Atomically Unveiling an Atlas of Polytypes in Transition-Metal Trihalides

Transition-metal trihalides MX₃ (M = Cr, Ru; X = Cl, Br, and I) belong to a family of novel two-dimensional (2D) magnets that can exhibit topological magnons and electromagnetic properties, thus affording great promises in next-generation spintronic devices. Rich magnetic ground states observed in the MX₃ family are believed to be strongly correlated to the signature Kagome lattice and interlayer van der Waals coupling raised from distinct stacking orders. However, the intrinsic air instability of MX₃ makes their direct atomic-scale analysis challenging. Therefore, information on the stacking-registry-dependent magnetism for MX₃ remains elusive, which greatly hinders the engineering of desired phases. Here, we report a nondestructive transfer method and successfully realize an intact transfer of bilayer MX₃, as evidenced by scanning transmission electron microscopy (STEM). After surveying hundreds of MX₃ thin flakes, we provide a full spectrum of stacking orders in MX₃ with atomic precision and calculated their associated magnetic ground states, unveiled by combined STEM and density functional theory (DFT). In addition to well-documented phases, we discover a new monoclinic C2/c phase in the antiferromagnetic (AFM) structure widely existing in MX₃. Rich stacking polytypes, including C2/c, C2/m, R3̅, P3₁12, etc., provide rich and distinct magnetic ground states in MX₃. Besides, a high density of strain soliton boundaries is consistently found in all MX₃, combined with likely inverted structures, allowing AFM to ferromagnetic (FM) transitions in most MX₃. Therefore, our study sheds light on the structural basis of diverse magnetic orders in MX₃, paving the way for modulating magnetic couplings via stacking engineering.