Ultrafast spin dynamics and coherent magnetization switching in ferromagnetic thin films

In this thesis we present an experimental study of the dynamics of nonequilibrium spin relaxation and coherent magnetization switching with a time-resolved magneto-optical pump-probe technique that we have developed.;We have demonstrated that significant modulation of magnetic ordering can be induced on 100 fs time scale by ferntosecond photoexcitaion of a non-equilibrium distribution of electrons and spins, and the subsequent relaxation dynamics has been time resolved, which can be described by a simplified rate-equation model.;We have also discovered that by photoexciting the exchange coupled FM/AF interface with ferntosecond laser pulses, significant modulation in the unidirectional exchange bias field (Hex.) can be induced on an ultrashort timescale. This leads to large coherent magnetization rotation in the permalloy film on a timescale of 100 psec. The dependence of the photoinduced coherent magnetization rotation dynamics on the external applied field and the "built-in" internal exchange bias field has been studied systematically. Theoretical considerations are offered from total magnetic energy arguments, and the temporal magnetization oscillations observed experimentally are well described by Landau-Liftshitz-Gilbert equations.;Our work has shown that the approach via ultrafast photoexcitation provides important insights on the spin relaxation dynamics, and opens up possibilities for very high speed control and study of coherent switching of magnetization through microscopic access to spins in magnetic systems.