Room-temperature electric polarization induced by phase separation in multiferroic GdMn$_2$O$_5$

It was generally accepted until recently that multiferroics RMn$_2$O$_5$ crystallized in the centrosymmetric $Pbam$ space group and ferroelectricity in them could exist only at low temperatures due to the magnetic exchange striction. Recent comprehensive structural studies (Phys. Rev. Lett. 114, 117601 (2015)) have shown that the actual symmetry of RMn$_2$O$_5$ at room temperature is a noncentrosymmetric monoclinic $Pm$ sp. gr., which allows room temperature ferroelectricity to exist. However, such a polarization has not yet been found. Our electric polarization loop studies of GdMn$_2$O$_5$ have revealed that a polarization does exist up to room temperature. This polarization occurs mainly in restricted polar domains that arise in the initial GdMn$_2$O$_5$ matrix due to phase separation and charge carrier self-organization. These domains are self-consistent with the matrix, which leads to the noncentrosymmetricity of the entire crystal. The polarization is controlled by a magnetic field, thereby demonstrating the presence of magnetoelectric coupling. The low-temperature ferroelectricity enhances the restricted polar domain polarization along the $b$-axis.