Determination of the Local Symmetry and the Multiferroic-ferromagnetic Crossover in Ni3-xCoxV2O8 by using Raman Scattering Spectroscopy

Comprehensive vibrational studies on the multiferroic-to-normal-ferromagnetic crossover in isostructural Ni3-xCoxV2O8 (NCVO, x = 0 - 3.0) are performed using Raman scattering spectroscopy. The systematically red-shifted phonon modes are discussed in terms of the mode Gr\"uneisen parameters, and are interpreted as a chemical pressure effect. In addition, we present evidence that the local symmetry is broken in the multiferroic phase.