We have studied the effects of substituting nonmagnetic Ga${}^{3+}$ for Mn${}^{3+}$ on dielectric and magnetic properties in multiferroic YMn${}_2$O${}_5$. Upon Ga substitution, whereby only the Mn${}^{3+}$ ions are selectively replaced, the commensurate magnetic phase eventually disappeared owing to the disconnection of the magnetic interaction between Mn${}^{3+}$ and Mn${}^{4+}$ ions. In contrast, the low-temperature incommensurate magnetic phase is hardly affected by Ga substitution. Precise crystal structure analyses of Ga-substituted YMn${}_2$O${}_5$ 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.

• Received 15 January 2013

DOI:https://doi.org/10.1103/PhysRevB.87.104414