Domain Control in Multiferroic BiFeO3 through Substrate Vicinality

Control over ferroelectric polarization variants in BiFeO3 films through the use of various vicinal SrTiO3 substrates is demonstrated. The ferroelectric polarization variants in these films are characterized by piezoelectric force microscopy and the corresponding structural variants are carefully analyzed and confirmed by X-ray diffraction. Implementation of this approach has given us the ability to create single domain BiFeO3 films on (001), (110), and (111) surfaces. The piezo/ ferroelectric properties of the BiFeO3 films, in turn, can be tailored through this approach. Such results are very promising for continued exploration of BiFeO3 films and provide a template for detailed multiferroic-coupling studies in the magnetoelectric BiFeO3 system. Magnetoelectric coupling in multiferroic materials has attracted much attention because of the intriguing science underpinning this phenomenon. Additionally, there is an exciting potential for applications and devices that take advantage of these materials with multiple order parameters. BiFeO3 (BFO) is a room temperature, single-phase magnetoelectric multiferroic with a ferroelectric Curie temperature of ∼ 1103 K and an antiferromagnetic Néel temperature of ∼ 643 K. Recent studies of BFO thin films have shown the existence of a large ferroelectric polarization, as well as a small net magnetization of the Dzyaloshinskii-Moriya type resulting from a canting of the antiferromagnetic sublattice. The ferroelectric polarization in BFO can have orientations along the four cube diagonals (<111>), and the direction of the polarization can be changed by ferroelectric and ferroelastic switching. Our previous studies have shown coupling between ferroelectricity and antiferromagnetism in BFO thin films resulting from the coupling of both antiferromagnetic and ferroelectric domains to the underlying ferroelastic domain switching events. Such a study was a crucial first step in the exploration of approaches to control and manipulate magnetic properties using an electric field. It was also noted, however, that these films exhibit a very complicated domain structure, which complicates the interpretation of the fundamental properties of this system as well as the interactions across hetero-interfaces. The lack of large single crystals of the desired crystallographic orientation provokes another motivation to explore approaches to create “single crystalline” epitaxial films that are free of ferroelectric/ferroelastic domains. Recent studies have explored the ability to control the ferroelectric domain configuration, which is formed after the phase transformation, through substrate engineering. In this study, we demonstrate an approach to control the ferroelectric domain structure in BFO films through the use of vicinal SrTiO3 (STO) substrates. This has enabled us to create thin films that “mimic” the primary crystal facets of the pseudo-cubic unit cell, namely single domain (100), (110), and (111) surfaces. The ferroelectric domain structure of an epitaxial BFO film on STO substrates with different orientations can be modeled using the phase-field method in which the spatial distribution of the polarization field and its evolution is described by the time dependent Ginzburg–Landau (TDGL) equations. For a BFO film grown on a (001)-oriented perovskite substrate, there are eight possible ferroelectric polarization directions corresponding to the four structural variants of the rhombohedral phase (Fig. 1a). In this case, the individual domains are energetically degenerate, and domain structures with twinning are expected in order to relax the elastic energy of the film. Experimentally, however, only the downward directed polarization variants are observed, indicating the existence of a self-poling effect (as a consequence of the bottom electrode). Figure 1b shows an in-plane (IP) piezoforce microscopy (PFM) image of a BFO film grown on (001) STO substrate. The three contrast levels observed in the IP-PFM images acquired along the two orthogonal <110> directions, together with the uniform out-of-plane (OP) PFM contrast (not shown), indicate that the domain structure of the BFO films is characterized by four polarization variants. The C O M M U N IC A TI O N