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.