridional, and equatorial effects, are obtained in a first approximation with respect to the magnetization. The dependences of these effects on the angle of incidence of the inducing wave obtained as a result of a numerical experiment are presented. Analytical formulas are found for them at small angles of incidence. A comparative analysis of the linear and nonlinear Kerr effects is made.

Anapole moment and spin-electric interactions in rare-earth nanoclusters

Spin-electric interactions and magnetoelectric properties of triangular rare-earth molecular nanoclusters are investigated by the example of Dy3. The effective spin-electric Hamiltonian is derived on the base of a developed quantum-mechanical model of the cluster spin structure. It is shown that a toroidal (anapole) moment is induced in such clusters with magnetic field exceeding some threshold value or with crossed electric and magnetic fields (or an electric current). Due to the considerable relaxation time of the state with non-zero toroidal moment, it is possible to observe the non-equilibrium quantum linear magnetoelectric effect.

Pinning of magnetic domain walls in multiferroics

The behavior of antiferromagnetic domain walls (ADW) against the background of a periodic ferroelectric domain structure has been investigated. It has been shown that the structure and the energy of ADW change due to the interaction with a ferroelectric domain structure. The ferroelectric domain boundaries play the role of pins for magnetic spins, the spin density changes in the vicinity of ferroelectric walls. The ADW energy becomes a periodical function on a coordinate which is the position of the ADW relative to the ferroelectric domain structure. It has been shown that the energy of the magnetic domain wall attains minimum values when the center of the ADW coincides with the ferroelectric wall and the periodic ferroelectric structure creates periodic coercitivity for the ADW. The neighbouring equilibrium states of the ADW are separated by a finite potential barrier.

Entropy first order transition in YbInCu4

Abstract Surprisingly, the elliptic shape and the parameters characterizing the line of the isostructural phase transition in YbInCu 4 and its alloys in the wide range of magnetic fields and temperatures, are perfectly described in terms of the entropy transition for free Yb ions. Assuming a more complicated model for the Yb 3+ ions considerable deviations from experimental data would appear.

New intensity magneto-optical effect in materials exhibiting giant magnetoresistance

The change in the reflectivity of a metallic magnetic multilayer that exhibits giant magnetoresistance for a monochromatic electromagnetic plane wave with polarization along the magnetization (s polarization) in response to a change from the antiferromagnetic magnetic configuration of the multilayer to the ferromagnetic configuration is investigated. This magneto-optical effect is treated in the effective-medium approximation, in which the dielectric constant needed is found analytically with consideration of the interface roughness scattering of electrons. It is shown in the example of an Fe/Cu multilayer that the effect amounts to ∼0.7%. The representation found for the complex conductivity is convenient in a special case for investigating the magnetoresistive effect.

Spin-current-induced classical and quantum effects in the dynamics of a mesoscopic magnet

The dynamics of a high-spin quantum system with magnetic anisotropy of the easy plane type under the action of spin-polarized current permeating this system is considered. The spin-polarized current (with electron spins polarized along the hard magnetic axis of the system) induces the reorientation of the magnetic moment of the system from the easy plane to the hard magnetic axis. Analytical expressions describing characteristics of the reorientation process in the limiting cases of strong and weak dissipation are obtained. Under strong dissipation conditions, the reorientation is shown to have a threshold character with “soft” (continuous) displacement of the magnetic moment from the easy plane. Under weak dissipation, the reorientation occurs as a discrete process, that is, is accompanied by magnetic moment jumps and hysteresis as the spin current increases and decreases. At a fairly low temperature and weak damping, quantum effects arise in the system. The spin current induces excitations quasi-anionic in character, Bloch oscillations of magnetic moment precession, and tunneling between different precession quantum modes. These quantum effects, in particular, manifest themselves in the system under consideration by magnetic moment jumps and magnetic susceptibility peaks.

Current-Voltage Characteristics of a Spin Half-Metallic Transistor

A new design of spin transistor based on half-metallic ferromagnets (referred to as a spin half-metallic transistor) is suggested, and its current-voltage characteristics are studied theoretically. Like a bipolar transistor, the new device can amplify current. At the same time, the properties of a spin half-metallic transistor depend considerably on the mutual orientation of the magnetizations of its three contacts. We also propose a device based on an F↑-F↓ junction. This device consists of two single-domain half-metallic parts with opposite magnetizations. There is a range of voltages where the current-voltage characteristics of an F↑-F↓ junction and a semiconductor diode are similar. The behavior of an F↑-F↓ junction under different conditions is studied.

Nonlinear dynamics of semiclassical spin in a time-dependent magnetic field

The dynamics of magnetic nanoclusters (or molecules) with a large spin in a magnetic field whose strength varies in proportion to time is analyzed. Such a field breaks the symmetry relative to rotations through 2π, as well as clockwise and counterclockwise rotations, and induces a number of new coherent quantum effects in the spin dynamics, such as the formation of a band energy spectrum with continuous spin states or the emergence of “Bloch” oscillations in spin precession and interband Zener tunneling. Bloch oscillations are manifested in experiment as equidistant identical jumps on the magnetization curve. The interband Zener tunneling gives rise to additional jumps and peaks on the susceptibility of the system.

Nonlinear domain-wall dynamics in a system of two magnetic layers

The nonlinear dynamics of the magnetization in a spin-valve structure is investigated. Equations describing the dynamics of the magnetization in such a structure are obtained. The stability of the solution corresponding to a motionless flat domain wall is investigated. The nonlinear domain-wall dynamics are investigated in the approximation of a strong exchange interaction between the magnetic layers and in the approximation of a large magnetostatic energy. In the former case the nonlinear dynamical equations are shown to be similar to the equations describing the dynamics of the magnetization in a weak ferromagnet, and in the latter case they are similar to the equations of motion of a magnetic vortex (i.e., a vertical Bloch line) in a domain wall.