Large strain contribution to the laser-driven magnetization response of magnetostrictive TbFe$_{2}$

TL;DR

This study demonstrates that longitudinal strain pulses significantly influence the transient magnetization response in laser-excited TbFe$_{2}$ films, highlighting the role of thermal expansion and magnetoelastic coupling in ultrafast magnetization dynamics.

Contribution

It introduces a comprehensive model combining strain pulse propagation and inverse magnetostriction to explain magnetization changes, emphasizing the impact of quasi-static strain in laser-induced magnetization dynamics.

Findings

Strain pulses are detected via reflectivity changes.

Delayed magneto-optical Kerr response indicates genuine magnetization dynamics.

Quasi-static strain from thermal expansion dominates the magnetization response.

Abstract

We investigate strain-induced contributions to the transient polar magneto-optical Kerr effect response in laser-excited terfenol. The tr-MOKE signals obtained from TbFe$_{2}$ films with and without glass capping exhibit distinct signatures associated with transient strain. We experimentally observe the arrival of strain pulses via the reflectivity change. The tr-MOKE response measured without changing the pump-probe geometry is delayed by several picoseconds. This suggests a genuine magnetization response as opposed to instantaneous changes of optical constants as the origin of the signal. We model the propagation of longitudinal acoustic picosecond strain pulses and incorporate the inverse magnetostriction effect via a magnetoelastic term in the effective field of the Landau-Lifshitz-Gilbert equation with large damping. This reproduces not only the delay of the pulsed response, but also unveils the dominant contribution of quasi-static strain to the magnetization dynamics due to the thermal expansion in the optically probed near-surface region. Our experiments exemplify that purely longitudinal strain along the out-of-plane direction of the thin film enables efficient magnetoelastic coupling via the shear strain components arising in the oblique crystallographic frame of reference.