Skin layer of BiFeO3 single crystals

TL;DR

This study uncovers a distinct surface layer in BiFeO3 single crystals with unique structural and electronic properties, undergoing a surface-specific phase transition at T* that could impact surface-dependent applications.

Contribution

It provides the first detailed characterization of a surface 'skin' in BiFeO3, revealing its structural, electronic, and phase transition properties distinct from the bulk.

Findings

Identified a surface layer with different structure and electron density

Detected a surface-specific phase transition at T* ~ 275°C

Showed the skin's properties influence surface-dominated applications

Abstract

A surface layer ("skin") that is functionally and structurally different from the bulk was found in single crystals of BiFeO3. Impedance analysis indicates that a previously reported anomaly at T* ~ 275 \pm 5 ^/circC corresponds to a phase transition confined at the surface of BiFeO3. X-ray photoelectron spectroscopy and X-ray diffraction as a function of both incidence angle and photon wavelength unambiguously confirm the existence of a skin with an estimated skin depth of few nanometres, elongated out-of-plane lattice parameter, and lower electron density. Temperature-dependent x-ray diffraction has revealed that the skin's out of plane lattice parameter changes abruptly at T*, while the bulk preserves an unfeatured linear thermal expansion. The distinct properties of the skin are likely to dominate in large surface to volume ratios scenarios such as fine grained ceramics and thin films, and should be particularly relevant for electronic devices that rely on interfacial couplings such as exchange bias.