Speed limits of the laser-induced phase transition in FeRh

TL;DR

This study investigates the speed limits of laser-induced phase transitions in FeRh films using ultrafast x-ray diffraction and magneto-optical Kerr effect, revealing intrinsic nucleation times and phase growth dynamics in different film thicknesses.

Contribution

It provides the first direct measurement of the intrinsic nucleation timescale and phase growth dynamics of laser-induced magnetic phase transitions in FeRh films.

Findings

Intrinsic nucleation time for ferromagnetic domains is about 8 ps.

Thicker films show delayed phase transformation due to heat transport.

Phase growth into the layer occurs over longer timescales for strong excitations.

Abstract

We use ultrafast x-ray diffraction (UXRD) and the polar time-resolved magneto-optical Kerr effect (tr-MOKE) to study the laser-induced metamagnetic phase transition in two FeRh films with thicknesses below and above the optical penetration depth. In the thin film, we identify an intrinsic timescale for the light-induced nucleation of ferromagnetic (FM) domains in the antiferromagnetic material of $8\,\text{ps}$ that is substantially slower than the speed of sound. For the inhomogeneously excited thicker film, only the optically excited near-surface part transforms within $8\,\text{ps}$. For strong excitations we observe an additional slow rise of the FM phase, which we experimentally relate to a growth of the FM phase into the depth of the layer by comparing the transient magnetization in front- and backside excitation geometry. In the lower lying parts of the film, which are only excited via near-equilibrium heat transport, the FM phase emerges significantly slower than $8\,\text{ps}$ after heating above the transition temperature.