N. D. Rizzo,* T. J. Silva, and A. B. Kos Electromagnetic Technology Division, National Institute of Standards and Technology, Boulder, Colorado 80303 (Received 17 March 1999) We used a magneto-optical Kerr effect microscope to measure 180± magnetization reversal in a high coercivity CoCr10Ta4 thin film subjected to nanosecond field pulses. Exponential magnetization decay occurs for pulse duration tp , 10 ns followed by logarithmic decay for tp . 10 ns, indicating a crossover from nonequilibrium magnetization relaxation at short tp to metastable equilibrium and thermal relaxation for longer tp . We conclude that the nonequilibrium magnetization relaxation time tn and that the average relaxation time of microscopic thermal fluctuations t0 is tn t0 5 ns.