Effects of nickel doping on the multiferroic and magnetic phases of MnWO₄

The various orders in multiferroic materials with a frustrated spiral spin modulation inducing a ferroelectric state are extremely sensitive to small perturbations such as magnetic and electric fields, external pressure, or chemical substitutions. A classical multiferroic, the mineral Hübnerite with chemical formula MnWO₄, shows three different magnetic phases at low temperature. The intermediate phase between 7.5 K < T < 12.7 K is multiferroic and ferroelectricity is induced by an inversion symmetry breaking spiral Mn-spin order and strong spin-lattice interactions. The substitution of Ni²⁺ (spin 1) for Mn²⁺ (spin 5/2) in MnWO₄ and its effects on the magnetic and multiferroic phases are studied. The ferroelectric phase is stabilized for low Ni content (up to 10%). Upon further Ni doping, the polarization in the ferroelectric phase is quickly suppressed while a collinear and commensurate magnetic phase, characteristic of the magnetic structure in NiWO₄, appears first at higher temperature, gradually extends to lower temperature, and becomes the ground state above 30% doping. Between 10% and 30%, the multiferroic phase coexists with the collinear commensurate phase. In this concentration region, the spin spiral plane is close to the a-b plane which explains the drop of the ferroelectric polarization. The phase diagram of Mn_1-xNi_xWO₄ is derived by a combination of magnetic susceptibility, specific heat, electric polarization, and neutron scattering measurements.