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

Ceramic samples of MnW 1– x Mo x O 4 ( x ≤ 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 Mn 2+ ions in the spin spiral multiferroic MnWO 4 . Samples were characterized by X-ray diffraction, optical spectroscopy, magnetization, and dielectric permittivity measurements. It was observed that the Néel temperature T N, the spin spiral ordering temperature T M2, and the ferroelectric phase-transition temperature T FE2 of MnWO 4 increased upon the nonmagnetic substitution of Mo 6+ for W 6+ . Like pure MnWO 4, the ferroelectric critical temperature T FE2 ( x ) coincides with the magnetic ordering temperature T M2 ( x ). A density functional analysis of the spin-exchange interactions for a hypothetical MnMoO 4 that is isostructural with MnWO 4 suggests that Mo substitution increases the strength of the spin-exchange couplings among Mn 2+ in the vicinity of a Mo 6+ ion. Our study shows that the Mo-doped MnW 1– x Mo x O 4 ( x ≤ 0.3) compounds are spin-frustrated materials that have higher magnetic and ferroelectric phase-transition temperatures than does pure MnWO 4 .