Magnon spectra and strong spin-lattice coupling in magnetically frustrated MnB_2O_4 (B = Mn,V): Inelastic light-scattering studies

摘要

The ferrimagnetic spinels MnB_2O_4 (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 MnB2O_4 (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 MnV_2Q_4 and Mn_3O_4. We observe a pair of q = 0 one-magnon modes at 74 and 81 cm~(-1) in MnV_2O_4, which is in contrast with the single 80-cm~(-1) q = 0 magnon that has been reported for MnV_2O_4 based on previous neutron-scattering measurements and spin-wave calculations. Additionally, we find that the two-magnon energy of MnV_2O_4 decreases ("softens") with decreasing temperature below T_N, which we attribute to strong coupling between magnetic and vibrational excitations near the zone boundary.