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

Bismuth ferrite (BiFeO3) is a lead-free magnetoelectric multiferroic with antiferromagnetic order and a large spontaneous polarization at room temperature. This antiferromagnetic order in BiFeO3 is complex, where, as a consequence of the Dzyaloshinskii-Moriya interaction (DMI), a small canting of the antiferromagnetic order forms a chiral spin cycloid. Understanding the interplay between the ferroelectric polarization and the spin cycloid, as well as its electric field manipulation, is of significant interest for antiferromagnetic spintronics and next generation computation. There is still much to learn about BiFeO3’s intrinsic antiferromagnetic structure in thin films, where epitaxial strain imposed by an underlying substrate can influence the spin texture. As a model system, we have synthesized epitaxial thin films (2-100 nm) of BiFeO3 on (110) TbScO3 substrates via oxide molecular-beam epitaxy (MBE). This allows us to look at BiFeO3 thin films with an unparalleled structural quality where even small amounts of strain (0.1% in the case of TbScO3) can have a large impact on the spin texture. In this work, we explore how the interplay between epitaxial strain from the substrate and elastic energy from the 109 degree domains affects the formation and orientation of the spin cycloid in BiFeO3 using scanning nitrogen-vacancy magnetometry as well as spin transport measurements. NV magnetometry uses a nitrogen vacancy implanted at the tip of a diamond cantilever which acts as a single-spin magnetometer to sensitively map nanoscale surface stray fields. Using this method, we have the resolution necessary to image BiFeO3’s spin cycloid. Electric-field-dependent spin transport shows that the spin cycloid is of critical importance to spin transport in BiFeO3 and can be manipulated by switching the ferroelectric polarization, which is of great interest in low dissipation magnonic devices. We find that 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 degree domains. As we further decrease the film thickness to 15 nm, we find that the spin cycloid is no longer coherent and any further decrease leads to an absence of the spin cycloid.