Physical Review B - CMMP

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

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We have demonstrated recently that ultrafast (∼100 ps) coherent magnetization rotation can be 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 support of the coherent magnetization model by studying the details of magnetization switching dynamics, including the dependence on the amplitude of exchange bias field. The exchange bias field is subject to large transient modulation by femtosecond pulsed laser excitation, providing the impulsive force for subsequent magnetization dynamics. Good agreement is obtained with a simple model of coherent magnetization processes based on the Landau-Lifshitz-Gilbert equations of motion. We have also measured a strong dependence of the phenomenological damping constant (α) on the exchange bias in the coupled ferromagnetic/antiferromagnetic spin system. © 2000 The American Physical Society.