Ferroelectricity in perovskite HoMnO3 and YMnO3

Ferroelectricity is observed in orthorhombic HoMnO3 and YMnO3 at the magnetic lock-in transitions into an E-type structure or an incommensurate phase with a temperature independent wave vector, respectively. In HoMnO3, the ferroelectric polarization strongly depends on the external magnetic field, indicating the involvement of the rare earth moment order in this compound. The results are discussed within the framework of recent theoretical models, in particular, the double exchange driven polar displacements predicted for E-type magnetic structures.