Dynamical stabilisation by spin transfer in a nearly isotropic magnet

ほとんど等方的な磁石におけるスピン移行による動的安定化現象

山本 慧   ; Barker, J.*; 紅林 秀和*

Yamamoto, Kei; Barker, J.*; Kurebayashi, Hidekazu*

Spin transfer between spin-polarised conduction electrons and ordered magnetic moments results in a dissipative torque acting on the macroscopic magnetisation. When the direction of the torque is such that it counteracts spin relaxation, the magnetisation can be distabilised from the equilibrium state corresponding to the minimum of free energy, which should take the system to another steady state. In this work, the asymptotic dynamics of the ferromagnetic film under the anti-damping torque is systematically investigated by applying dynamical systems theory. We find that the well-documented auto-oscillation regime constitutes only a part of the parameter space and several other attractors appear depending on the sign and magnitude of uniaxial anisotropy. We discuss the effect of thermal fluctuations by introducing a stochastic noise and solving the dynamica equations numerically. The steady state expectation values of squared magnetisation components are computed and compared with recent experimental data.