X-ray detection of ultrashort spin current pulses in synthetic antiferromagnets

TL;DR

This study demonstrates ultrafast spin current generation in synthetic antiferromagnets using femtosecond laser pulses, revealing superdiffusive spin transport effects and hot electron depolarization in magnetic multilayers.

Contribution

It provides experimental evidence of ultrafast spin current transfer and distinguishes between superdiffusive spin transport and hot electron effects in synthetic antiferromagnets.

Findings

Fe layer demagnetizes following Ni excitation

Spin current influences demagnetization amplitude

Hot electron effects cause depolarization independent of magnetization alignment

Abstract

We explore the ultrafast generation of spin currents in magnetic multilayer samples by applying fs laser pulses to one layer and measuring the magnetic response in the other layer by element-resolved x-ray spectroscopy. In Ni(5~nm)/Ru(2~nm)/Fe(4~nm), the Ni and Fe magnetization directions couple antiferromagnetically due to the RKKY interaction, but may be oriented parallel through an applied magnetic field. After exciting the top Ni layer with a fs laser pulse, we find that also the Fe layer underneath demagnetizes, with a $4.1 \pm 1.9$\% amplitude difference between parallel and antiparallel orientation of the Ni and Fe magnetizations. We attribute this difference to the influence of a spin current generated by the fs laser pulse that transfers angular momentum from the Ni into the Fe layer. Our results confirm that superdiffusive spin transport plays a role in determining the sub-ps demagnetization dynamics of synthetic antiferromagnetic layers, but also evidence large depolarization effects due to hot electron dynamics, which are independent of the relative alignment of the magnetization in Ni and Fe.