Strain-induced structural instability in FeRh

TL;DR

This study uses density functional theory to explore how epitaxial strain affects the structural and magnetic phases of FeRh, revealing a metastable fcc-like structure and strain-induced instabilities relevant for memory device applications.

Contribution

It demonstrates the strain-dependent structural energetics of FeRh, identifies a metastable fcc-like phase, and explains the instability using non-linear elasticity theory.

Findings

Metastable fcc-like AFM structure exists under strain

Strain causes softening and sign change of shear modulus

Additional lattice instability predicted at low temperature

Abstract

We perform density functional calculations to investigate the structure of the inter-metallic alloy FeRh under epitaxial strain. Bulk FeRh exhibits a metamagnetic transition from a low-temperature antiferromagnetic (AFM) phase to a ferromagnetic (FM) phase at 350K, and its strain dependence is of interest for tuning the transition temperature to the room-temperature operating conditions of typical memory devices. We find an unusually strong dependence of the structural energetics on the choice of exchange-correlation functional, with the usual local density approximation (LDA) yielding the wrong ground-state structure, and generalized gradient (GGA) extensions being in better agreement with the bulk experimental structure. Using the GGA we show the existence of a metastable face-centered-cubic (fcc)-like AFM structure that is reached from the ground state body-centered-cubic (bcc) AFM structure by following the epitaxial Bain path. We predict that this metastable fcc-like structure has a significantly higher conductivity than the bcc AFM phase. We show that the behavior is well described using non-linear elasticity theory, which captures the softening and eventual sign change of the orthorhombic shear modulus under compressive strain, consistent with this structural instability. Finally, we predict the existence of an additional unit-cell-doubling lattice instability, which should be observable at low temperature.