Evolution of structural distortion in BiFeO 3 thin films probed by second-harmonic generation

BiFeO 3 thin films have drawn much attention due to its potential applications for novel magnetoelectric devices and fundamental physics in magnetoelectric coupling. However, the structural evolution of BiFeO 3 films with thickness remains controversial. Here we use an optical second-harmonic generation technique to explore the phase-related symmetry evolution of BiFeO 3 thin films with the variation of thickness. The crystalline structures for 60 and 180-nm-thick BiFeO 3 thin films were characterized by high-resolution X-ray diffractometry reciprocal space mapping and the local piezoelectric response for 60-nm-thick BiFeO 3 thin films was characterized by piezoresponse force microscopy. The present results show that the symmetry of BiFeO 3 thin films with a thickness below 60 nm belongs to the point group 4 mm . We conclude that the disappearance of fourfold rotational symmetry in SHG s -out pattern implies for the appearance of R- phase. The fact that the thinner the film is, the closer to 1 the tensor element ratio χ 31 / χ 15 tends, indicates an increase of symmetry with the decrease of thickness for BiFeO 3 thin films.