Résumé
NV-center magnetometry emerges as a powerful technique to investigate complex magnetic textures at the nanoscale under ambient conditions. It makes use of the response to magnetic field of the single spin of an NV-center [1], which is a defect in the crystalline structure of diamond consisting of a nitrogen atom and a vacancy, allowing to reach a sensitivity down to a few µT/Hz-1/2. We demonstrate here its imaging capabilities on antiferromagnets, materials which attract a growing interest in spintronics. In particular, we investigate the effect of epitaxial strain on the cycloidal modulation of the antiferromagnetic order in BiFeO3 and exploit the sensitivity of the NV center to magnetic noise to image antiferromagnetic domain walls, spirals and skyrmions in a synthetic antiferromagnet (SAF).BiFeO3 exhibits a large electrical polarization and an antiferromagnetic order with a cycloidal modulation with a period of 64 nm in the bulk. The propagation direction of the cycloid is strongly coupled to the direction of the electrical polarization. This magnetoelectric coupling allows the electric field control of the magnetic order, which is a desired feature in the view of developing new devices with a low power consumption. Here we study the effect of epitaxial strain on the magnetic order [2]. By using different substrates to grow BFO thin films, we tune the epitaxial strain and then perform direct imaging of the resulting magnetic state. Our measurements reveal that a moderate strain modifies the propagation direction of the cycloid with respect to the electrical polarization. In highly strained films, the cycloidal modulation disappears and only antiferromagnetic domains can be observed.We also apply NV-center magnetometry to SAF samples hosting domain walls, spin spirals and magnetic skyrmions [3]. Besides measurements of the stray field produced by the structures, we identified an additional imaging mechanism which relies on the enhancement of the NV center spin relaxation by magnetic noise having a component at its magnetic transition frequency. We detect a reduction of the NV center PL above the non-collinear magnetic structures which we can correlate with an increase of its relaxation rate. This effect arises from the magnetic noise produced by the excitations inside the structures, which have a different spectrum than the spin waves in the ferromagnetic background. This new relaxometry-based measurement mode allows a fast and simple investigation of various spin states in antiferromagnets.This work was done in collaboration with the Unité Mixte de Physique CNRS/Thalès in Palaiseau, France, the SPEC at the CEA in Gif-sur-Yvette, France and the Center for Nanoscience and Nanotechnology (C2N) in Palaiseau, France. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 846597 and from the DARPA TEE Program.References[1] Rondin et al, Rep. Prog. Phys 77 056503 (2014). [2] Haykal et al., Nat. Commun., 11 1704 (2020).[3] Legrand et al, Nat. Mater., 19 34-42 (2020).