Evolution of Antiferromagnetic Spin Texture and Magnon Transport in MBE-Grown Epitaxial Multiferroic BiFeO3

Bismuth ferrite (BFO) is a room temperature magnetoelectric multiferroic with antiferromagnetic order and a large spontaneous polarization. Understanding the interplay between the ferroelectric polarization and the spin cycloid, as well as manipulating this interplay by an applied electric field, is of significant interest for antiferromagnetic spintronics and next-generation computation. We have synthesized epitaxial thin films (5-100 nm) of BFO on (110) terbium scandate substrates via oxide molecular-beam epitaxy (MBE). We explore how the interplay between epitaxial strain from the substrate and elastic energy from the 109° domains affects the formation and orientation of the spin cycloid in BFO using NV magnetometry as well as spin transport measurements. Electric-field-dependent spin transport shows that the spin cycloid is of critical importance to spin transport in BFO and can be manipulated by switching the ferroelectric polarization, which is of great interest in low-dissipation magnonic devices. Reducing the film thickness from 100 nm to 30 nm changes the propagation direction of the spin cycloid from perpendicular to parallel to the 109° domain walls. By 15 nm, we find that the spin cycloid is extremely disordered, and any further decrease leads to an absence of the spin cycloid.