P. E. Zil’berman

Current-induced inverse population of spin subbands in magnetic junctions

The spin flux matching conditions that should be satisfied at the boundaries between contacting layers of a magnetic junction to ensure the inverse population of spin subbands at experimentally achievable current densities of −107–108 A/cm2 are determined. The essence of the conditions is that an efficient spin injection takes place at the input boundary, whereas the injection at the output boundary is blocked. The fulfillment of the matching conditions for contacting layers with antiparallel magnetization orientations leads to stability in magnetic fluctuations for any forward currents.

‘‘Exchange’’ spin waves in nonuniform yttrium iron garnet films

Spin wave resonance spectra have been observed in yttrium iron garnet films. The spectra exhibit as many as 120 strongly excited modes with large deviation from quadratic spacing, which are interpreted in terms of a thickness variation of the effective magnetization 4πM(x) depending both on the magnetization and the anisotropy field. Profiles of 4πM(x) are calculated from the spectra measured both in parallel and in perpendicular bias magnetic fields. It has been shown that the films tested have monotonic variations of the effective magnetization. The difference 4πΔM between upper and lower 4πM(x) values is as large as 330 G. It is shown that exchange‐dominated spin waves (ESW) with the maximum wave number of 2.5×105 cm−1 have been excited under experiment conditions. Microwave pulse delay due to ESW propagation across the film thickness has been observed. Delay versus frequency characteristics strongly depend on the profile M(x) and the direction of the applied magnetic field. Profiles M(x) have been calculated to provide both constant and linear delay‐versus‐frequency characteristics.

Current-induced spin injection and surface torque in ferromagnetic metallic junctions

The joint influence of two current-induced effects, namely, longitudinal nonequilibrium spin injection and surface torque, on spin-valve-type ferromagnetic metallic junctions is considered theoretically. The current flows normally to layer boundaries. The analysis is based on solving a system of coupled equations of motion for mobile electron and lattice magnetizations. The boundary conditions for the equations of motion are derived from the continuity condition for the total magnetization flux in these subsystems. A dispersion relation is derived for spin wave fluctuations depending on the current through the junction. The fluctuations become unstable at currents exceeding some threshold value (usually, 106−3 × 107 A/cm2). The joint action of longitudinal spin injection and torque lowers the instability threshold. Current-induced spin injection decreases spin wave frequencies near the threshold and can strengthen magnetization pinning at the injecting contact.

Principal Mode of the Nonlinear Spin-Wave Resonance in Perpendicular Magnetized Ferrite Films

The paper reports a theoretical and experimental study of the nonlinear spin-wave resonance (SWR) modes in normally magnetized ferrite films. Particular attention is focused on the principal, lowest frequency, SWR mode. It is shown theoretically that, as the precession amplitude increases, the profile of the principal mode changes to make the excitation distribution across the film thickness more uniform. The nonlinear shift of the resonance field depends on the surface-spin pinning parameters. An experimental study has been made of YIG films with a strong uniaxial anisotropy field gradient over the film thickness, as well as of YIG films of submicron thickness. As the microwave power was increased, the principal-mode resonance field was observed undergoing a sublinear shift accompanied by a superlinear growth of absorbed power. This behavior is attributed to a change in the profile of the spatial distribution of ac magnetization.

Current-induced spin injection from probe to ferromagnetic film

The distribution of the spin polarization of conduction electrons is calculated under the conditions for the spin injection from a probe to a ferromagnetic film in the presence of current. It is demonstrated that the main parameters that determine the deviation of the spin polarization from equilibrium are the current density and spin polarization of the probe material, whereas the ratio of the probe diameter to the spin diffusion length weakly affects the result in certain ranges. The population inversion of the spin subbands can be reached at distances of about spin diffusion length from the probe-film interface.

The Influence of a Current on the Magnetization Dynamics in a Ferromagnet-Antiferromagnet Structure: Simulation

A ferromagnet-antiferromagnet junction in the presence of an in-plane magnetostatic field and a spin-polarized current flowing perpendicularly to the junction layers is considered. In the macrospin approximation, the system of nonlinear equations describing the dynamics of magnetization of the antiferromagnet layer in such a junction is numerically solved with allowance for the spin torque transfer and the spin equilibrium disturbance caused by spins injected by the current into the antiferromagnet. It is shown that, when the current exceeds a certain threshold, the magnetization becomes instable and that, beyond the instability region, the results are in complete agreement with the theory using linearization in small deviations from the equilibrium. It is found that the development of instability causes switching from the antiparallel configuration to the parallel one and that, in the instability region, nondecaying oscillations of the longitudinal and transverse components of the antiferromagnet magnetization are formed.

Magnetostatic energy and stripe domain structure in a ferromagnetic plate of finite width with in-plane anisotropy

The magnetostatic energy and domain structure (DS) in a long ferromagnetic plate of a finite width with in-plane anisotropy are calculated for the case of the domain magnetization vectors lying in the plane of the plate. The situation where the DS period is much shorter than the width but is considerably larger than the thickness of the plate is analyzed in detail. The equilibrium DS period and the width ratio of two adjacent domains are determined as functions of an external magnetic field parallel to the plane of the plate by minimizing the energy. The DS period is found to be proportional to the plate width and the domain wall energy and inversely proportional to the squared saturation magnetization. While the width of the favorable domains (with the magnetization parallel to the field) grows with increasing field, the unfavorable domains, rather than disappearing completely, form relatively narrow transition regions between the favorable domains, i.e., 360° domain walls.

Terahertz emission in the ferromagnetic-antiferromagnetic structure

Current flow in the structure consisting of a hard-magnetic ferromagnetic rod (hardened steel) with a diameter of up to 100 μm that contacts a thin (10–100 nm) ferromagnetic (FeMn) film is studied at room temperature. Effective magnetization is induced in the FeMn film in the presence of the spin-polarized current in spite of the absence of the intrinsic magnetization. The terahertz radiation that is detected in this work is caused by the precession of the induced magnetization. The emission is interpreted using the recent theoretical predictions on the effect of the sd exchange on the lattice dynamics of antiferromagnetic layer.

sd-Exchange emission in ferromagnetic junctions

The spin-injection emission in a ferromagnetic junction at terahertz frequencies is theoretically analyzed. It is demonstrated that the efficiency of the emission caused by the sd-exchange interaction of the injected spin in which the electromagnetic field is involved strongly depends on the orientation angle of the magnetization of the active region relative to the magnetization direction of the injecting region. The fact that the calculated radiation frequency and power are close to the experimental results shows that the sd-exchange emission must be taken into account in the interpretation of the observed terahertz emission in magnetic junctions.

Spin-injection radiation of terahertz waves in ferromagnetic structures

We propose and discuss the mechanisms of terahertz sd exchange radiation during transmission of the spin-polarized current in a structure from a thin steel rod pressured to an ultrathin ferromagnetic film. The two mechanisms considered are quantum transitions between the quasi-Fermi levels in the film near the rod and precession of injected spins around the film magnetization.