Probing nanoscale magnetism with quantum sensors: from antiferromagnets to 2D materials

NV centers are defects in diamond which can be used as quantum sensors to probe magnetism at the nanoscale when integrated in an atomic force microscope. Such a measurement relies on the spin S = 1 of the NV center: the static stray field produced by a magnetic state induces a Zeeman shift on the spin sublevels, which can be detected optically. In addition, NV centers are also sensitive to spin waves, as the magnetic noise originating from thermally activated spin waves accelerates their spin relaxation. In this case, the enhanced relaxation leads to a decrease of the photoluminescence emitted by the NV center [1], which allows an easy localization of spin waves interacting with magnetic textures. We applied this approach to the study of Co-based synthetic antiferromagnets [2], in which we were able to detect spin waves channeled inside the domain walls [3].

We report here on a more detailed investigation of domain walls and skyrmions in synthetic antiferromagnetic layers using this approach. Our measurements reveal that the spatial distribution of the detected magnetic noise and its amplitude are related to the chirality of the magnetic texture. In particular, we found by looking at the magnetic stacks from both sides that counter-clockwise rotating Néel walls generate a much stronger noise than clockwise rotating ones, while they produce similar stray field distributions. This type of relaxometry measurement thus constitutes a new way to determine the rotational sense of Néel domain walls and skyrmions.

Besides NV centers, negatively charged boron vacancies in h-BN can also be used as quantum sensors to investigate the magnetic state of bidimensional materials, by integrating an ensemble of sensors directly into the heterostructure. As a proof-of-concept, we image the magnetic field produced by exfoliated flakes of CrTe₂, a van der Waals ferromagnet with a Curie temperature slightly above 300 K [4], demonstrating the ease of use and high flexibility of this new approach.

[1] M. Rollo et al, Physical Review B 103, 235418 (2021).

[2] W. Legrand et al,  Nature Materials 19, 34–42 (2020).

[3] A. Finco et al,  Nature Communications 12, 767 (2021).

[4] P. Kumar et al, Physical Review Applied 18, L061002 (2022).