How to Measure and Model Light-Induced Spin Transfer

TL;DR

This study investigates ultrafast light-induced spin transfer in magnetic materials, revealing that spin lifetimes and disorder significantly influence the magnetic response beyond initial and final state considerations.

Contribution

It demonstrates that including spin lifetimes and disorder effects is essential to accurately model ultrafast spin transfer dynamics in Heusler compounds.

Findings

Spin lifetimes and disorder affect ultrafast spin transfer.

Ordered Co2MnGa shows strong laser-induced magnetic signals.

No intra-site spin transfer observed in pure Co due to short spin lifetimes.

Abstract

Femtosecond laser light can transfer spin angular momentum between magnetic subspecies that exhibit hybridized valence bands within an alloy or compound, and represents the fastest route for manipulating the magnetization of a material. To date, ultrafast spin transfer has predominantly been explained in terms of the initial and final states available for laser excitation. Here, by comparing the measured and calculated dynamics across the entire $M$-edges of two very similar Heusler compounds, $Co_2MnGa$ and $Co_2MnGe$ as well as a sample of elemental Co, we find that simply accounting for the initial and final electron states available for laser excitation cannot alone explain the experimental observations. The influence of spin lifetimes must also be included, due to the shifting of the Fermi level upon replacing Ga with Ge, or the presence of crystalline disorder. This explains why the ordered $L2_1$ phase of $Co_2MnGa$ demonstrates strong laser-induced magnetic signal enhancements across the entire Co-edge, while similar enhancements were not observed in partially disordered $Co_2MnGe$. Although intra-site spin-transfers were expected in the minority channel in pure Co due to the presence of many more available states in the minority channel above the Fermi level, no such signal was observed due to very short few-femtosecond spin lifetimes in a metal. Finally, we identify key regions in the magnetic asymmetry where a transiently enhanced signal could be misinterpreted as a light-induced spin-transfer signature.