Abstract
In functional oxide films, different electrical/mechanical boundaries near film surfaces induce rich phase diagrams and exotic phenomena. In this paper, we review some key points which underpin structure, phase transition and related properties in BiFeO3 ultrathin films. Compared with the bulk counterparts, we survey the recent results of epitaxial BiFeO3 ultrathin films to illustrate how the atomic structure and phase are markedly influenced by the interface between the film and the substrate, and to emphasize the roles of misfit strain and depolarization field on determining the domain patterns, phase transformation and associated physical properties of BiFeO3 ultrathin films, such as polarization, piezoelectricity, and magnetism. One of the obvious consequences of the misfit strain on BiFeO3 ultrathin films is the emergence of a sequence of phase transition from tetragonal to mixed tetragonal & rhombohedral, the rhombohedral, mixed rhombohedral & orthorhombic, and finally orthorhombic phases. Other striking features of this system are the stable domain patterns and the crossover of 71° and 109° domains with different electrical boundary conditions on the film surface, which can be controlled and manipulated through the depolarization field. The external field-sensitive enhancements of properties for BiFeO3 ultrathin films, including the polarization, magnetism and morphotropic phase boundary-relevant piezoelectric response, offer us deeper insights into the investigations of the emergent properties and phenomena of epitaxial ultrathin films under various mechanical/electrical constraints. Finally, we briefly summarize the recent progress and list open questions for future study on BiFeO3 ultrathin films.
Highlights
Nowadays, there is a growing demand for materials exhibiting various functional electronic properties for plenty of applications involving sensing, actuation and information/energy storage.A fundamental basis for understanding the occurrence of these intriguing functional properties in materials is the evolution of their symmetry as functions of external stimuli such as temperature, stress, electrical/magnetic field and pressure [1], which is subsequent to phase transitions due to the broken symmetry
In comparison with the effect of normal misfit strain, it has been shown that the shear strain from the substrates could induce significant shifts of the stable points for phase transitions and the occurrences of extra novel properties in ferroelectric PbTiO3 films [59,60]
Despite the numerous works mentioned, in this review, we summarize here the effects of interfaces that significantly influence the domain structure, phase diagram and related properties of BFO ultrathin films
Summary
There is a growing demand for materials exhibiting various functional electronic properties for plenty of applications involving sensing, actuation and information/energy storage. The interface induced new exotic behaviors of BFO thin films are of scientific interest and technological importance for applications and raises fundamental questions on understanding the roles of external mechanical and electrical constraints near the interface, on manipulating the atomic structure, phase transition and corresponding functional properties. Particular emphasis is placed on investigations of epitaxial BFO ultrathin films, since new phase, novel ferroelectric, ferromagnetic and piezoelectric properties emerge while keeping the extrinsic factors such as electrical/mechanical boundary conditions at the interface between the film and the substrate under control. After discussing the extrinsic effect of misfit strain on crystal structure and corresponding phase transition of BFO films, we will illustrate how properties such as ferroelectricity, ferromagnetism and piezoelectricity are affected markedly by extrinsic misfit strain. Domain pattern (including domain stability, its size and coexistence and crossover of domains) is another fundamental aspect of BFO films, which is closely connected to most macroscopic properties (polarization, piezoelectricity, conductivity, and photovoltaic etc.)
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