Rhombohedral-Orthorhombic Ferroelectric Morphotropic Phase Boundary Associated with a Polar Vortex in BiFeO3 Films.

Strongly correlated oxides exhibit multiple degrees of freedoms, which can potentially mediate exotic phases with exciting physical properties, such as the polar vortex recently found in ferroelectric oxide films. A polar vortex is stabilized by competition between charge, lattice, and/or orbital degrees of freedom, which displays vortex-ferroelectric phase transitions and emergent chirality, making it a potential candidate for designing information storage and processing devices. Here, by a combination of controlled film growth and aberration-corrected scanning transmission electron microscopy, we obtain nanoscale vortex arrays in [110]-oriented BiFeO3 films. These vortex arrays are stabilized in ultrathin BiFeO3 layers sandwiched by two coherently grown orthorhombic scandate layers, exhibiting a ferroelectric morphotropic phase boundary constituted by a mixed-phase structure of polar orthorhombic BiFeO3 and rhombohedral BiFeO3. Clear polarization switching and piezoelectric signals were observed in these multilayers as revealed by piezoresponse force microscopy. This work presents a feature of a polar vortex in BiFeO3 films showing morphotropic phase boundary character, which offers a potential degree of manipulating phase components and properties of ferroelectric topological structures.