The first material studied in EXAFONIS was BiFeO3 (BFO), an insulating multiferroic compound which has emerged as a promising platform for spintronic applications because its multiferroic order is preserved well above room temperature. In addition to a strong ferroelectric polarization, BFO exhibits a cycloidal AF order that can be deterministically controlled by electric fields through magnetoelectric coupling, thus opening perspectives for low-power reconfigurable AF spintronics. An important objective of EXAFONIS was to image, understand and control the AF order in BFO thin films. Three main results have been obtained during the project. We have first imaged topological defects emerging from the cycloidal AF order at the surface of a bulk BFO crystal. At the junctions between magnetic domains characterized by a cycloidal AF order propagating along different wavevectors, we observed the formation of topological line defects identical to those found in a broad variety of lamellar physical systems with rotational symmetries. This work established, for the first time, the presence of these magnetic objects in a multiferroic AF material, offering additional topological AF textures for future use in spintronics. We then investigated in details how the cycloidal AF order evolves in BFO thin films grown on different substrates allowing to tune epitaxial strain. Importantly, we demonstrated the stabilization of a single domain ferroelectric and antiferromagnetic spin cycloid state in epitaxial BFO(111) thin films. This single domain multiferroic configuration in a BFO thin film opens an avenue both for electrically-controlled non-collinear antiferromagnetic spintronics. Last, westudied ferroelectric centre states BFO thin films. We have shown that such polar textures contain flux closures of AF spin cycloids, with distinct AF entities at their cores depending on the electric field polarity. By tuning the epitaxial strain, quadrants of canted AF domains can also be electrically designed. These results open the path to reconfigurable topological states in multiferroic AF materials.
The second objective was to demonstrate the detection and manipulation of magnetic skyrmions in AF materials, thus integrating topology in the vibrant area of AF spintronics. We have first shown that non-collinear AF spin textures can be imaged with nanoscale spatial resolution by probing the magnetic noise they locally produce via thermal populations of magnons. This was achieved by adding a relaxometry-based imaging mode to the scanning-NV magnetometry toolbox, which relies on measurements of variations in the photoluminescence signal of the NV defect induced by magnetic noise. As a proof-of-concept, the efficiency of this novel method was first demonstrated by imaging domain walls and spin spirals in synthetic antiferromagnets (SAF). We then showed that NV-based relaxometry enables to image isolated skyrmions in SAF, which was an important goal of EXAFONIS. We then demonstrated that skyrmions in SAF can be moved by current along the current direction at velocities of up to 900 meters per second. This result opens an important path toward the realization of logic and memory devices based on the fast manipulation of skyrmions in tracks. Last, we also shown that the magnetic noise distribution measured around the contour of magnetic skyrmions in SAF reveals their Néel/Bloch nature, giving therefore also insight into the strength of Dzyaloshinkii-Moriya interaction involved in their stabilization.
