Magnetoelastic coupling and spin contributions to entropy and thermal transport in biferroic yttrium orthochromite.

Direct engineering of material properties through exploitation of spin, phonon, and charge-coupled degrees of freedom is an active area of development in materials science. However, the relative contribution of the competing orders to controlling the desired behavior is challenging to decipher. In particular, the independent role of phonons, magnons, and electrons, quasiparticle coupling, and relative contributions to the phase transition free energy largely remain unexplored, especially for magnetic phase transitions. Here, we study the lattice and magnetic dynamics of biferroic yttrium orthochromite using Raman, infrared, and inelastic neutron spectroscopy techniques, supporting our experimental results with first-principles lattice dynamics and spin-wave simulations across the antiferromagnetic transition at $T_{\rm N}\sim 138$\,K. Spectroscopy data and simulations together with the heat capacity ($C_p$) measurements, allow us to quantify individual entropic contributions from phonons (0.01$\pm0.01$\,$k_{\rm B}$\,atom$^{-1}$), dilational (0.03$\pm0.01$\,$k_{\rm B}$\,atom$^{-1}$), and magnons (0.11$\pm0.01$\,$k_{\rm B}$\,atom$^{-1}$) across $T_{\rm N}$. High-resolution phonon measurements conducted in a magnetic field show that anomalous $T$-dependence of phonon energies across $T_{\rm N}$ originates from magnetoelastic coupling. Phonon scattering is primarily governed by the phonon-phonon coupling, with little contribution from magnon-phonon coupling, short-range spin correlations, or magnetostriction effects; a conclusion further supported by our thermal conductivity measurements conducted up to 14 Tesla, and phenomenological modeling.