Characterizing the Nonequilibrium Response of FeRh Thin Films using Time-Domain Thermoreflectance (TDTR)

TL;DR

This study uses Time-Domain Thermoreflectance (TDTR) to investigate the ultrafast magnetic phase-dependent thermal and optical responses of FeRh thin films during their AF to FM transition, revealing phase-specific dynamics and properties.

Contribution

The paper demonstrates how TDTR can distinguish and analyze the transient responses of AF and FM phases in FeRh, providing new insights into their ultrafast thermal and magnetic behaviors.

Findings

AF phase shows a large negative transient reflectance response.

FM phase exhibits a positive transient reflectance response.

Strong subpicosecond signal observed in AF phase.

Abstract

Time-Domain Thermoreflectance (TDTR) characterization of FeRh throughout its first-order antiferromagnetic (AF) to ferromagnetic (FM) transition shows that the transient reflectance, $\Delta$R(t)/R, strongly depends on the magnetic order of the sample. Using TDTR, which uses optical pulses to induce small temperature excursions, we have found that the $\Delta$R(t)/R of the AF phase exhibits a large negative response, while the response of the FM phase is positive. This magnetic phase sensitivity has allowed us to study the transient response of both the AF and FM phase to the pump pulse excitation and the mixed phase of the material. These results are significant since the ultrafast properties of antiferromagnetic materials and mixed antiferromagnetic and ferromagnetic materials are difficult to detect using other conventional techniques.We have found that the AF phase exhibits a strong subpicosecond signal not observed in the FM phase. The magnetic phase dependence of the sign of $\Delta$R(t)/R is qualitatively explained using the results of ab-initio density functional theory (DFT) calculations. Using the two-temperature model, we found that the change in the thermalization time across the transition is caused by differences in both the electronic heat capacity and the electron-phonon coupling factor of the AF and FM phases. The electron-phonon coupling constant in the AF phase is also determined using the two-temperature model conducted using the NTMpy code package. For the FM phase, we provide boundaries for the magnitude of the electron-phonon coupling factor for the FM phase. These results indicate that TDTR can be used to study the transient properties of magnetic materials that are otherwise challenging to probe.