Determination of the local symmetry and the multiferroic-ferromagnetic crossover in Ni 3− x Co x V 2 O 8 by using Raman scattering spectroscopy

Comprehensive vibrational studies on the multiferroic-to-normal-ferromagnetic crossover in isostructural Ni 3− x Co x V 2 O 8 (NCVO, x = 0–3.0) are performed using Raman scattering spectroscopy. The systematically red-shifted phonon modes are discussed in terms of the mode Grüneisen parameters, and are interpreted as a chemical pressure effect. In addition, we present evidence that the local symmetry is broken in the multiferroic phase.