  Phonon properties and the lattice thermal conductivity are not generally
understood to be sensitive to magnetic fields. Using an applied field to change
MnBi2Te4 between antiferromagnetic (AFM), canted (CAFM) and ferromagnetic (FM)
phases we discovered a new way to control the lattice thermal conductivity,
generating both a positive and a negative magnetic field dependence. We report
the field dependence of the thermal conductivity, k, in the in-plane direction
under an applied magnetic field along the cross-plane direction in MnBi2Te4
from 2K to 30K. k decreases with field in the AFM phase, saturates 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 which magnon-phonon
scattering processes are allowed by energy conservation. We also report
magneto-Seebeck coefficient, Nernst coefficient and thermal Hall data measured
in the same configuration. This finding may open a way to design magnetically
controlled heat switches.
