Observation of vortex domain structures in multiferroic hexagonal manganites RMnO3 by transmission electron...

Multiferroic hexagonal manganite RMnO3 (R = rare-earth elements) shows improper ferroelectricity accompanied by tilting of MnO5 hexahedra as the primary order parameter. The ferroelectricity is originated from displacements of rare-earth ions along c-axis triggered by the MnO5 hexahedra tilting. Although coupling between ferroelectric and antiferromagnetic domains below the magnetic transition temperature of ∼90 K has been reported from previous work[1], the relationship between the ferroelectric domains and structural domains due to the MnO5 hexahedra tilting has not been well-studied. In this talk, we will report our studies on unique patterns of ferroelectric antiphase domains with a vorticity in hexagonal RMnO3, obtained from the results of transmission electron microscopy [2].The electron diffraction patterns obtained at room temperature exhibit superlattice reflection spots due to the MnO5 hexahedra tilting and displacements of rare-earth ions along c-axis, in addition to the fundamental reflections associated with the high symmetry structure with the space group of P63/mmc. Unique antiphase/ferroelectric "cloverleaf-like" domain patterns are clearly observed in dark-field images taken using superlattice spots. The fundamental and superlattice dark-field imaging combined with high-resolution imaging clearly demonstrates that in the cloverleaf-like domain patterns the antiphase and ferroelectric domains arrange periodically with certain rotation direction. In addition, there exist two types of cloverleaf-like domain patterns with the opposite rotations next to each other in the superlattice dark-field images. These results indicate that the cloverleaf-like domain patterns can be considered as the aggregation of vortices and antivortices consisting of ferroelectric and antiphase domains.