  The effect of thermal fluctuations on spin-transfer switching has been
studied for a broad range of time scales (sub-ns to seconds) in a model system,
a uniaxial thin film nanomagnet. The nanomagnet is incorporated into a
spin-valve nanopillar, which is subject to spin-polarized current pulses of
variable amplitude and duration. Two physical regimes are clearly
distinguished: a long pulse duration regime, in which reversal occurs by
spin-transfer assisted thermal activation over an energy barrier, and a short
time large pulse amplitude regime, in which the switching probability is
determined by the spin angular momentum in the current pulse.
