Impact of lattice dynamics on the phase stability of metamagnetic FeRh: Bulk and 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) phases, 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 nonvibrational 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 vibrations contribute with the same sign and in similar magnitude to the isostructural AF-FM phase transition as excitations of the electronic and magnetic subsystems demonstrating that lattice degrees of freedom need to be included in thermodynamic modeling.

Hysteresis Design of Magnetocaloric Materials—From Basic Mechanisms to Applications

“…Both properties may change at a magnetostructural transition. This is expressed quantitatively in terms of the vibrational density of states (VDOS) g ( ϵ ), which can be measured by inelastic neutron scattering or nuclear resonant inelastic X‐ray scattering . VDOS and phonon dispersion relations have been calculated from first principles achieving excellent agreement with experimental data .…”

“…This is expressed quantitatively in terms of the vibrational density of states (VDOS) g ( ϵ ), which can be measured by inelastic neutron scattering or nuclear resonant inelastic X‐ray scattering . VDOS and phonon dispersion relations have been calculated from first principles achieving excellent agreement with experimental data . The expression for S lat resembles S el , where the expression in the parentheses takes into account the Bosonic character of the phonons through the Bose–Einstein distribution function n ( ϵ , T )=(exp( ϵ / k B T )−1) −1 for the occupation numbers, which becomes large for high T or small phonon energies ϵ : …”

“…Thus, we can expect considerable contributions from S el to the magnetocaloric effect if strong features in the DOS right at appear or disappear during a magnetostructural transition. This is for instance the case for La(Fe x Si 1− x ) 13 or FeRh . For second‐order transitions, S el can be considered to be unimportant.…”

“…As we will discuss below, the three contributions to Δ S can have the same sign. This is the case for the magnetocaloric systems La(Fe x Si 1− x ) 13 and FeRh . However, one strictly cannot assume that the degrees of freedom are totally independent.…”

Properties and Decomposition of Heusler Alloys

“…It has been shown that a magnetic state change of Rh between antiferromagnetism (with a Rh moment of 0 μ B ) and ferromagnetism (≈0.9 μ B ), accompanied by a significant change in electronic structure, combine as the main origin of the metamagnetic transition . To be more precise, it is the cooperative contribution of electronic, magnetic, and vibrational degrees of freedom that is responsible for the giant magnetocaloric effect in FeRh . The alloy Ni 45 Co 5 Mn 32 Cr 5 In 13 is stable with respect to decomposition into all individual elements.…”