Time-resolved magnetic relaxation of a nanomagnet on subnanosecond time scales

We present a two-current-pulse temporal correlation experiment to study the intrinsic subnanosecond nonequilibrium magnetic dynamics of a nanomagnet during and following a pulse excitation. This method is applied to a model spin-transfer system, a spin-valve nanopillar with perpendicular magnetic anisotropy. Two pulses separated by a short delay ($<500$ ps) are shown to lead to the same switching probability as a single pulse with a duration that depends on the delay. This demonstrates a remarkable symmetry between magnetic excitation and relaxation and provides a direct measurement of the magnetic relaxation time. The results are consistent with a simple finite-temperature Fokker-Planck macrospin model of the dynamics, suggesting more coherent magnetization dynamics in this short-time nonequilibrium limit than near equilibrium.