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

TL;DR

This study uses time-resolved nonlinear magneto-optics to investigate ultrafast, inhomogeneous magnetization dynamics in epitaxial Co/Cu(001) films, revealing how film thickness influences demagnetization profiles at interfaces.

Contribution

It introduces a novel interface-sensitive method to analyze ultrafast spin dynamics and demonstrates thickness-dependent magnetization redistribution in ferromagnetic films.

Findings

Stronger surface demagnetization in films up to 3 nm thick.

Reversal of magnetization profile in thicker films.

Demagnetization influenced by spin mean free paths.

Abstract

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 find in films of up to 3 nm thickness a stronger demagnetization at the surface, because the film thickness is smaller than the effective mean free path of the spin current mediating the demagnetization, i.e. the difference between the mean free paths of the majority and minority carriers. For film thicknesses larger than 3 nm, the magnetization profile reverses, since majority spins can escape into the conducting substrate only from the interface-near region.