Optical spectroscopic signatures of the magnetic and the structural phase transitions in the dielectric functions of the MnV₂O₄ spinel ferrimagnet

The temperature-dependent complex dielectric functions of the MnV₂O₄ ferrimagnetic spinel were investigated at temperatures from 15 K to 300 K in the spectral range of 0.7–4.5 eV by using a spectroscopic ellipsometry. The imaginary part of the dielectric function exhibited an optical gap of approximately 0.8 eV and a broad peak around 1.8 eV. The origin of the peak was assigned as the charge excitation between neighboring V sites with the same spin orientation. The dielectric functions at 1.2 eV, at 1.8 eV, and at 2.5 eV were investigated by increasing the temperature in 0.4 K steps at the temperatures near the structural phase transition temperature (58 K) and the magnetic phase transition temperature (61 K). The temperature-scanned dielectric functions exhibited the sharp and clear signatures of phase transitions, however the temperature dependencies at the three photon energies were different. Analysis of the temperature depedence shows that the width of the spin-polarized transition between the vanadium Hubbard bands can be quite narrow and that the interpretation of multiple vanadium Hubbard bands with different orbital correlation cannot be appropriate for the origin of the full spectral band from the optical gap to an energy of about 3 eV.