Magnon gap mediated lattice thermal conductivity in MnBi₂Te₄

In magnetic materials with strong spin-lattice coupling, magnon-phonon interactions can change the sensitivity of the lattice thermal conductivity in an applied magnetic field. Applying an out-of-plane magnetic field to change MnBi₂Te₄ between antiferromagnetic (AFM), canted antiferromagnetic (CAFM), and ferromagnetic (FM) phases, we controlled the lattice thermal conductivity, generating both a positive and a negative magnetic field dependence. The in-plane thermal conductivity decreases with field in the AFM phase, remains approximately constant in the CAFM phase, and increases with field in the FM phase. We explain this in terms of the field-induced changes of the magnon gap which modifies magnon-phonon scattering. We also report thermal Hall data measured in the same configuration.