Coupled Spin-lattice Dynamics across a Magnetostructural Phase Transition

TL;DR

Problem: The microscopic role of lattice dynamics in first-order magnetocaloric materials remains debated, particularly across the MnAs magnetostructural transition around $T_C \approx 318$ K. Approach: finite-temperature first-principles spin–lattice dynamics (SLD) simulations capture the coupled evolution of magnetization and phonons, enabling separation of lattice and magnetic contributions to the isothermal entropy change $\Delta S_T = \Delta S_M + \Delta S_L + \Delta S_{PT}$. Key findings: two lattice contributions have opposite signs and comparable magnitudes: magnetic-field–driven phonon hardening yields $\Delta S_L \approx -9.35\ J\,kg^{-1}\,K^{-1}$, while lattice entropy change from the hexagonal–orthorhombic transition gives $\Delta S_{PT} \approx +9.64\ J\,kg^{-1}\,K^{-1}$, together with a magnetic contribution $\Delta S_M$, leading to a total $\Delta S_T$ near $-30\ J\,kg^{-1}\,K^{-1}$ at $H=5$ T; phonon dispersions are highly field-tunable, enabling magnetic-field–controlled thermal transport. Significance: establishes a unified microscopic framework for spin–lattice coupling in first-order magnetocaloric materials and suggests design principles for enhanced caloric effects and switchable thermal conductivity in MnAs and related compounds.

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.

Figures (4)

• Figure 1: Magnetostructural transition in MnAs. (a) Schematic illustrating the structural transition in MnAs from the hexagonal phase to the orthorhombic phase at $T_C$ of 318 K. Arrows in the orthorhombic phase highlight the atomic distortions during the transition. (b) Simulated lattice constants of both hexagonal ("Hex") and orthorhombic ("Ortho") phases of MnAs along different directions as a function of temperature. The directions are labeled according to the convention shown in (a). The blue dashed line marks the experimental $T_C$ of 318 K. (c) Simulated Gibbs free energy of both hexagonal and orthorhombic phases of MnAs as a function of temperature. The blue dashed line marks the crossing that occurs around 323 K, close to the experimental $T_C$ of 318 K.
• Figure 2: Simulated magnetization and isothermal entropy change in hexagonal MnAs. (a) Simulated zero-field magnetization of hexagonal MnAs as a function of temperature using both ASD and SLD. Simulated isothermal entropy change in hexagonal MnAs as a function of temperature and magnetic field using (b) ASD and (c) SLD.
• Figure 3: Phonon properties of MnAs near the magnetostructural transition. (a) Simulated phonon dispersions of hexagonal MnAs at zero-field as a function of temperature, showing softening of the TA phonon at $M$ as the temperature approaches $T_C$. (b) Simulated phonon dispersions of hexagonal MnAs at 320 K as a function of magnetic field, showing the hardening effect of the TA phonon by the magnetic field. (c) Simulated phonon dispersion of the orthorhombic MnAs at 320 K. (d) Simulated zero-field phonon density of states of hexagonal and orthorhombic MnAs at 320 K.
• Figure 4: Thermal conductivity of MnAs near the magnetostructural transition. (a) Lattice thermal conductivity along different directions and the electronic thermal conductivity ("$\kappa_{\mathrm{elec}}$") of both phases of MnAs as a function of temperature. The electronic thermal conductivity is calculated using Wiedemann-Franz law using electrical conductivity data from bean1962magnetic. (b) Lattice thermal conductivity along different directions and the electronic thermal conductivity ("$\kappa_{\mathrm{elec}}$") of hexagonal MnAs at 320 K as a function of magnetic field. The electronic thermal conductivity is calculated using Wiedemann-Franz law using magnetoresistance data from mira2003structural.