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

摘要

The ferrimagnetic spinels MnB2O4 (B = Mn,V) exhibit a similar series ofclosely spaced magnetic and structural phase transitions at low temperatures,reflecting both magnetic frustration and a strong coupling between the spin andlattice degrees of freedom. Careful studies of excitations in MnB2O4 (B =Mn,V), and the evolution of these excitations with temperature, are importantfor obtaining a microscopic description of the role that magnetic excitationsand spin-lattice coupling play in the low temperature phase transitions ofthese materials. We report an inelastic light (Raman) scattering study of thetemperature and magnetic field dependences of one- and two-magnon excitationsin 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=0magnon that has been reported for MnV2O4 based on previous neutron scatteringmeasurements and spin wave calculations. Additionally, we find that thetwo-magnon energy of MnV2O4 decreases ("softens") with decreasing temperaturebelow T_{N}, which we attribute to strong coupling between magnetic andvibrational excitations near the zone boundary.