Nonequilibrium magnetic domain configurations assisted by thermally excited magnons

Dynamic behavior in the evolving structure of domains in mesoscopic ferromagnets (Ni/sub 80/Fe/sub 20/) undergoing ultrafast magnetization reversal is studied with scanning Kerr microscope and micromagnetic modeling based on Landau-Liftshitz-Gilbert equation. A faint branchlike fine structure is observed in the initial state of the magnetization reversal. Micromagnetic simulation at 300 K reveals that complex domain structures evolve out of these fine structures, in agreement with the experimental observations. This result implies that thermally excited magnons at finite temperatures are effectively involved in the formation of complex nonequilibrium domain structures. Furthermore, the speed at which the magnetization reversal is driven by switching fields is found to play an essential role in determining these dynamic domain patterns.