Multiferroicity and spiral magnetism inFeVO4with quenched Fe orbital moments

Abstract

${\text{FeVO}}_4$ 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 ${\text{Fe}}^{3+}$ ions, linked by ${\text{VO}}_4^{3-}$ groups. Two long-range magnetic ordering transitions occur at $T_{N1}=22\text{K}$ and $T_{N2}=15\text{K}$. Both magnetic structures are incommensurate and below $T_{N2}$, ${\text{FeVO}}_4$ becomes weakly ferroelectric coincidentally with the loss of the collinearity of the magnetic structure in a very similar fashion than in the classical ${\text{TbMnO}}_3$ multiferroic material. However we argue that the symmetry considerations and the mechanisms invoked to explain these properties in ${\text{TbMnO}}_3$ do not straightforwardly apply to ${\text{FeVO}}_4$. First, the magnetic structures, even the collinear structure, are all acentric so that ferroelectricity in ${\text{FeVO}}_4$ is not correlated with the fact magnetic ordering is breaking inversion symmetry. Regarding the mechanism, ${\text{FeVO}}_4$ has quenched orbital moments that questions the exact role of the spin-orbit interactions.