Abstract

The family of 2D van der Waals (vdw) materials shows a wide range of highly interesting properties. We discuss these materials in the context of spintronic phenomena and potential devices. In particular, vdw layers allow for novel ultrathin tunnel barriers for magnetic tunnel junctions (MTJs).

Magnetic tunnel junctions are used today as magnetic field sensors in magnetic disk drives that store 70% of all digital data today. MTJs are also the basis of high performance, non-volatile magnetic random access memories, that is a memory technology available from the major semiconductor foundries today. Such MTJs are formed from complex multilayers formed from ultrathin ferromagnetic layers each of which generates stray magnetic fields that results in coupling within and between MTJs.

Today’s MTJs use synthetic antiferromagnetic sandwiches that eliminate these stray fields. We have demonstrated an all-antiferromagnetic tunnel junction that is formed from two bilayers of the insulating van der Waals antiferromagnet CrSBr that are twisted at a non-zero angle. These junctions exhibit two (or more) non-volatile states in zero magnetic field with very large tunneling magnetoresistance values (TMR)Nexceeding 1,000 %1. We discuss the origin of these giant TMR values and show how the magnitude depends on the twist angle. These high TMR values depend on the structural perfection of the vdw layers that is very difficult to achieve in conventional tunnel barriers formed from insulating oxides or nitrides. We show that the perfection of vdw ferromagnetic metallic layers allows for very high current induced domain wall mobilities in racetracks formed from such vdw materials. These mobilities are even higher than those found in state of the art racetrack memory devices2. On the other hand, we find structural defects in many vdw materials that appear to be intrinsic to such materials. For example, we find Fe vacancies in specific Wyckoff sites within the vdw layers in the ferromagnetic Fe3GeTe2. These result, in a non-centrosymmetric crystal structure that, thereby, allows for the presence of Neel-like skyrmions3. Perhaps, even more interestingly the presence of Fe atoms randomly distributed within the vdw gaps in Fe3GeTe2 behave as the first 2D spin glass4.

• Funded by European Research Council Advanced Grant “SUPERMINT” (2022-2027).

1: Chen, Y. et al. Twist-assisted all-antiferromagnetic tunnel junction in the atomic limit. Nature 632, 1045–1051 (2024). 2: Yang, S.-H., Ryu, K.-S. & Parkin, S. S. P. Domain-wall velocities of up to 750 ms−1 driven by exchange-coupling torque in synthetic antiferromagnets. Nat. Nanotechnol. 10, 221–226 (2015). https://doi.org/10.1038/nnano.2014.324 3: Chakraborty, A. et al. Magnetic skyrmions in a thickness tunable 2D ferromagnet from a defect driven Dzyaloshinskii-Moriya interaction. Adv. Mater. 34, 2108637 (2022). https://doi.org/10.1002/adma.202108637 4: Pal, B. et al. Realization of a Spin Glass in a two-dimensional van der Waals material. Science (accepted) (2025).