Impact of lattice dynamics on the phase stability of metamagnetic FeRh: Bulk and thin films

TL;DR

This study combines DFT and NRIXS to analyze phonon behavior and thermodynamics in FeRh, revealing strong spin-phonon coupling, lattice instabilities, and the influence of epitaxial strain on phase stability.

Contribution

It provides a detailed microscopic understanding of phonon and lattice effects on FeRh's metamagnetic transition, including the prediction of a new monoclinic phase from first principles.

Findings

Distinct VDOS differences confirm strong spin-phonon coupling.

Epitaxial strain significantly affects the ferromagnetic phase's lattice dynamics.

A stable monoclinic phase is predicted but not observed in thin films.

Abstract

We present phonon dispersions, element-resolved vibrational density of states (VDOS) and corresponding thermodynamic properties obtained by a combination of density functional theory (DFT) and nuclear resonant inelastic X-ray scattering (NRIXS) across the metamagnetic transition of B2 FeRh in the bulk material and thin epitaxial films. We see distinct differences in the VDOS of the antiferromagnetic (AF) and ferromagnetic (FM) phase which provide a microscopic proof of strong spin-phonon coupling in FeRh. The FM VDOS exhibits a particular sensitivity to the slight tetragonal distortions present in epitaxial films, which is not encountered in the AF phase. This results in a notable change in lattice entropy, which is important for the comparison between thin film and bulk results. Our calculations confirm the recently reported lattice instability in the AF phase. The imaginary frequencies at the $X$-point depend critically on the Fe magnetic moment and atomic volume. Analyzing these non vibrational modes leads to the discovery of a stable monoclinic ground state structure which is robustly predicted from DFT but not verified in our thin film experiments. Specific heat, entropy and free energy calculated within the quasiharmonic approximation suggest that the new phase is possibly suppressed because of its relatively smaller lattice entropy. In the bulk phase, lattice degrees of freedom contribute with the same sign and in similar magnitude to the isostructural AF-FM phase transition as the electronic and magnetic subsystems and therefore needs to be included in thermodynamic modeling.