The temperature-dependent ferroelectric properties of super-tetragonal ${\mathrm{BiFeO}}_3$ are investigated using surface-sensitive low-energy electron microscopy (LEEM). We use epitaxial oxide ${\mathrm{BiFeO}}_3/{\mathrm{Ca}}_{0.96}{\mathrm{Ce}}_{0.04}{\mathrm{MnO}}_3$ bilayers grown by pulsed laser deposition on ${\mathrm{YAlO}}_3$ substrates. Ferroelectric, micrometer-scale domains are written by piezoresponse force microscopy and subsequently observed by LEEM from room temperature up to about 950 K. Kelvin probe force microscopy and LEEM spectroscopy reveal that the surface potential is efficiently (>50%) screened by adsorbates that are only released after annealing above 873 $\pm$ 50 K in ultrahigh vacuum. The surface structure and chemistry of the ferroelectric thin films are analyzed using scanning transmission electron microscopy, electron energy loss spectroscopy, and x-ray photoelectron spectroscopy, discarding the occurrence of a putative “skin layer” effect. While its magnetic and structural transitions were reported in the literature, the true, ferroelectric Curie temperature of super-tetragonal ${\mathrm{BiFeO}}_3$ has not been determined so far. Here, we measure a Curie temperature of 930 $\pm$ 30 K for the super-tetragonal ${\mathrm{BiFeO}}_3$ surface and corroborate it with volume-sensitive, temperature-dependent x-ray diffraction measurements. These results suggest that LEEM can be used as a powerful tool to probe surface charge and ferroelectric transitions in ultrathin films.

中文翻译：

---

超四方BiFeO3薄膜的表面和体铁电相变

超四边形随温度变化的铁电特性 ${\mathrm{二氧化铁}}_3$使用表面敏感的低能电子显微镜（LEEM）进行了研究。我们使用外延氧化物${\mathrm{二氧化铁}}_3/{钙}_{0.96}{策}_{0.04}{\mathrm{二氧化锰}}_3$ 脉冲激光沉积在 ${\mathrm{亚罗}}_3$基材。铁电的微米级区域由压电响应力显微镜写入，随后在室温至约950 K的温度范围内由LEEM观察。开尔文探针力显微镜和LEEM光谱表明，被吸附剂有效地筛选了表面电势（> 50％）。仅在873以上退火后才释放$\pm$超高真空下为50K。使用扫描透射电子显微镜，电子能量损失光谱和X射线光电子能谱分析铁电薄膜的表面结构和化学性质，从而消除了假定的“表皮层”效应的发生。尽管在文献中报道了其磁性和结构转变，但超四边形的真实铁电居里温度${\mathrm{二氧化铁}}_3$到目前为止尚未确定。在这里，我们测量的居里温度为930$\pm$ 超四角形为30 K ${\mathrm{二氧化铁}}_3$表面，并通过体积敏感的，与温度相关的X射线衍射测量来证实。这些结果表明，LEEM可以用作探测超薄膜中表面电荷和铁电跃迁的有力工具。