Multiferroicity and spiral magnetism in FeVO4 with quenched Fe orbital moments

FeVO4 has been studied by heat capacity, magnetic susceptibility, electric polarization and single-crystal neutron-diffraction experiments. The triclinic crystal structure is made of S-shaped clusters of six Fe3+ ions, linked by VO43- groups. Two long-range magnetic ordering transitions occur at T-N1=22 K and T-N2=15 K. Both magnetic structures are incommensurate and below T-N2, FeVO4 becomes weakly ferroelectric coincidentally with the loss of the collinearity of the magnetic structure in a very similar fashion than in the classical TbMnO3 multiferroic material. However we argue that the symmetry considerations and the mechanisms invoked to explain these properties in TbMnO3 do not straightforwardly apply to FeVO4. First, the magnetic structures, even the collinear structure, are all acentric so that ferroelectricity in FeVO4 is not correlated with the fact magnetic ordering is breaking inversion symmetry. Regarding the mechanism, FeVO4 has quenched orbital moments that questions the exact role of the spin-orbit interactions.