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.
Highlights
During recent years, ordered B2 FeRh (CsCl structure) has received increased attention due to its extraordinary properties, in particular its temperature-driven isostructural transition between a ferromagnetic (FM) and antiferromagnetic (AF) phase at TM ∼ 350 K, which was discovered more than seven decades ago [1,2,3,4]
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
The FM VDOS exhibits a particular sensitivity to the slight tetragonal distortions present in epitaxial films, which is not encountered in the AF phase
Summary
During recent years, ordered B2 FeRh (CsCl structure) has received increased attention due to its extraordinary properties, in particular its temperature-driven isostructural transition between a ferromagnetic (FM) and antiferromagnetic (AF) phase at TM ∼ 350 K, which was discovered more than seven decades ago [1,2,3,4]. The advantage is that the first-order phase transition of FeRh can be tailored well below the Curie temperature of FePt, which relaxes the lifetime and reliability problem of the NFCs. during recording, the magnetic moment of FeRh is still high overcoming the problem of thermally written-in errors [14]. Deak and coworkers evaluated the magnetic and electronic contributions to G in their relativistic disordered local moment (DLM) approach [53] They were able to reproduce a transition from AF to FM, albeit at a rather large temperature and atomic volume. A direct comparison of these calculated quantities with experimental data is inevitable to evaluate a specific model This requires the detailed knowledge of all individual contributions to the Gibbs free energy difference between the AF and FM phase G(T ,p), which is usually divided into vibrational, magnetic and electronic degrees of freedom ( G = Gvib + Gmag + Gel). The calculated results are found to be in excellent agreement with available experimental data
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