Magnetism and Magneto-Transport in Covalent 2D Magnets by Self-Intercalation

The recent realization of atomically thin van der Waals (vdW) magnets by exfoliation, and characterization of 2D magnets down to the monolayer limit, has rekindled interest in 2D magnetism [1-3]. However, most of the existing research has focused on a limited set of exfoliated vdW systems, many of which possess relatively low magnetic ordering temperatures and often suffer from chemical instability. In this talk, I will introduce a class of less-explored materials dubbed covalent 2D magnets [4,5]. These magnets emerge from the self-intercalation of native 3d transition metal atoms between the vdW layers of transition metal dichalcogenides, forming ultrathin, covalently bonded layered magnets, denoted by the chemical formula M_1+dX₂. The degree of self-intercalation provides a unique mechanism to tune interlayer exchange coupling, magnetic order, and spin texture, providing a new pathway for manipulating 2D magnetism.

I will begin by discussing the synthesis of these materials, focusing on Cr₂Te₃ and Fe₃Se₄, as well as 2D magnet/2D semiconductor heterostructures. Subsequently, I will delve into their magnetic properties and unconventional anomalous Hall effect (AHE), characterized by humps and dips near the coercive field in AHE hysteresis. We attribute the observation to an intrinsic mechanism, distinct from previously proposed explanations such as topological Hall effect, or two-channel AHE arising from spatial inhomogeneities. Crucially, multiple Weyl-like nodes near the Fermi surface emerge in the electronic band structure, whose positions are sensitively modulated by the spin canting angles of the self-intercalated Cr cations. These nodes contribute strongly to the Berry curvature and AHE conductivity. Our findings provide compelling evidence for the intrinsic origin of the unconventional AHE in Cr₂Te₃ and further establish self-intercalation as a versatile approach for tailoring magnetic and magnetotransport properties, opening new avenues for spintronic applications.

We acknowledge support from the US National Science Foundation (Grant No. DMR-2242796, ECCS-2042085, MRI-1229208, MRI-1726303) and University at Buffalo Center for Advanced Semiconductor Technology.

References
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(3) Mak, K. F. et al. Nat. Rev. Phys. 1, 646-661 (2019).
(4) Bian, M. et al. Mater. Res. Lett. 9, 205-212 (2021).
(5) Bian, M. et al. Adv Mater 34, 2200117 (2022).
(6) He, K. et al. Adv Sci (2024).