Control of magnetic interaction and ferroelectricity by nonmagnetic Ga substitution in multiferroic YMn₂O₅

We have studied the effects of substituting nonmagnetic Ga3⁺ for Mn3⁺ on dielectric and magnetic properties in multiferroic YMn ₂O₅. Upon Ga substitution, whereby only the Mn3 ⁺ ions are selectively replaced, the commensurate magnetic phase eventually disappeared owing to the disconnection of the magnetic interaction between Mn3⁺ and Mn4⁺ ions. In contrast, the low-temperature incommensurate magnetic phase is hardly affected by Ga substitution. Precise crystal structure analyses of Ga-substituted YMn ₂O₅ have revealed that the bonds between Mn ions along the ab plane changed markedly with increasing Ga concentration, while those along the c axis did not change at all. This indicates that the magnetic interactions along the ab plane were altered dramatically, while those along the c axis were robust against Ga substitution. From the dielectric and magnetic phase diagrams as a function of Ga substitution, we conclude that the electric polarization is predominantly induced by exchange-striction in the commensurate magnetic phase, and by an inverse Dzyaloshinskii-Moriya interaction in the incommensurate phase.