Magnon spectra and strong spin-lattice coupling in magnetically frustrated MnV$_2$O$_4$: Inelastic light scattering studies*

The spinel MnV$_2$O$_4$ exhibits a series of closely spaced magnetic and structural transitions at low temperatures, reflecting magnetic frustration and strong spin-lattice coupling. MnV$_2$O$_4$ has a canted ferrimagnetic ground state with an undetermined orbital configuration. Temperature dependent studies of magnetic and vibrational excitations in MnV$_2$O$_4$ are important for determining the role that spin-lattice coupling plays in the low temperature phase transitions of this material and setting constraints on the orbital ground state configuration. We report an inelastic light (Raman) scattering study of the temperature and magnetic field dependences of magnetic excitations in MnV$_2$O$_4$. We observe a pair of $\textbf{q}=0$ one-magnon modes at 74 cm$^{-1}$ and 81 cm$^{-1}$, which is in contrast with the single 80 cm$^{-1}$ $\textbf{q}=0$ magnon that has been reported for MnV$_2$O$_4$ from previous neutron scattering measurements and spin wave calculations. Additionally, we find that the two-magnon energy of MnV$_2$O$_4$ decreases with decreasing temperature below $T_N$, which we attribute to strong coupling between zone-boundary magnons and phonons. These results offer important clues to the orbital ground state and the nature of spin-lattice coupling in MnV$_2$O$_4$.