Narrow Results

We develop a theory applicable in the regime, where the wavelength of spin up and down electrons, λ_↑,↓, is smaller than the DW width L (smooth DW). This regime is appropriate for strongly doped magnetic semiconductors and metallic ferromagnets. We develop an approach related to a weak (adiabatic) perturbation arising after a local transformation to the uniform magnetization. In the limit of L ≫ λ_↑,↓ we find an exponentially small reflection and spin-flip transition amplitudes. The theory is useful in the regime, where the DW width is not so large, and the perturbation corrections to the wave functions are quite essential. We calculate the current-induced spin density and the spin torque acting locally on the magnetic moments within the DW. The spatially-distributed torque acts as a force pushing the DW in one direction as a response to the external current, and also as an internal force, which can distort the static ordering of magnetic moments in the DW.

In recent years, electrical spin injection and detection has grown into a lively area of research in the field of spintronics. Spin injection into a paramagnetic material is usually achieved by means of a ferromagnetic source, whereas the induced spin accumulation or associated spin currents are detected by means of a second ferromagnet or the reciprocal spin Hall effect, respectively. This article reviews the current status of this subject, describing both recent progress and well-established results. The emphasis is on experimental techniques and accomplishments that brought about important advances in spin phenomena and possible technological applications. These advances include, amongst others, the characterization of spin diffusion and precession in a variety of materials, such as metals, semiconductors and graphene, the determination of the spin polarization of tunneling electrons as a function of the bias voltage, and the implementation of magnetization reversal in nanoscale ferromagnetic particles with pure spin currents.

We consider theoretically a current flowing perpendicular to interfaces of a spin-valve type ferromagnetic metallic junction. For the first time a simultaneous action of the two current effects is investigated, namely, the nonequilibrium longitudinal spin injection and the transverse spin surface torque. Dispersion relations for fluctuations are derived and solved under the proper boundary conditions. Joint action of the two effects mentioned lowers the instability threshold, its typical value being 1 × 10⁶–3 × 10⁷ A/cm². Spin wave excitations may soften near the threshold. A nonlinear problem is solved about steady state arising due to instability development.

We show in this paper that the technologically relevant field-like spin–orbit torque (SOT) shows resilience against the geometrical effect of electron backscattering. As a device grows smaller in size, the effect of geometry on physical properties like spin torque, and hence switching current could place a physical limit on the continued shrinkage of such a device — a necessary trend of all memory devices (MRAM). The geometrical effect of curves has been shown to impact quantum transport and topological transition of Dirac and topological systems. In our work, we have ruled out the potential threat of line curves degrading the effectiveness of SOT switching. In other words, SOT switching will be resilient against the influence of curves that line the circumferences of defects in the events of electron backscattering, which commonly happens in the channel of modern electronic devices.

Motion of a planar domain wall in a nanoscale ferromagnetic wire under electric current is studied based on microscopic description. In the adiabatic case, domain wall is driven by spin torque (spin transfer), and there is an intrinsic pinning arising from hard-axis anisotropy energy. Extrinsic pinning does not affect the threshold current in this case. A resonating oscillation of domain wall occurs under AC current, and this was recently used for a spectroscopy of a single "domain wall particle". Nucleation of domain walls by spin torque is discussed. Fundamental mechanism of spin transfer effect is understood in terms of spin Josephson effect.

The spin-Hall effect (SHE) and the inverse spin-Hall effect (ISHE) coupled with magnetization dynamics were investigated using a simple Ni₈₁Fe₁₉/Pt film. A spin current generated by magnetization dynamics was detected electrically using ISHE. The observed magnetic field angle dependence of the ISHE signal is well reproduced by a model calculation based on the dc spin pumping and ISHE. In the same system, we found that spin relaxation in the Ni₈₁Fe₁₉ layer is manipulated electrically using SHE. An electric current applied to the Pt layer exerts the spin torque on the entire magnetization of the Ni₈₁Fe₁₉ layer via the macroscopic spin transfer induced by SHE, which modulates spin relaxation in the Ni₈₁Fe₁₉ layer. This spin-relaxation modulation enables quantitative measurements of spin currents without assuming any microscopic parameters.

Interplay between magnetization dynamics and electric current in a conducting ferromagnet is theoretically studied based on a microscopic model calculation. First, the effects of the current on magnetization dynamics (spin torques) are studied with special attention to the "dissipative" torques arising from spin-relaxation processes of conduction electrons. Next, an analysis is given of the "spin motive force", namely, a spin-dependent 'voltage' generation due to magnetization dynamics, which is the reaction to spin torques. Finally, an attempt is presented of a unified description of these effects.

Spin transport in a magnetically nonuniform structure can lead to transfer of angular momentum from conduction electrons to local magnetization. This, in turn, gives rise to a spin-transfer torque, which can modify magnetic state of the system. Another mechanism of current-induced spin torque relies on spin-orbit interactions. However, the spin torque can be induced not only by electric field driving the current, but also by a temperature gradient which drives a thermocurrent due to the thermoelectric phenomena. Both current- and thermallyinduced spin torques allow to control and manipulate magnetic moments.