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, which allows an easy localization of spin waves interacting with magnetic textures. We applied this approach to the study of Co-based synthetic antiferromagnets, in which we were able to detect spin waves channeled inside the domain walls.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 the sensor 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 300K.

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, in which we were able to detect spin waves channeled inside the domain walls [2].We report here on a more detailed investigation of domain walls in synthetic antiferromagnetic layers using this approach. Our measurements reveal that the amplitude of the detected magnetic noise is 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.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 the sensor 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 300K [3].[1] M. Rollo et al, Phys. Rev. B 103 (2021) 235418.[2] A. Finco et al, Nat. Commun. 12 (2021) 767.[3] P. Kumar et al, Phys. Rev. Appl. 18 (2022) L061002.

In magnetic thin films, in presence of an interfacial Dzyaloshinkii-Moriya interaction (DMI), homochiral Néel domain walls and skyrmions are stabilized, with the chirality fixed by the sign of the DMI constant D. In addition, the DMI also affects the dispersion of spin waves, with the introduction of non-reciprocity, meaning that spin waves with opposite wavevectors have different energies.We report here on the detection of this non-reciprocity effect in spin waves confined within domain walls in synthetic antiferromagnets using scanning NV center relaxometry. Nitrogen-Vacancy (NV) centers in diamond are powerful quantum sensors used to probe magnetic fields -- by monitoring the Zeeman effect on their spin sublevels -- and magnetic fluctuations -- by measuring their spin relaxation time. Our NV centers are integrated in a scanning probe, allowing us to map the magnetic stray field and the magnetic noise at the surface of a sample with nanoscale spatial resolution.We have previously demonstrated with this technique that thermally activated spin waves channeled inside domain walls accelerate the spin relaxation of an NV center, which leads to a decrease of the photoluminescence emitted and therefore an easy localization of domain walls in synthetic antiferromagnets [1]. With this work, we show that the intensity of the magnetic noise detected at a domain wall is strongly dependent on its magnetic chirality, by measuring it from both sides of a stack grown on a membrane. While a strong noise signal is present above counter clockwise rotating walls, almost no noise is found at clockwise walls. With the help of numerical simulations, we attribute this effect to a DMI induced filtering of spin waves with either positive or negative wavevectors depending on the sign of D. Yet, in a thin film, spin waves propagating with positive wavevectors produce a strong stray field below and a weak one above the sample, and vice versa for negative wavevectors [5], which explains our experimental observations.Finally, we also investigated both numerically and experimentally the magnetic noise distribution above synthetic antiferromagnetic skyrmions and found, in addition to a similar chirality-related amplitude effect, that a specific noise pattern can be attributed to Néel or Bloch skyrmions.[1] A. Finco et al, Nat. Commun., 12 (2021) 767[2] T. Devolder, Phys. Rev. Applied, 20 (2023) 054057

In magnetic thin films, in presence of an interfacial Dzyaloshinkii-Moriya interaction (DMI), homochiral Néel domain walls and skyrmions are stabilized, with the chirality fixed by the sign of the DMI constant D. This chirality plays an important role in spintronics, as it sets the direction of the spin-orbit torque driven motion of these magnetic objects.In addition, the DMI also affects the dispersion of spin waves, with the introduction of non-reciprocity, meaning that spin waves with opposite wavevectors have different energies [1].We report here on the detection of this non-reciprocity effect in spin waves confined within domain walls in synthetic antiferromagnets using scanning NV center relaxometry. Nitrogen-Vacancy (NV) centers in diamond are powerful quantum sensors used to probe magnetic fields (by monitoring the Zeeman effect on their spin sublevels) and magnetic fluctuations (by measuring their spin relaxation time). By integrating a single NV center inside an atomic force microscope, one can thus map the magnetic stray field and the magnetic noise at the surface of a sample with nanoscale spatial resolution [2,3].We have previously demonstrated with this technique that thermally activated spin waves channeled inside domain walls accelerate the spin relaxation of an NV center, which leads to a decrease of the photoluminescence emitted and therefore an easy localization of domain walls in synthetic antiferromagnets [4]. With this work, we show that the intensity of the magnetic noise detected at a domain wall is strongly dependent on its magnetic chirality, by measuring it from both sides of a stack grown on a membrane. While a strong noise signal is present above counter clockwise rotating walls, almost no noise is found at clockwise walls. With the help of numerical simulations, we attribute this effect to a DMI induced filtering of spin waves with either positive or negative wavevectors depending on the sign of D. Yet, in a thin film, spin waves propagating with positive wavevectors produce a strong stray field below and a weak one above the sample, and vice versa for negative wavevectors [5], which explains our experimental observations.Finally, we also investigated both numerically and experimentally the magnetic noise distribution above synthetic antiferromagnetic skyrmions and found, in addition to a similar chirality-related amplitude effect, that a specific noise pattern can be attributed to Néel or Bloch skyrmions.[1]: D. Cortés-Ortuño and P. Landeros, J. Phys.: Condens. Matter, 25 (2013) 156001[2]: L. Rondin et al, Rep. Prog. Phys., 77 (2014) 056503[3]: F. Casola et al, Nat Rev Mater., 3 (2018) 17088 [4]: A. Finco et al, Nat. Commun., 12 (2021) 767[5]: T. Devolder, Phys. Rev. Applied, 20 (2023) 054057

In magnetic thin films, in presence of an interfacial Dzyaloshinkii-Moriya interaction (DMI), homochiral Néel domain walls and skyrmions are stabilized, with the chirality fixed by the sign of the DMI constant D. In addition, the DMI also affects the dispersion of spin waves, with the introduction of non-reciprocity, meaning that spin waves with opposite wavevectors have different energies.We report here on the detection of this non-reciprocity effect in spin waves confined within domain walls in synthetic antiferromagnets using scanning NV center relaxometry. Nitrogen-Vacancy (NV) centers in diamond are powerful quantum sensors used to probe magnetic fields -- by monitoring the Zeeman effect on their spin sublevels -- and magnetic fluctuations -- by measuring their spin relaxation time. Our NV centers are integrated in a scanning probe, allowing us to map the magnetic stray field and the magnetic noise at the surface of a sample with nanoscale spatial resolution.We have previously demonstrated with this technique that thermally activated spin waves channeled inside domain walls accelerate the spin relaxation of an NV center, which leads to a decrease of the photoluminescence emitted and therefore an easy localization of domain walls in synthetic antiferromagnets [1]. With this work, we show that the intensity of the magnetic noise detected at a domain wall is strongly dependent on its magnetic chirality, by measuring it from both sides of a stack grown on a membrane. While a strong noise signal is present above counter clockwise rotating walls, almost no noise is found at clockwise walls. With the help of numerical simulations, we attribute this effect to a DMI induced filtering of spin waves with either positive or negative wavevectors depending on the sign of D. Yet, in a thin film, spin waves propagating with positive wavevectors produce a strong stray field below and a weak one above the sample, and vice versa for negative wavevectors [5], which explains our experimental observations.Finally, we also investigated both numerically and experimentally the magnetic noise distribution above synthetic antiferromagnetic skyrmions and found, in addition to a similar chirality-related amplitude effect, that a specific noise pattern can be attributed to Néel or Bloch skyrmions.[1] A. Finco et al, Nat. Commun., 12 (2021) 767[2] T. Devolder, Phys. Rev. Applied, 20 (2023) 054057

The high versatility and nanoscale spatial resolution of scanning NV center microscopy makes it a very attractive technique for the investigation of skyrmions and complex magnetic textures in a broad range of materials, in particular antiferromagnets. In this talk, I will present recent results about multiferroic solitons in bismuth ferrite and show that we are able to estimate the sign and strength of DMI in magnetic thin films through the measurement of thermal spin wave noise. Bismuth ferrite is the most studied multiferroic material, and scanning NV magnetometry is among the very few techniques allowing the imaging of its very complex antiferromagnetic state, which is strongly coupled to the ferroelectric order. Here, we look at nanodisks in which we electrically write centre-convergent or divergent ferroelectric states, and probe the resulting magnetic whirl forming at the center. We observe flux closures of antiferromagnetic spin cycloids, with distinct antiferromagnetic entities at their cores depending on the electric field polarity, thus forming multiferroic solitons [1].In a second part, I will report on the detection of DMI-induced spin wave non-reciprocity in synthetic antiferromagnets using scanning NV center relaxometry. We probe the thermal spin wave noise produced by thermal spin waves confined in domain walls and skyrmions [2] and find that the intensity of this noise is strongly dependent on the magnetic chirality of the texture, and thus the DMI sign, by measuring it from both sides of a stack grown on a membrane. We explain this effect by the filtering in frequency and wavevector intrinsic to the NV-based noise detection, which is thus sensitive to the spin wave non-reciprocity.[1] A. Chaudron et al, Nature Materials (2024)[2] A. Finco et al, Nature Communications 12, 767 (2021)