Separation of ultrafast spin currents and spin-flip scattering in Co/Cu(001) driven by femtosecond laser excitation via the complex MOKE

TL;DR

This study uses complex MOKE to distinguish between ultrafast spin currents and spin-flip scattering in Co/Cu(001) films, revealing their temporal dominance and providing insights into microscopic spin dynamics after femtosecond laser excitation.

Contribution

It introduces a method to separate local and non-local spin contributions in ultrafast magnetization dynamics using polarization-resolved MOKE measurements.

Findings

Spin-dependent transport dominates before 100 fs

Spin-flip scattering dominates after 200 fs

Effective depth sensitivity of MOKE enables spatial profiling

Abstract

Ultrafast magnetization dynamics in metallic heterostructures consists of a combination of local demagnetization in the ferromagnetic constituent and spin-dependent transport contributions within and in between the constituents. Separation of these local and non-local contributions is essential to obtain microscopic understanding and for potential applications of the underlying microscopic processes. By comparing the ultrafast changes of the polarization rotation and ellipticity in the magneto-optical Kerr effect (MOKE) we observe a time-dependent magnetization profile M(z,t) in Co/Cu(001) films by exploiting the effective depth sensitivity of the method. By analyzing the spatio-temporal correlation of these profiles we find that on time scales before hot electron thermalization (<100 fs) the transient magnetization of Co films is governed by spin-dependent transport effects, while after hot electron thermalization (>200 fs) local spin-flip processes dominate.