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.