Strain-induced structural instability in FeRh

We perform density functional calculations to investigate the structure of the intermetallic alloy FeRh under epitaxial strain. Bulk FeRh exhibits a metamagnetic transition from a low-temperature antiferromagnetic (AFM) phase to a ferromagnetic phase at 350 K, 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 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-like AFM structure that is reached from the ground-state body-centered-cubic-like AFM structure by following the epitaxial Bain path. We show that the behavior is well described using nonlinear 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.

Figure 1

Magnetic phases of FeRh: (a) antiferromagnetic (AFM-II) and (b) ferromagnetic order.

Figure 2

Energy vs cubic lattice parameter for the nonmagnetic (NM, blue), antiferromagnetic (AFM, black) and ferromagnetic (FM, red) phases of FeRh, computed using (a) the LDA and (b) the GGA.

Figure 3

Calculated strain dependence of the energy of the AFM phase computed at (a) the GGA-PBE and (b) the LDA level along with (c) their respective $c/a$ ratios (solid lines, left axis) and volume (dashed lines, right axis).

Figure 4

Electronic densities of states (DOS) for (a) the bcc-like FM, (b) the bcc-like AFM, and (c) the metastable fcc-like AFM phases.

Figure 5

(a) Compressive strain dependence of the phonon dispersion in AFM FeRh computed at the GGA level. In (b) we show the evolution of the frequency as a function of strain at the unstable R point. In (c) and (d) we show the high-symmetry points in the tetragonal Brillouin zone and the resulting structural distortion, respectively. Panel (e) shows the evolution of the phonon dispersion in FM FeRh as a function of compressive strain.

Figure 6

(a) GGA+$U$ relative energy of the AFM phase with $U$ values between 0 and 1 eV and (b) the corresponding magnetic moments.

Figure 7

(a) Change in elastic energy (per formula unit) at a finite isotropic strain with respect to a volume-conserving orthorhombic strain for the AFM FeRh system. For each color the points show the DFT data and the lines the Eq. (A5) fit for a specific value of isotropic strain. (b) Dependence of the AFM shear moduli $C^{\prime }$ on isotropic strain according to Eq. (2). Here different orders with respect to the isotropic strain are shown. Convergence under compression is achieved at fourth order.