  The spin-transfer torque from a DC spin-polarized current can generate
highly-coherent magnetic precession in nanoscale magnetic-multilayer devices.
By measuring linewidths of spectra from the resulting resistance oscillations,
we argue that the coherence time can be limited at low temperature by thermal
deflections about the equilibrium magnetic trajectory, and at high temperature
by thermally-activated transitions between dynamical modes. Surprisingly, the
coherence time can be longer than predicted by simple macrospin simulations.
