Spatially modulated morphotropic phase boundaries in a compressively strained multiferroic thin film

TL;DR

This study reveals complex interphase boundary structures in strained BiFeO3 thin films, including flat and zig-zag boundaries, with implications for strain engineering in multiferroic devices.

Contribution

It uncovers a new type of zig-zag phase boundary and elucidates their atomic-scale polarization and strain characteristics using advanced electron microscopy and modeling.

Findings

Flat MPBs extend over >1 mm with ~20 um spacing.

Zig-zag boundaries consist of alternating R'/R' and T'/T' twin domains.

Atomic-scale polarization rotation and >15% strain variation across MPBs.

Abstract

The coexisting rhombohedral-like (R', MA) and tetragonal-like (T', MC) monoclinic phases in compressively strained bismuth ferrite thin films exhibit exceptional piezoelectric and magnetic properties. While previous studies have largely focused on probing the morphotropic phase boundaries (MPBs) comprising ordered R'/T' twins, their self-organizing structures remain not fully explored. Here, we observed two types of interphase boundaries in a 60 nm-thick BiFeO3 film epitaxially grown on a LaAlO3 substrate by employing multi-modal diffraction-based electron microscopy. First, the flat MPBs form lines extending >1 mm, and repeat almost every ~20 um. Additionally, we uncover a new type of phase boundary with zig-zag regions of alternating R'/R' and T'/T' twin domains. Cross-sectional multislice electron ptychography confirms the atomic-scale polarization rotation across the MPB, with out-of-plane strain varying >15%. Plan-view electron backscatter diffraction reveals the lattice disclination of ~1.5-degrees across the zig-zag interphase boundaries, while having >2.5 degrees within the MPB. Phase-field modeling suggests that the formation of zig-zag phase boundaries arises from balancing between Landau and elastic energies. We speculate that such well-ordered interphase boundaries are associated with mesoscale in-plane strain modulations, thus providing a way to engineer and harness their properties for potential device applications.