Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China.
University of Chinese Academy of Sciences, Beijing 100049, China.
Sci Rep. 2016 Dec 1;6:38268. doi: 10.1038/srep38268.
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.
BiFeO 薄膜由于其在新型磁电设备和磁电耦合基础物理中的潜在应用而受到广泛关注。然而，BiFeO 薄膜厚度的结构演变仍存在争议。在这里，我们使用光学二次谐波产生技术来研究厚度变化时 BiFeO 薄膜的与相相关的对称演变。通过高分辨率 X 射线衍射倒易空间映射对 60nm 和 180nm 厚 BiFeO 薄膜的晶体结构进行了表征，并通过压电力显微镜对 60nm 厚 BiFeO 薄膜的局部压电响应进行了表征。结果表明，厚度低于 60nm 的 BiFeO 薄膜的对称度属于点群 4mm。我们得出结论，SHG s-out 图案中四重旋转对称的消失意味着 R 相的出现。薄膜越薄，张量元素比 χ/χ 越接近 1，这表明 BiFeO 薄膜的对称度随着厚度的减小而增加。
