Spin Textures in 2D Magnets

The continuous reduction of the structural size in nanotechnology slowed down over the last decade, approaching the natural limit of single atoms as building blocks of matter. Therefore, intensive research is directed toward exploring new frontiers, in particular to further enhance the energy efficiency of computing devices. One promising strategy for energy optimisation and further decreasing the device size is the use of 2D materials as building blocks. 2D materials offer the specific advantages of atomically flat interfaces, the possibility to exploit proximity effects to tailor material properties, potentially low defect densities, as well as the capability to induce intriguing quantum states by adjusting the twist angle of two adjacent layers. Atomically thin layers of 2D materials are especially suited as components of spintronic devices, as they potentially allow for efficient control of spin generation and transport by an all-electrical means. Recently, the discovery of 2D magnets has opened novel perspectives along these directions. 2D magnets are currently studied with respect to a wide range of magnetic phenomena, including 2D ferromagnetism and anti-ferromagnetism, and frustrated magnets or other exotic topological magnetic materials.

The present PhD work investigates spin textures in various 2D magnetic systems by combining scanning transmission x-ray microscopy (STXM) with electrical control of the spin textures. Determining the detailed magnetic phase diagram of the 2D ferromagnet Fe3GeTe2 (FGT) provided the basis for controllably nucleating single skyrmions with single nano-second voltage pulses within a FGT-based heterostructure. As another relevant result, higher topological spin textures such as skyrmioniums, disykrmioniums and skyrmion bags could be observed especially in FGT with an optimised iron content (iron deficiency). Moreover, evaluation of the current-driven skyrmion movement in an exfoliated FGT wire revealed the temperature-dependent activation of the spin-transfer torque that underlies the skyrmion drive.

Furthermore, skyrmioniums could also be observed in another 2D out-of-plane ferromagnet, namely Cr2Ge2Te6, which is characterised by a relatively small magnetocrystalline anisotropy, indicating the generic nature of the skyrmionium formation mechanism.

Finally, by evaluating the x-ray magnetic circular/linear dichroism (XMCD/XMLD) of the 2D antiferromagnetic CrSBr, we could separate the materials' spin and orbital degree of freedom and determine the anisotropic magnetic and nonmagnetic components of the linear dichroic signal. Moreover, the spectra provided valuable insights into its air sensitivity, essential for novel functions in future 2D spintronic devices.

In conclusion, the rich details contained in the determined magnetic phase diagrams and the functional devices realised from 2D magnets open novel perspectives for the fields of 2D spintronics and neuromorphic computing, both of which are promising t