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

BiFeO 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 films with thickness remains controversial. Here we use an optical second-harmonic generation technique to explore the phase-related symmetry evolution of BiFeO thin films with the variation of thickness. The crystalline structures for 60 and 180-nm-thick BiFeO thin films were characterized by high-resolution X-ray diffractometry reciprocal space mapping and the local piezoelectric response for 60-nm-thick BiFeO thin films was characterized by piezoresponse force microscopy. The present results show that the symmetry of BiFeO 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/χ tends, indicates an increase of symmetry with the decrease of thickness for BiFeO thin films.