Thermal Effects on Precessional States in Nanomagnets Driven by DC Spin-Transfer Torques

A DC current passing through a nanoscale magnetic multilayer can excite steady-state microwave-frequency magnetization precession by transferring spin angular momentum from one layer to the other [1-3]. In frequency-domain measurements, the spectra generated by such excitations consist of peaks with a non-zero width in frequency, $\Delta f$, indicating that the oscillatory signal produced by the precessing magnet is not perfectly periodic. Here we measure the temperature ($T)$ dependence of $\Delta f$. We argue that at least two mechanisms contribute to $\Delta f$: thermal deflections of the magnetic moment within a precessional orbit (for which $\Delta f\sim T^{1/2})$ and thermally-activated transitions between different dynamical states (for which ln($\Delta f)\sim $1/$T)$. \newline [1] S. I. Kiselev, J. C. Sankey et al., Nature \textbf{425}, 380 (2003). \newline [2] W. H. Rippard et al., Phys. Rev. Lett. \textbf{92}, 027201 (2004). \newline [3] I. N. Krivorotov et al., in press, Science.