Optical control of ultrafast spin -wave relaxation by magnetic anisotropy in a ferromagnet

This thesis presents an investigation of the damping of spin waves in ferromagnetic Au(3 nm)/Ni(10 nm)/MgO(001) thin films using the time-resolved Magneto-optical Kerr Effect (TR-MOKE) and ferromagnetic resonance (FMR) techniques. In the optical measurements, a 150 fs, 800 nm laser beam pulse is split into pump and probe components. The pump pulse, containing most of the beam energy, thermally excites coherent spin precession. The weaker probe pulse, time-delayed by a variable beam path, captures the magnetization dynamics via the polar MOKE effect, and oscillations are observed as a function of external field amplitude and direction. The extracted precession frequency is consistent in both the optical and resonance techniques; however, additional damping is observed in the TR-MOKE measurements that is strongly correlated to the orientation of the magnetization with respect to the magnetic anisotropy. The damping is identical in TR-MOKE and FMR only when the external field is applied near the easy axis of magnetization. The enhanced damping in TR-MOKE is shown to be a consequence of pump-induced inhomogeneous broadening in the presence of magnetic anisotropy, a result of differing temperature recovery profiles for the magnetization and magnetic anisotropy. Finally, a simple model is developed which explains the anisotropic damping: mode broadening occurs in regions where the magnetization changes rapidly with respect to changes in the external field, as determined by the curvature of the magnetic free energy. We thus introduce a novel damping effect in TR-MOKE: pump-induced anisotropic damping (PIAD)