Depth-resolved magnetization dynamics in Fe thin films after ultrafast laser excitation

TL;DR

This study uses time-resolved x-ray magnetic reflectivity to investigate ultrafast, depth-dependent magnetization changes in Fe thin films after laser excitation, revealing inhomogeneous dynamics and stress-induced oscillations.

Contribution

It provides the first detailed depth-resolved analysis of magnetization dynamics in Fe films on femtosecond timescales, combining experimental measurements with theoretical modeling.

Findings

Magnetization is strongly inhomogeneous near the interface within the first picoseconds.

Local and non-local angular momentum transfer phenomena occur simultaneously.

The film exhibits periodic thickness oscillations due to laser-induced stresses.

Abstract

We performed time-resolved x-ray resonant magnetic reflectivity measurements on a laser-excited ferromagnetic Fe thin film to simultaneously probe the transient structural and magnetic depth profiles with nanometer spatial and femtosecond temporal resolution. Our results show that during the first picoseconds after optical excitation, the magnetization of the Fe layer is strongly inhomogeneous, especially in the vicinity of the buried interface. By comparing our experimental results to predictions based on the microscopic three-temperature model and simulations of laser-induced spin-currents, we demonstrate that local and non-local angular momentum transfer phenomena take place simultaneously. After a few picoseconds, the magnetization relaxes back to equilibrium while the total thin film thickness starts oscillating periodically, with a maximum dilation of approximately 1.3% of the entire thin film thickness due to laser-induced stresses.