Ferroelectricity in the multiferroic delafossite CuFeO2 induced by ion doping or magnetic field

On the basis of a microscopic model and using the Green's function technique we have studied the induced ferroelectric polarization P[110] along the [110] direction in the triangular CuFeO2. It is shown that P[110] is induced by an external magnetic field parallel to the c axis hc or a nonmagnetic ion substitution which both can break the inversion symmetry of the system. Our Hamiltonian consists of Heisenberg antiferromagnetic interactions in and out of the ab plane and anharmonic spin-phonon interactions. P[110] decreases with increasing the temperature T. It appears at T ∼ 9 K and begins to increase below T ∼ 9 K, which is the transition temperature to a noncollinear phase. This behavior is an evidence for the multiferroic character of CuFeO2. By doping with Rh ions the polarization has a maximum in the curve P[110](x) with increasing Rh concentration x whereas by Sc doping it decreases. The observed results are in qualitative agreement with the experimental data.