Effect of Nonmagnetic Substituents Mg and Zn on the Phase Competition in the Multiferroic Antiferromagnet MnWO₄

The effects of substituting nonmagnetic Mg 2+ and Zn 2+ ions for the Mn 2+ ( S = 5/2) ions on the structural, magnetic, and dielectric properties of the multiferroic frustrated antiferromagnet MnWO 4 were investigated. Polycrystalline samples of Mn 1− x Mg x WO 4 and Mn 1− x Zn x WO 4 (0 ≤ x ≤ 0.3) solid solutions were prepared via a solid-state route and characterized via X-ray and neutron diffraction, magnetization, and dielectric permittivity measurements. Mg and Zn substitutions result in very similar effects. The Néel temperature ( T N ), the AF3-to-AF2 magnetic phase-transition temperature ( T 2 ), and the critical ferroelectric temperature ( T c = T 2 ) of MnWO 4 are reduced upon nonmagnetic doping. At the lowest temperature ( T = 1.5 K), the incommensurate magnetic structure for x (Mg) = 0.15 and x (Zn) = 0.15 corresponds to either a sinusoidal spin arrangement or an elliptical spin-spiral phase similar to the polar AF2 structure observed in MnWO 4 . These findings were discussed by considering the effects of the Mg and Zn substitutions on the crystal lattice and on the spin exchange network of MnWO 4 .