Temperature Dependent Magnon-Phonon Coupling in bcc Fe from Theory and Experiment

Abstract

An ab initio based framework for quantitatively assessing the phonon contribution due to magnonphonon interactions and lattice expansion is developed. The theoretical results for bcc Fe are in very good agreement with high-quality phonon frequency measurements. For some phonon branches, the magnonphonon interaction is an order of magnitude larger than the phonon shift due to lattice expansion, demonstrating the strong impact of magnetic short-range order even significantly above the Curie temperature. The framework closes the previous simulation gap between the ferro-and paramagnetic limits. DOI: 10.1103/PhysRevLett.113.165503 PACS numbers: 63.20.K-, 63.20.dd, 63.20.dk, 75.50.Bb An understanding of the mutual interaction between different temperature-induced excitations in solids is a pivotal challenge for the simulation of thermodynamic properties of many materials. Experimental studies of phonons at elevated temperatures can help elucidate magnon-phonon coupling. Neutron scattering experiments of phonon dispersions have provided important data at selected temperatures [11]. Nuclear resonant inelastic x-ray scattering measurements are more amenable for showing thermal trends with measurements at many temperatures, and we have recently performed nuclear resonant inelastic x-ray scattering measurements of the phonon density of states of bcc Fe at 38 temperatures through the Curie transition [12]. Nonharmonic changes in the phonon DOS and vibrational entropy were found to track the change in magnetization with temperature. Since experimental analysis of phonon DOS broadening suggests explicit anharmonic contributions from phonon-phonon interactions to be an order of magnitude smaller, these new results are suggestive of large magnon-phonon interactions in bcc Fe.Parameter-free electronic structure calculations like density functional theory (DFT) in principle provide access to interatomic forces, spin-polarized energetics, and their interactions. Force-constant calculations and spin simulations have indeed been performed for decades [10,13]. However, most studies have been restricted to separate investigations of the two effects, whereas their mutual coupling could only be addressed in recent years [1,[14][15][16]. The T ¼ 0 K limit of a ferromagnetic system like Fe is the most straightforward case [17] since calculating force constants for a single magnetic configuration with all spins pointing in the same direction is sufficient ("FM limit" in Fig. 1). The infinite-temperature limit of a paramagnetic system with fully disordered spins ("PM limit") is significantly more challenging due to the large magnetic phase space that needs to be sampled for an accurate prediction of the coupling. Significant progress has been made only very recently with techniques based on DLM and spin molecular dynamics [1,18], a spin-spiral approach [15], dynamical mean field theory [14], and a spin-space averaging procedure [16].Given the complexity of the problem, present day methods are currently applied only at very low temperat...

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