Résumé
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