Nanoscale topological defects and improper ferroelectric domains in multiferroic barium hexaferrite nanocrystals

Multiferroic materials that demonstrate magnetically driven ferroelectricity have fascinating properties such as magnetic (electric) field-controlled ferroelectric (magnetic) response that can be used in transformative applications including fast-writing, power-saving, and nondestructive data storage technologies in next-generation computing devices. However, since multiferroicity is typically observed at low temperatures, it is highly desirable to design multiferroic materials that can operate at room temperature. Here we show that BaFe12O19 is a promising room-temperature multiferroic material, and we unravel in three dimensions (3D) the dynamics of topological defects, strain, and improper ferroelectric domains driven by electric fields in individual BaFe12O19 nanocrystals. Using Bragg coherent diffractive imaging in combination with group-theoretical analysis, first-principles density functional calculations of phonons, and Landau phase-field theory we uncover in 3D the dynamics of topological defects, strain, and improper ferroelectric domains driven by electric fields in individual BaFe12O19 nanocrystals. Our results show that BaFe12O19 is an improper ferroelectric, in contrast to the current paradigm that adheres to the absence of improper ferroelectricity. Moreover, the fine structure of the reconstructed Bragg electron density suggests that BaFe12O19 may be able to harbor novel topological quantum states of matter and a pathway to transform information technologies.