Coherent magnetization rotation induced by optical modulation in ferromagnetic/antiferromagnetic exchange-coupled bilayers

We have demonstrated recently that ultrafast (similar to 100 ps) coherent m agnetization rotation can he induced by optical modulation of the exchange coupling in an exchange coupled bilayer (NiFe/NiO) with short laser pulses [Phys. Rev. Lett. 82, 3705 (1999)]. In this paper, we acquire further suppo rt of the coherent magnetization model by studying the details of magnetiza tion switching dynamics, including the dependence on the amplitude of excha nge bias field. The exchange bias field is subject to large transient modul ation by femtosecond pulsed laser excitation, providing the impulsive force for subsequent magnetization dynamics. Good agreement is obtained with a s imple model of coherent magnetization processes based on the Landau-Lifshit z-Gilbert equations of motion. We have also measured a strong dependence of thr phenomenological damping constant (a) on the exchange bias in the coup led ferromagnetic/antiferromagnetic spin system.