Ultrafast inhomogeneous magnetization dynamics analyzed by interface-sensitive nonlinear magneto-optics

We analyze laser-induced ultrafast, spatially inhomogeneous magnetization dynamics of epitaxial Co/Cu(001) films in a 0.4–10 nm thickness range with time-resolved magnetization-induced second harmonic generation, which probes femtosecond spin dynamics at the vacuum/Co and Co/Cu interfaces. The interference of these two contributions makes the overall signal particularly sensitive to differences in the transient magnetization redistribution between the two interfaces, i.e., ultrafast magnetization profiles in the ferromagnetic film. We conclude that the magnetization dynamics within the first several hundred femtoseconds is characteristically dependent on the Co film thickness. In films up to 3 nm thickness, we find a stronger demagnetization at the film surface compared to the Cu/Co interface, which we explain by a spin current from Co into the Cu substrate with an effective mean free path of about 3 nm. For film thicknesses larger than 3 nm, the transient magnetization profile over the Co film reverses its sign since spins can be transferred into the substrate only from the interface near region. Our work emphasizes that spatial inhomogeneities in the dynamic magnetic response to femtosecond laser excitation allow conclusions on the underlying microscopic processes.