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Magnon spectra and strong spin-lattice coupling in magnetically frustrated MnB2O4 (B = Mn,V): Inelastic light scattering studies

The ferrimagnetic spinels MnB2O4 (B = Mn,V) exhibit a similar series of closely spaced magnetic and structural phase transitions at low temperatures, reflecting both magnetic frustration and a strong coupling between the spin and lattice degrees of freedom. Careful studies of excitations in MnB2O4 (B = Mn,V), and the evolution of these excitations with temperature, are important for obtaining a microscopic description of the role that magnetic excitations and spin-lattice coupling play in the low temperature phase transitions of these materials. We report an inelastic light (Raman) scattering study of the temperature and magnetic field dependences of one- and two-magnon excitations in MnV2O4 and Mn3O4. We observe a pair of q=0 one-magnon modes at 74 cm^{-1} and 81 cm^{-1} in MnV2O4, which is in contrast with the single 80 cm^{-1} q=0 magnon that has been reported for MnV2O4 based on previous neutron scattering measurements and spin wave calculations. Additionally, we find that the two-magnon energy of MnV2O4 decreases ("softens") with decreasing temperature below T_{N}, which we attribute to strong coupling between magnetic and vibrational excitations near the zone boundary.