Micromagnetic simulations of persistent oscillatory modes excited by spin-polarized current in nanoscale exchange-biased spin valves

We perform three-dimensional micromagnetic simulations of current-driven magnetization dynamics in nanoscale exchange biased spin valves that take account of (i) back action of spin-transfer torque on the pinned layer, (ii) nonlinear damping, and (iii) random thermal torques. Our simulations demonstrate that all these factors significantly impact the current-driven dynamics and lead to a better agreement between theoretical predictions and experimental results. In particular, we observe that at a nonzero temperature and a subcritical current, the magnetization dynamics exhibits nonstationary behavior in which two independent persistent oscillatory modes are excited which compete for the angular momentum supplied by spin-polarized current. Our results show that this multimode behavior can be induced by combined action of thermal and spin transfer torques. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3057912]