A multiferroic on the brink: Uncovering the nuances of strain-induced transitions in BiFeO3

Abstract

Bismuth ferrite (BiFeO3) is one of the very few known single-phase multiferroic materials. While the bulk compound is rhombohedral (R), the discovery of an epitaxial strain-induced structural transition into a so-called 'super tetragonalphase' (T-phase) in this material incited a flurry of research activity focused on gaining an understanding of this phase transition and its possible functionalities. This metastable phase of BiFeO3 is also multiferroic, with giant ferroelectric polarization and coexisting antiferromagnetic order, but above all it is the strain relaxation-induced phase mixtures and their outstanding piezoelectric and magnetoelectric responses which continue to intrigue and motivate the physicist and materials scientist communities. Here, we review the research into the T-phase and mixed-phase BiFeO3 system. We begin with a brief summary of the history of the T-phase and an analysis of the structure of the various phases reported in the literature. We then address important questions regarding the symmetry and octahedral rotation patterns and the (as yet underexplored) important role of chemistry in the formation of the metastable T-phase. We follow by describing the phase transitions in this material, and how these may hold promise for large magnetoelectric responses. Finally we point out some experimental challenges inherent to the study of such a system, and potential pathways for how they may be overcome. It is our intention with this work to highlight important issues that, in our opinion, should be carefully considered by the community in order to use this fascinating materials system for a new paradigm of functionality.

Strain-Driven Nanoscale Phase Competition near the Antipolar–Nonpolar Phase Boundary in Bi0.7La0.3FeO3 Thin Films

“…The uneven intensity distribution between the observed quarter-order peaks suggests that there is a preferred orientation, which is likely the result of the orthorhombic nature of the DyScO 3 substrate. 37 The lattice parameters of the second phase observed in the heterostructures grown on DyScO 3 of additional reflections in the diffraction pattern supports this conclusion as both superstructured phases would have clear superlattice spots. The second region, within a lighter contrast stripe, has a SAED pattern (Figure 3c), which is consistent with the RSM studies, where superlattice spots corresponding to the highlighted 1/4 (01̅ 1) reflections (as marked by the arrows, Figure 3c) indicate that the region exhibits an antipolar structure with a space group of Pbam.…”

“…Figure S2). This stripe morphology and its thickness-dependent evolution are reminiscent of a strainmediated, mixed-phase structure with enhanced electromechanical response in highly strained BiFeO 3 films 34 and suggest that there could be strain-induced phase coexistence in heterostructures grown on DyScO 3 .…”

“…The second region, within a lighter contrast stripe, has a SAED pattern (Figure 3c), which is consistent with the RSM studies, where superlattice spots corresponding to the highlighted 1/4 (01̅ 1) reflections (as marked by the arrows, Figure 3c) indicate that the region exhibits an antipolar structure with a space group of Pbam. Thus, it appears that at small strains, as is the case for films grown on DyScO 3 substrates, an intimate mixture of the two nanoscale phases is observed. This demonstrated ability to control phase stability using epitaxial strain in Bi 0.7 La 0.3 FeO 3 has not been previously reported.…”

“…Below, we will describe additional studies that confirm that this diffraction pattern arises from two phases. The data together with additional RSM studies at different diffraction conditions (Supporting Information, Figures S3− S5) allow for the extraction of the unit-cell parameters for the various phases, which are thus summarized ( Figure 2d); this includes two distinct phases for the films grown on SrTiO 3 and GdScO 3 and a mixture of phases on DyScO 3 . To further understand the nature of the crystal structures, various RSM studies were conducted to investigate quarterorder diffraction peaks (as their presence indicates a structure which is compatible with antipolar order).…”

“…It is important to note that the heterostructures grown on GdScO 3 are expected to show antiferroelectric-like behavior as the measurement started from negative bias, poling the sample into the antipolar (and potentially antiferroelectric-like) state for the remainder of the measurement. Heterostructures grown on SrTiO 3 and DyScO 3 are expected to show antipolar (and potentially antiferroelectric-like) behavior regardless of starting voltage because of their as-grown Pbam phase that is further reinforced by the applied bias. This is all consistent with what is expected for the antipolar Pbam phase.…”