Structural transformation and charge transfer induced ferroelectricity and magnetism in annealed YMnO3

Multiferroic materials such as YMnO3, which uniquely exhibit ferroelectricity and magnetism simultaneously, have been extensively studied for spintronic device applications. However, the origin of multiferroicity remains poorly understood. In this study, the structural phases of YMnO3 ceramics and their lattice distortions after careful annealing were investigated to explain the origins of their multiferroicity. A structural transition from the orthorhombic to the hexagonal phase was observed when the annealing temperature reached around 1100 °C. This structural transformation also results in a magnetic transition from 3D Mn-O-Mn to 2D Mn-O-Mn superexchange coupling. The ferroelectricity was enhanced by escalation of the structural distortion caused by the rising annealing temperature. The annealing effect also results in the re-hybridization of the electronic structure of YMnO3. X-ray absorption near-edge spectra suggest that there is charge transfer from the Y-OT (apical oxygen) bonds of Y 4d-O 2p hybridized states to the OT-Mn bonds of Mn 3d-O 2p hybridized states, which is responsible for the enhanced ferroelectricity. This approach could be used to probe the origin of the ferroelectricity and multiferroic properties in rare-earth manganites.