Résumé
Magnetic textures in various types of materials are at the core of the development of energy-efficient spintronic devices, and researchers follow many promising paths towards their improvement, looking at domain walls, magnetic skyrmions, antiferromagnets, multiferroics, bidimensional van der Waals heterostructures, etc. A common challenge in the study of all these objects is the need for a highly sensitive magnetic imaging technique offering nanometer scale spatial resolution, preferably under ambient conditions. These performances are offered by scanning-NV magnetometry, a scanning probe technique which uses a single Nitrogen-Vacancy (NV) defect in diamond as a quantum sensor [1]. NV centers can be used to measure magnetic fields with a high sensibility, by detecting the Zeeman shift of their electronic spin resonance. Alternatively, measurements of their spin relaxation time allows us to probe spin waves through the magnetic noise which they generate [2,3].I will concentrate here on the imaging of antiferromagnetic states in bismuth ferrite, a room-temperature multiferroic hosting a spin cycloid locked to the direction of the ferroelectric polarization. At the surface of bulk BiFeO3 single crystals, we demonstrate the coexistence of several cycloid propagation directions within a single ferroelectric domain. At the junction between these cycloids, we observe the formation of topological defects (±π-disclinations and edge dislocations) which are typically found in lamellar materials [4]. The presence of these objects in a multiferroic antiferromagnet provides new possibilities towards their energy-efficient electric control.[1] L. Rondin et al, Rep. Prog. Phys. 77 (2014) 056503[2] M. Rollo et al., Phys. Rev. B 103 (2021) 235418 [3] A. Finco et al, Nat. Commun 12 (2021) 767[4] A. Finco et al, arXiv:2202:02243, in press in Phys. Rev. Lett.