Growth of Highly Insulating Bulk Single Crystals of Multiferroic BiFeO3 and Their Inherent Internal Strains in the Domain-Switching Process

Single crystals free from external strains are inevitable for research of the anisotropic intrinsic properties of BiFeO3. We have established a procedure to grow highly insulating bulk single crystals of BiFeO3 for the first time by a newly developed method called the laser-diode heating floating-zone (LDFZ) method, as well as by the control of the oxygen partial pressure in the atmosphere. The quality of the crystals is much improved, as demonstrated by the X-ray diffraction and the magnetic and the dielectric measurements. In the magnetization, no weak-ferromagnetism, localized-spin paramagnetism, or spin glass is essentially observed, and antiferromagnetism dominates. Enormous electric polarization with significant anisotropy is observed, indicating that by increasing the number of the coexisting ferroelastic variants, internal strains (structural mismatch) from the surrounding domains increase and the domain growth is constrained more. The internal strains play a crucial role in the domain switching in the bulk crystals.