Increasing the Phase-Transition Temperatures in Spin-Frustrated Multiferroic MnWO₄ by Mo Doping

Abstract

Ceramic samples of MnW_1–<i>x</i>Mo_<i>x</i>O₄ (<i>x</i> ≤ 0.3) solid solution were prepared by a solid-state route with the goal of increasing the magnitude of the spin-exchange couplings among the Mn²⁺ ions in the spin spiral multiferroic MnWO₄. Samples were characterized by X-ray diffraction, optical spectroscopy, magnetization, and dielectric permittivity measurements. It was observed that the Néel temperature <i>T</i>_N, the spin spiral ordering temperature <i>T</i>_M2, and the ferroelectric phase-transition temperature <i>T</i>_FE2 of MnWO₄ increased upon the nonmagnetic substitution of Mo⁶⁺ for W⁶⁺. Like pure MnWO₄, the ferroelectric critical temperature <i>T</i>_FE2(<i>x</i>) coincides with the magnetic ordering temperature <i>T</i>_M2(<i>x</i>). A density functional analysis of the spin-exchange interactions for a hypothetical MnMoO₄ that is isostructural with MnWO₄ suggests that Mo substitution increases the strength of the spin-exchange couplings among Mn²⁺ in the vicinity of a Mo⁶⁺ ion. Our study shows that the Mo-doped MnW_1–<i>x</i>Mo_<i>x</i>O₄ (<i>x</i> ≤ 0.3) compounds are spin-frustrated materials that have higher magnetic and ferroelectric phase-transition temperatures than does pure MnWO₄