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. We demonstrate here its imaging capabilities both on ferromagnetic skyrmions stabilized at zero external field and on antiferromagnetic domain walls and skyrmions in a synthetic antiferromagnet.To observe non-collinear textures in ferromagnets, we operate the NV magnetometer in photoluminescence quenching mode, allowing the detection of stray field producing areas by measuring the spatial variations of the emitted NV center photoluminescence. In addition, the use of a diamond probe ensures that the experiment is carried out in the absence of external magnetic perturbation. To illustrate this, we show that skyrmions with a diameter about 60 nm [2] are stabilized by the exchange bias coming from the interface between the antiferromagnet IrMn and the ferromagnet NiFe in an optimized Pt/Co/NiFe/IrMn stack.In addition to ferromagnets, antiferromagnets have recently attracted a great interest in spintronics owing to the robustness of their magnetic textures and their fast dynamics. However, since they exhibit no net magnetization, antiferromagnets are challenging to work with. Therefore, we introduce a new imaging mode of the scanning NV-center microscope which does not rely on the measurement of the static magnetic stray field but on the detection of magnetic noise originating from spin waves inside the non-collinear antiferromagnetic textures of interest. The presence of magnetic noise accelerates the NV spin relaxation. As a consequence, the emitted photoluminescence is reduced, allowing a simple detection of the noise sources [3].We demonstrate this new technique on synthetic antiferromagnets [4] consisting of two ferromagnetic Co layers antiferromagnetically coupled through a Ru/Pt spacer. We first image domain walls and prove that we perform noise-based imaging by measuring a shorter NV spin relaxation time above an antiferromagnetic domain than above a domain wall. Calculations of the spin waves dispersion both in the antiferromagnetic domains and in the domains walls as well as maps of simulated magnetic noise intensity enable us to conclude that the noise which we probe arises from spin waves channelled in the domain walls.Going further, we tune the composition of the synthetic antiferromagnet stacks in order to stabilize spin spirals or antiferromagnetic skyrmions. In both cases, our relaxometry-based technique is able to image the non-collinear structures, demonstrating its efficiency and opening new avenues of exploration in the characterization of complex structures in magnetically-compensated materials.This work was done in collaboration with Spintec in Grenoble, France, the Unité Mixte de Physique CNRS/Thalès in Palaiseau, 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.[1] L. Rondin et al, Rep. Prog. Phys 77, 056503 (2014)[2] K. Gaurav Rana et al, Phys. Rev. Appl., 13, 044079 (2020)[3] A. Finco et al, arXiv:2006.13130 (2020)[4] W. Legrand et al, Nat. Mater., 19, 34 (2020)