| The multiferroic properties of hexagonal HoMnO3 and YMnO 3 and orthorhombic RMn2O5 ( R=Ho, Dy and Tb) were investigated via dielectric, magnetic, strain, and neutron scattering measurements. These magneto-electric materials have recently attracted attention since they show a wealth of fundamental physical phenomena as well as potential for applications as magnetodielectric sensors or multiple state memory media. The possibility of controlling the magnetic (dielectric) properties in these compounds by external electric (magnetic) fields indicates a strong cross-correlation between magnetic and ferroelectric orders coexisting in some temperature range. In this work, it was discovered via anomalies in the linear thermal expansivities and longitudinal magnetostriction coefficients that there exists extraordinarily strong spin-lattice coupling in these systems that allows for the indirect coupling of the magnetic to the ferroelectric sub-system. Also, a scenario is presented which explains how the ferroelectric order in RMn2O5's arises at the magnetic lock-in transition in these compounds as a consequence of frustration in the Mn-spin system. In addition, an interesting behavior was observed in the RMn2O5's once isotropic pressure is applied as revealed by the pressure dependent polarization and dielectric measurements. In this system, a pressure induced stabilization of the ferroelectric order was observed at low temperature. |
