Two-dimensional (2D) van der Waals (vdW) magnetic materials have attracted significant attention due to their tunable magnetic properties and potential applications in spintronic devices. In layered vdW systems, magnetism is especially sensitive to small structural modifications, since interlayer spacing, stacking configuration, exchange coupling, and magnetic anisotropy all influence the stability of magnetic ground state [1]. Among the available tunning methods, pressure is particularly useful because it provides a clean and controllable way to tune magnetism in van der Waals halides by modifying interlayer distance, stacking, and exchange interactions. To capture this physics, we model the magnetic behaviour using a layered Heisenberg Hamiltonian that includes intralayer exchange, interlayer exchange, and single-ion anisotropy. The finite-temperature properties are then studied using atomistic spin dynamics based on stochastic Landau-Lifshitz-Gilbert equation implemented in UppASD [2]. In this work, we study how pressure drives phase transitions in NiBr₂ and CrBr₃ using DFT-derived magnetic parameters. For NiBr₂, we focus on the pressure-induced suppression of the helimagnetic phase and stabilization of collinear antiferromagnetic order. At ambient pressure, the triangular Ni lattice supports a delicate helimagnetic ground state, stabilized by frustrated in-plane exchange interactions. However, this helical order is already close to its stability limit. Under pressure, the van der Waals gap contracts and the second-nearest interlayer exchange NiBr₂ , grows strongly, shifting the system toward collinear AFM order. Our simulation therefore successfully explains the opposite pressure trends: the helimagnetic transition is suppressed between 0.5~1.0 GPa, while the collinear AFM transition temperature rises steeply toward ~75 K near 3 Gpa which agrees with experimental findings [3]. This makes NiBr₂ fundamentally different from NiI₂, where the helimagnetic phase remains stable to much higher pressure [4]. For CrBr₃, the pressure response reveals a stacking-controlled route to destroy ferromagnetism. The experimentally observed suppression of T_C , and saturated magnetization, ending in a ferromagnetic collapse near 5.8 GPa, cannot be explained by pressure-modified exchange interactions in the ideal structure alone [5]. Instead, simulations indicate that pressure promotes AA-stacked regions whose interlayer coupling is antiferromagnetic. These AFM-coupled stacking blocks compete with the original FM-coupled stacking, gradually breaking apart the ferromagnetic order while the local Cr moments remain intact.

References

[1] Burch, K.S., Mandrus, D. & Park, JG. Magnetism in two-dimensional van der Waals materials. Nature 563, 47–52 (2018). https://doi.org/10.1038/s41586-018-0631-z
[2] O. Eriksson, A. Bergman, L. Bergqvist, and J. Hellsvik, Atomistic Spin Dynamics: Foundations and Applications. Oxford, U.K.: Oxford University Press, 2017.
[3] Qureshi, P.A., Pokhrel, K.K., Prchal, J. et al. Opposite pressure effects on magnetic phase transitions in NiBr2. commun Matter (2026). https://doi.org/10.1038/s43246-026-01138-5
[4] Liu, Q., Su, W., Gu, Y. et al. Surprising pressure-induced magnetic transformations from helimagnetic order to antiferromagnetic state in NiI2. Nat Commun 16, 4221 (2025). https://doi.org/10.1038/s41467-025-59561-0
[5] Misek, M., Dutta, U., Kral, P., Hovancik, D., Kastil, J., Pokhrel, K., Ray, S., Valenta, J., Prchal, J., Kamarad, J., Borodavka, F., Eigner, V., Dusek, M., Holy, V., Carva, K., Kamba, S., Sechovsky, V., & Pospisil, J. (2025). Suppression of ferromagnetism in van der Waals insulator due to pressure-induced layer stacking variation. arXiv preprint arXiv:2501.1344