Antiferromagnetic spin textures in ferroelectric single domain BiFeO₃: bulk versus thin films

BiFeO₃ is the prototypical room temperature multiferroic with rhombohedral symmetry because the polarization is lying along the [111] directions of the pseudo-cubic unit cell. In each ferroelectric domain, the competition between the superexchange and the magnetoelectric interactions results in the stabilization of an incommensurate cycloidal rotation of the Fe³⁺ magnetic moments. For a given polarization direction, the BiFeO₃ symmetry allows in principle three equivalent propagation directions for this spin cycloid.

In bulk single crystals, thanks to real-space scanning NV magnetometry images of the magnetic pattern supported by resonant elastic X-ray scattering, we indeed observe the coexistence of three rotational domains with different propagation directions of the antiferromagnetic cycloid. These directions are not strictly locked to the preferred crystallographic axes, with continuous rotation bridging different antiferromagnetic domains and formation of topological defects typical for lamellar materials¹.

In epitaxial thin films harboring single ferroelectric domains, the degeneracy between the three possible cycloids is lifted thanks to in-plane strain anisotropy, and a single spin cycloid is propagating throughout the whole sample. Nevertheless, when thickness is reduced down to the nanometer scale, this periodic cycloid is progressively destabilized and turns into a maze landscape before eventually forming antiferromagnetic bubbles.

¹ Phys. Rev. Lett. 128, 187201 (2022)