Inelastic light scattering measurements of structural phase coexistence in ferrimagnetic spinel Mn$_3$O$_4$

The ferrimagnetic spinel Mn$_3$O$_4$ has a number of functional properties, e.g., magnetodielectricity, that are ascribed to a coupling between the spins and lattice of this material. Such a coupling is manifested in the symmetry-lowering structural distortion that occurs when Mn$_3$O$_4$ magnetically orders at $T=33$~K. A recent x-ray diffraction study$^2$ of polycrystalline Mn$_3$O$_4$ found that this distortion is not fully realized, i.e., the high-symmetry and low-symmetry structures coexist below $T=33$~K due to strains from lattice mismatch. To extend this work, we use variable-pressure and variable-magnetic-field inelastic light scattering spectroscopy to study structural phase coexistence in single crystals of Mn$_3$O$_4$. We confirm the coexistence of tetragonal (high-symmetry) and orthorhombic (low-symmetry) phases below $T=33$~K. Furthermore, we demonstrate that the application of hydrostatic pressure suppresses the remnant tetragonal phase, while the application of magnetic field can bolster this phase. These results indicate that microscopic descriptions of functional behavior in Mn$_3$O$_4$ should consider effects due to structural phase coexistence. [2]~M.~C. Kemei, {\it et~al.\/}, {\it Phys. Rev. B\/} {\bf 89}, 174410 (2014).