Signatures of ballistic and diffusive transport in the time-dependent Kerr-response of magnetic materials

We calculate the influence of diffusive and ballistic transport on ultrafast magnetization in thick metallic films. When only diffusive transport is present, gradients of magnetization in the material remain up to picosecond timescales. In contrast, in the extreme superdiffusive limit where ballistic transport dominates, the magnetization changes homogeneously in space. We calculate the measurable magneto-optical responses for a $\SI{40}{\nano\meter}$ Nickel film. Although the resulting Kerr rotation dynamics are found to be very similar in the two limits of transport, our simulations reveal a clear signature of magnetization gradients in the Kerr ellipticity dynamics, namely a strong probe-angle dependence for the case when diffusive transport allows gradients to persist. We then perform probe-angle dependent complex magneto-optical Kerr effect (CMOKE) measurements on an excited \SI{40}{\nano\meter} Nickel film. The angle dependence of the measured Kerr signals closely matches the simulated response with diffusive transport. Therefore we conclude that the influence of ballistic transport on ultrafast magnetization dynamics in such films is negligible.