Ferroelectricity in perovskite HoMnO$_{3}$ and YMnO$_{3}$ induced by magnetic order.

Ferroelectricity is observed in orthorhombic HoMnO$_{3}$ and YMnO$_{3 }$at the magnetic lock-in transitions into an E-type structure or an incommensurate phase with a temperature independent wave vector, respectively. In HoMnO$_{3}$ 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. The ferroelectricity observed in YMnO$_{3}$ cannot be explained within the current picture of the magnetic order and a refinement of the magnetic structure seems to be necessary.