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)

We will present our modules to control a scanning NV center microscope which we use to map weak magnetic fields at the nanoscale through the optical reading of the magnetic resonance of a defect in diamond, the nitrogen vacancy center. Our system thus combines an AFM controller, piezoelectric scanners, a microwave source and a single photon detector.

The negatively-charged boron vacancy (V - B ) center in hexagonal boron nitride (hBN) is currently garnering considerable attention for the design of two-dimensional (2D) quantum sensing units. Such developments require a precise understanding of the spin-dependent optical response of V - B centers, which still remains poorly documented despite its key role for sensing applications. Here we investigate the spin-dependent photodynamics of V - B centers in hBN by a series of time-resolved photoluminescence (PL) measurements. We first introduce a robust all-optical method to infer the spin-dependent lifetime of the excited states and the electron spin polarization of V - B centers under optical pumping. Using these results, we then analyze PL time traces recorded at different optical excitation powers with a seven-level model of the V - B center and we extract all the rates involved in the spin-dependent optical cycles, both under ambient conditions and at liquid helium temperature. These findings are finally used to study the impact of a vector magnetic field on the optical response. More precisely, we analyze PL quenching effects resulting from electron spin mixing induced by the magnetic field component perpendicular to the V - B quantization axis. All experimental results are well reproduced by the seven-level model, illustrating its robustness to describe the spin-dependent photodymanics of V - B centers. This work provides important insights into the properties of V - B centers in hBN, which are valuable for future developments of 2D quantum sensing units.

The polarized photoluminescence emitted on the edge of a series of aluminum‐rich (Al,Ga)N‐AlN quantum wells (QWs) grown by molecular beam epitaxy on AlN templates deposited by metal organic chemical vapor deposition on c‐plane sapphire is measured. The contrast and the principal axis of the emission diagrams for 2 nm‐thick (Al,Ga)N QWs grown for aluminum compositions between 40% and 90% are studied. The light is emitted on the edge of the QWs at wavelengths going from 280 nm down to 209 nm. The emission diagram, a change from oblate to prolate with respect to the in‐plane orientation, for an aluminum composition is found to occur around 72%, that is, at an emission wavelength of about 235 nm. The orientations and shapes of the edge‐emission diagrams indicate that the fluctuations of the composition of the (Al,Ga)N confining layer are deep enough for producing intravalence band mixings. This property, that acts in concert with the built‐in strain and quantum‐confined Stark effect, contributes to the anisotropy of the light emission when the aluminum composition reaches 60–70%, that is, for an emission wavelength of 260–235 nm.