Visualization of magnetic domain structure changes induced by interfacial strain in CoFe₂O₄/BaTiO₃ heterostructures

We visualized the evolution of the magnetic domain structure in CoFe2O4 films on different structural phases of BaTiO3 substrates with {0 0 1} orientation, using variable temperature magnetic force microscopy. When BaTiO3 underwent transitions from rhombohedral to orthorhombic to and from orthorhombic to tetragonal structures with increasing temperature, only local variations to the magnetic domains were observed. At the BaTiO3 tetragonal-cubic transition however, the magnetic domain size increased by 75% with an overall decrease in stray field contrast (33% decrease) because of reorientation of the magnetization to the in-plane directions. The reorientation of magnetization during the tetragonal to cubic phase transition of BaTiO3 was induced by release of asymmetric interfacial strain between the CoFe2O4 film and the BaTiO3 substrate, as non-uniformly distributed a and c surface ferroelectric domains in the BaTiO3 tetragonal phase disappeared at the paraelectric BaTiO3 cubic phase transition. Strain analysis based on macroscopic magnetization measurements was correlated with the microscopic magnetic domain structure to help understand the coupling in two-phase multiferroicheterostructure mediated by interfacial strain.