Optical properties and magnetic-field-induced phase transitions in the ferroelectric state of Ni3V2O8:: Experiments and first-principles calculations

We use a combination of optical spectra, first-principles calculations, and energy-dependent magneto-optical measurements to elucidate the electronic structure and to study the phase diagram of Ni3V2O8. We find a remarkable interplay of magnetic field and optical properties that reveals additional high magnetic-field phases and an unexpected electronic structure, which we associate with the strong magnetodielectric couplings in this material over a wide energy range. Specifically, we observed several prominent magnetodielectric effects that derive from changes in the crystal-field environment around Ni spine and cross tie centers. This effect is consistent with a field-induced modification of local structure. Symmetry-breaking effects are also evident with temperature. We find Ni3V2O8 to be an intermediate-gap, local-moment band insulator. This electronic structure is particularly favorable for magnetodielectric couplings, because the material is not subject to the spin-charge separation characteristic of strongly correlated large-gap Mott insulators, while at the same time remaining a magnetic insulator independent of the particular spin order and temperature.