Surface and bulk ferroelectric phase transition in super-tetragonal ${\mathrm{BiFeO}}_3$ thin films

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.

Figure 1

(a) $2\theta \text{−}\omega$ x-ray diffraction scan of a ${\mathrm{BiFeO}}_3$ thin film on ${\mathrm{Ca}}_{0.96}{\mathrm{Ce}}_{0.04}{\mathrm{MnO}}_3/{\mathrm{YAlO}}_3$ (001). (b) Reciprocal space mapping around the ${\mathrm{YAlO}}_3$ (116) peak. These structural characterizations indicate that ${\mathrm{BiFeO}}_3$ is only in its super-tetragonal phase. (c) Local piezoelectric loop as a function of the bias voltage applied to the ${\mathrm{Ca}}_{0.96}{\mathrm{Ce}}_{0.04}{\mathrm{MnO}}_3$ electrode. The imprint to the right indicates a preferential downward polarization orientation. (d) Dark-field (DF) and electron energy loss spectroscopy images at the Fe and O edges. (e) Spectra at the Fe L edge showing that the Fe valence is not modified at the surface. BFO, ${\mathrm{BiFeO}}_3$; CCMO, ${\mathrm{Ca}}_{0.96}{\mathrm{Ce}}_{0.04}{\mathrm{MnO}}_3$; r.l.u., reciprocal lattice units; YAO, ${\mathrm{YAlO}}_3$.

Figure 2

(a) Room-temperature detection of ferroelectric domains using PFM (top) and MEM (bottom) at a start voltage of $-0.95$ V. (b) Electron reflectivity as a function of the start voltage showing the MEM-LEEM transition for up and down ferroelectric domains. In the inset, KPFM surface potential image at the ${\mathrm{BiFeO}}_3$ surface on an area where upward ferroelectric domains were preliminary written. The up domains show a lower electrostatic surface potential than pristine down domains. (c) X-ray photoemission spectroscopy showing the presence of 0.1 monolayer of carbon contamination.

Figure 3

(a) LEEM images of written domains in ${\mathrm{BiFeO}}_3$ with polarization pointing upwards as a function of temperature. A contrast inversion occurs between 774 and 873 K. The domains disappear completely from 926 K. (b)–(d) Temperature dependencies of (b) the LEEM reflectivity contrast from (a) and of the out-of-plane lattice parameters of (c) ${\mathrm{BiFeO}}_3$ and (d) ${\mathrm{YAlO}}_3$ from room temperature to 960 K. While the lattice constant of ${\mathrm{YAlO}}_3$ (black circles) increases linearly, that of ${\mathrm{BiFeO}}_3$ (red circles) starts decreasing abruptly from 800 K until 940 K. LEEM and x-ray diffraction suggest a ferroelectric-to-paraelectric phase transition around 930 $\pm$ 30 K in the film (yellow area). The jump at about 375 K is an artifact due to sample misalignment.

Figure 4

LEEM images of written domains in ${\mathrm{BiFeO}}_3$ with polarization pointing upwards at (a) room temperature; (b) 873 K, after desorption of screening species; (c) 382 K, after cooling in UHV from 873 K; and (d) room temperature after cooling from above the Curie temperature.