Exploring two-dimensional magnetism by scanning nitrogen-vacancy magnetometry

Qubits represent the fundamental building blocks of emerging quantum technologies. Their properties are highly sensitive to the environment, making them well suited for sensing applications and enabling the exploration of new physics at the frontiers of current research. The nitrogen-vacancy (NV) is a particularly promising qubit for quantum sensing applications because its spin ground state can be manipulated, initialized, and read out in a simple setup, its Zeeman splitting allows for quantitative magnetic field measurements, its long spin coherence enables highly sensitive measurements, and diamond, as a host material, provides a robust framework for the fabrication of nanoscale sensors. Embedded in an all-diamond atomic force microscopy tip, the NV can be brought into close contact with a magnetic surface. This enables the quantitative recording of the arising stray magnetic field with high sensitivity and spatial resolution. Thus, scanning NV magnetometry (S-NVM) is a viable tool for studying various phenomena in solid state physics.
The discovery of two-dimensional (2D) magnets has opened a new field of research that requires novel imaging techniques to characterize them. S-NVM is a suitable technique to study these materials due to its sensitivity to detect the weak magnetic signals from even a single atomic layer, typically without perturbing the fragile magnetic state of these materials. Despite the constraints imposed by the photophysics and the quantization axis of the NV, S-NVM is able to operate over the full temperature and external field range relevant to 2D magnets.
S-NVM is applied to study the nanoscale magnetism of single-layer EuGe2 grown by molecular beam epitaxy on a pre-patterned substrate. It is found that its average magnetization is described by a disordered 2D magnet with easy-plane anisotropy and low uniaxial in-plane anisotropy with a paramagnetic contribution. The disorder manifests itself in a complex magnetic texture on the film and can be described by a grain structure with a different critical temperature for each grain, replicating the observed statistics of the texture. Notably, the paramagnetic contribution is a property of each grain, indicating an internal structure below the spatial resolution of the technique.
S-NVM is also used to investigate the origin of the exchange bias in a MnPS3/Fe3GeTe2 heterostructure observed in anomalous Hall effect transport measurements. Although MnPS3 exhibits the magnetic structure of an ideal Heisenberg antiferromagnet with a perfectly compensated magnetic moment, a phase transition occurs below 40 K, giving rise to an uncompensated moment in the bulk crystal. This moment persists down to a few atomic layers and induces the exchange bias in the heterostructure.
These two examples demonstrate the great potential of S-NVM in exploring the rich physics of 2D magnetism. Thereby, it will contribute to new discoveries in the field and support the integration of 2D magnets into novel architectures for innovative spintronic devices.