Skin Layer of ${\mathrm{BiFeO}}_3$ Single Crystals

A surface layer (“skin”) different from the bulk was found in single crystals of ${\mathrm{BiFeO}}_3$. Impedance analysis and grazing incidence x-ray diffraction reveal a phase transition at $T^{*}\sim 275\pm 5°\mathrm{C}$ that is confined within the surface of ${\mathrm{BiFeO}}_3$. X-ray photoelectron spectroscopy and refraction-corrected x-ray diffraction as a function of incidence angle and photon wavelength indicate a reduced electron density and an elongated out-of-plane lattice parameter within a few nanometers of the surface. The skin will affect samples with large surface to volume ratios, as well as devices that rely on interfacial coupling such as exchange bias.

Figure 1

X-ray reflectivity measurement (blue dots) and theoretical fit (solid red line) yielding a roughness of 1 nm for the surface of the $\mathrm{BiFe}{\mathrm{O}}_3$ crystal used in the diffraction study. Inset: Polarized optical microscopy view graph of the same mirror-polished single crystal, with habitus parallel to $⟨001{⟩}_{\mathrm{pseudocubic}}$, showing a quadrantlike arrangement of the four types of ferroelastic domain present in the sample.

Figure 2

Capacitive impedance measurements [panels (a), (b), and (c)]. The index in the curves indicates the frequency of the measurement in kHz. There is a peak at $\mathrm{T}\cong 272°\mathrm{C}$ in the low-frequency region of the spectrum, which corresponds to the contact capacitance. This peak is absent at higher frequencies, so the transition does not affect the bulk. (c) The anomaly was also observed in unpolished crystals, so it is not caused by the polishing process. Panels (d) and (e): the XPS intensity at glancing detection (d) is shifted towards higher energies than that at normal incidence (e), consistent with a charge-depleted surface.

Figure 3

(a) Reciprocal space tomograph around the (102) reflection. Calculated penetration depth (b) and refractive shift (c) as a function of incidence angle for two different beam energies, showing that for an angle of $0.22°$, changing the energy allows tuning the penetration depth without changing the refractive shift. The $Q$-histogram analysis of the $HKL$ measured by changing the photon energy (d) and the incidence angle (e), both of which confirm that the lattice parameter is expanded at the surface.

Figure 4

Comparison between the reciprocal thermal expansion at the skin (empty symbols) and inside the crystal (solid symbols) evidencing the local phase transition at $T^{*}$ in the surface of ${\mathrm{BiFeO}}_3$.