Incommensurate orbital modulation behind ferroelectricity in CuFeO2.

ABSTRACT

CuFeO(2) is one of the multiferroic materials and is the first case that the electric polarization is not explained by the magnetostriction model or the spin-current model. We have studied this material using soft x-ray resonant diffraction and found that superlattice reflection 0 1-2q 0 appears in the ferroelectric and incommensurate magnetic ordered phase at the Fe L(2,3) absorption edges and moreover that the rotation of the x-ray polarization such as from σ to π or from π to σ is allowed at this reflection. These findings definitely provide direct evidence that the 3d t(2g↓) orbital state of Fe ions has a long-range order in the ferroelectric state. The spin-orbit interaction in Fe ions plays a crucial role to the ferroelectricity in CuFeO(2), coupling two nontrivial spin and orbital orders, both of which break the crystal symmetry.