Résumé
We report on the study of the cycloidal antiferromagnetic state in bulk single crystals of the multiferroic bismuth ferrite using a combination of real-space and reciprocal magnetic imaging techniques [1]. We first discuss the origin of the magnetic stray field allowing us to image the cycloidal modulation of the antiferromagnetic state. This stray field is produced by the small uncompensated moment arising from the spin density wave which is locked to the cycloid [2]. We then use scanning NV center magnetometry to image the magnetic texture, with additional resonant X-ray diffraction measurements to obtain a larger scale insight. We observe the unexpected coexistence inside a single ferroelectric domain of magnetic domains hosting different cycloid wavevectors. We show that the direction of these wavevectors is not strictly locked to the preferred crystallographic axes as continuous rotations bridge different wavevectors. Making profit of the nanoscale spatial resolution of scanning NV-center magnetometry, we show that topological defects form at the junction between the magnetic domains. These defects are identical to those found in a broad variety of lamellar physical systems with rotational symmetries (liquid crystals, magnetic helix [3], ferromagnetic garnets, copolymers, etc.). The presence of these objects in such a multiferroic antiferromagnet offers new opportunities in terms of robustness and electrical control towards their use in spintronic devices.[1] Finco et al, Phys. Rev. Lett. 128, 187201 (2022)[2] Ramazanoglu et al, Phys. Rev. Lett. 107, 207206 (2011)[3] Schönherr et al, Nat. Phys. 14, 465 (2018)