Programmable Magnetic Hysteresis in Orthogonally-Twisted Two-Dimensional CrSBr Magnets via Stacking Engineering

Twisting two-dimensional van der Waals magnets allows the formation and control of different spin-textures, as skyrmions or magnetic domains. Beyond the rotation angle, different spin reversal processes can be engineered by increasing the number of magnetic layers forming the twisted van der Waals heterostructure. Here, we consider pristine monolayers and bilayers of the A-type antiferromagnet CrSBr as building blocks. By rotating 90 degrees these units, we fabricate symmetric (monolayer/monolayer and bilayer/bilayer) and asymmetric (monolayer/bilayer) heterostructures. The magneto-transport properties reveal the appearance of magnetic hysteresis, which is highly dependent upon the magnitude and direction of the applied magnetic field and is determined not only by the twist-angle but also by the number of layers forming the stack. This high tunability allows switching between volatile and non-volatile magnetic memory at zero-field and controlling the appearance of abrupt magnetic reversal processes at either negative or positive field values on demand. The phenomenology is rationalized based on the different spin-switching processes occurring in the layers, as supported by micromagnetic simulations. Our results highlight the combination between twist-angle and number of layers as key elements for engineering spin-switching reversals in twisted magnets, of interest towards the miniaturization of spintronic devices and realizing novel spin textures.

💡 Research Summary

In this work the authors explore magnetic hysteresis engineering in orthogonally‑twisted van‑der‑Waals (vdW) heterostructures built from the A‑type antiferromagnet CrSBr. Pristine monolayers (ML) and bilayers (BL) of CrSBr serve as the basic building blocks. By rotating one block by 90° relative to the other, three distinct stacks are fabricated: symmetric ML/ML, symmetric BL/BL, and asymmetric ML/BL. All devices are encapsulated in h‑BN, contacted with few‑layer graphene electrodes, and measured in a vertical transport geometry at low temperature (2 K).

The magnetic anisotropy of CrSBr is strongly in‑plane, with the easy axis along the crystallographic b‑direction and a hard axis out‑of‑plane. In a pristine monolayer the resistance is insensitive to fields applied along the easy axis, while a field along the intermediate a‑axis produces a positive magnetoresistance (MR) that saturates near ±1 T. In a pristine bilayer, a field along the easy axis triggers a sharp resistance drop at ±0.2 T (spin‑flip of the two ferromagnetic layers), whereas a field along the a‑axis yields a smoother decrease reflecting a combined spin‑reorientation and spin‑flip process.

When the two blocks are orthogonally twisted, the high‑field (>1 T) MR of all three stacks converges to the behavior of the pristine bilayer: a positive MR that decays to zero. The low‑field regime, however, displays strikingly different hysteresis patterns. The ML/ML stack shows a gradual, smooth MR change dominated by spin‑reorientation in both layers. By contrast, any stack containing a bilayer (ML/BL or BL/BL) exhibits abrupt resistance jumps, indicating that the spin‑flip mechanism of the bilayer dominates the overall response.

A key observation is the pronounced asymmetry of the hysteresis loops in the asymmetric ML/BL device. When sweeping the magnetic field upward, the resistance plateau and subsequent drop are shifted toward positive fields (+0.01 T and +0.22 T); the opposite shift occurs on the downward sweep. The BL/BL stack also shows asymmetry, though less extreme (drops at –0.06 T and +0.17 T). This asymmetry originates from the fact that the external field is parallel to the easy axis of one block while being perpendicular to that of the other, leading to different energy barriers for spin‑flip versus spin‑reorientation.

First‑order reversal curve (FORC) measurements further quantify the emergence of hysteresis. In the ML/ML stack, hysteresis appears only when the maximum reversal field B_max exceeds ~0.08 T, and disappears for larger B_max where the curves become symmetric. In the ML/BL stack, hysteresis is already present for negative B_max values as small as –0.2 T, and a zero‑field memory effect is observed for B_max between 20 mT and 220 mT. The BL/BL stack never shows zero‑field hysteresis, confirming its purely volatile behavior.

Micromagnetic simulations reproduce these findings by varying the interlayer antiferromagnetic exchange J⊥ and the in‑plane anisotropy K. Increasing the number of layers raises the effective anisotropy, favoring spin‑flip over smooth reorientation. The competition between the two mechanisms is strongly dependent on the angle θ between the applied field and the easy axes of the two blocks. Angular scans reveal that the switching fields and ΔMR can be tuned over a ±0.5 T range simply by selecting the stack composition and the field direction. Minor deviations between nominally equivalent angles (θ = 0° vs 90°) are attributed to slight misalignments, strain‑induced exchange modulation, or twist‑angle imperfections.

Overall, the study demonstrates that, even with a fixed 90° twist, the number of constituent layers provides a powerful knob to program magnetic hysteresis. By choosing the appropriate stack, one can switch between volatile (smooth, reversible) and non‑volatile (abrupt, memory‑retaining) magnetoresistive states at zero field, a capability highly relevant for ultra‑compact spin‑valve devices, magnetic random‑access memory, and the creation of more complex spin textures such as merons or skyrmion bubbles in 2D magnets.