Macrospin models of spin transfer dynamics

The current-induced magnetization dynamics of a spin valve are studied using a macrospin (single-domain) approximation and numerical solutions of a generalized Landau-Lifshitz-Gilbert equation. For the purpose of quantitative comparison to experiment S. I. Kiselev, J. C. Sankey, I. N. Krivortov, N. C. Emley, R. J. Schoelkopf, R. A. Buhrman, and D. C. Ralph, Nature 425, 380 (2003), we calculate the resistance and microwave power as a function of current and external field, including the effects of anisotropies, damping, spin-transfer torque, thermal fluctuations, spin-pumping, and incomplete absorption of transverse spin current. Although many features of experiment appear in the simulations, there are two significant discrepancies: the current dependence of the precession frequency and the presence and/or absence of a microwave quiet magnetic phase with a distinct magnetoresistance signature. Comparison is made to micromagnetic simulations designed to model the same experiment.