Ferroelectricity in the cycloidal spiral magnetic phase of MnWO4

We investigate the relationships among magnetic, dielectric, and ferroelectric properties of a frustrated spin system MnWO4, which undergoes several magnetic phase transitions including a commensurate-incommensurate and a collinear-noncollinear transition. Dielectric and pyroelectric measurements show that the transition into a spiral magnetic ordered phase produces a ferroelectric state. The direction of the electric polarization is perpendicular to the spin rotation axis and the propagation vector of the spiral. These observations agree well with recent theoretical predictions that a cycloidal spiral magnetic ordering can result in electric polarization. In a material where ferroelectricity is induced by magnetic order, we can magnetically tune the ferroelectric transitions by exploiting the difference of the net magnetization between the ferroelectric and neighboring paraelectric phases