Coupled Spin-lattice Dynamics across a Magnetostructural Phase Transition

TL;DR

This study uses first-principles simulations to elucidate how spin-lattice coupling influences magnetocaloric effects and thermal conductivity in MnAs across a phase transition, revealing competing entropy contributions and tunable thermal properties.

Contribution

It provides a microscopic understanding of spin-lattice interactions during phase transitions in magnetocaloric materials, highlighting the role of phonon dynamics and magnetic fields.

Findings

Magnetic-field-induced phonon hardening increases lattice entropy by ~23%.

Lattice entropy change from structural transition partially cancels phonon hardening effects.

Phonon spectrum's magnetic-field dependence enables thermal conductivity tunability.

Abstract

First-order magnetostructural phase transitions underpin giant magnetocaloric effects, yet the microscopic role of lattice dynamics in these transitions remains controversial. Here we use first-principles spin-lattice dynamics simulations to investigate the coupled evolution of magnetization and phonon dispersions across the magnetostructural transition in MnAs. Our simulations quantitatively reproduce the experimentally observed Curie temperature, lattice contraction, and free-energy crossing between hexagonal and orthorhombic phases. We show that below the Curie temperature, magnetic-field-induced hardening of soft phonon modes gives rise to a sizable lattice entropy contribution that enhances the total isothermal entropy change by approximately 23% under a 5 T field. In contrast, the lattice entropy change associated with the structural phase transition itself has an opposite sign and partially compensates the lattice contribution due to field-induced phonon hardening. This competition reconciles long-standing discrepancies in the interpretation of magnetocaloric entropy measurements across first-order transitions. In addition, we demonstrate that the strong magnetic-field dependence of the phonon spectrum near the transition enables large tunability of lattice thermal conductivity, highlighting MnAs as a promising platform for magnetic-field-controlled thermal switching. Our results establish a unified microscopic picture of spin-lattice coupling in first-order magnetocaloric materials and provide design principles for engineering enhanced caloric and thermal transport functionalities.