Quantitative Imaging of 2D Magnets Down to the Monolayer Limit

The discovery of 2D magnetic van-der-Waals systems lead to investigations of novel physical phenomena at a reduced dimensionality and enables the realization of novel heterostructures for spintronic applications [1]. Due to their intrinsic weak magnetic signal strength, advanced characterization techniques are required. Here, we use the single spin of a nitrogen-vacancy (NV) in diamond embedded into an all-diamond AFM tip to obtain images of the stray field arising from 2D magnetic systems [2]. In a scanning configuration, we obtain a spatial resolution of approximately 50 nm and a field sensitivity of 1 μT/Hz^0.5. Complementary to other techniques, scanning NV magnetometry is a non-invasive method that can operate in the full temperature range from mK to room-temperature and is not restricted to a specific type of material.<br/><br/>We investigated the properties of mechanically exfoliated chromium trihalides. We determined the out-of-plane magnetization of a single layer of CrI3 quantitatively and confirmed the unexpected antiferromagnetic interlayer coupling. We relate this behavior to its monoclinic phase at low-temperatures and could switch it to a ferromagnetic coupling by inducing a phase transition to its rhombohedral structure [3]. In preliminary work, we also studied the monolayer of CrCl3 that exhibits an easy-plane anisotropy and shows characteristics of an ideal XY magnet. We study its magnetic phase transition and show how its magnetization is stabilized below its critical temperature.<br/><br/>In contrast to mechanical exfoliation, molecular-beam-epitaxy can be used to grow 2D magnets on larger scales with a well-defined geometry. We studied the lanthanide metalloxene EuGe2 that is a layered 2D antiferromagnet with ferromagnetic interlayer coupling. We show that such a system can be grown on a prepatterned substrate allowing us to define its geometry to an arbitrary shape. We find that the magnetization of the monolayer is reduced compared to its bulk value consistent with previous film characterization [4] and that the monolayer behaves like an XY magnet. However, our technique reveals that the film consists of independent magnetic grains of around 100 nm size resulting in a complex stray field pattern. The field dependence of that pattern suggests that these grains possess a varying critical temperature that can explain the previous observed anomalies [5].<br/><br/>Overall, our technique quantitatively determines the magnetic properties of the 2D magnets down to the monolayer limit and is capable of capturing anomalies at the nanoscale. As such, our technique provides a basis for both a deeper fundamental understanding of magnetization in 2D systems and the development of future engineering of 2D magnets.<br/> <br/>[1] Wang et al., The Magnetic Genome of Two-Dimensional van der Waals Materials, ACS Nano 16(5), 2022.<br/>[2] Hedrich et al., Parabolic Diamond Scanning Probes for Single-Spin Magnetic Field Imaging, Phys. Rev. Applied 14(6), 2020.<br/>[3] Thiel et al., Probing magnetism in 2D materials at the nanoscale with single-spin microscopy, Science 364(6444), 2019.<br/>[4] Tockmachev et al., Lanthanide f7 metalloxenes – a class of intrinsic 2D ferromagnets, Mater. Horiz. 6(7), 2019.<br/>[5] Reiser et al., in preparation.