Atomically Unveiling an Atlas of Polytypes in Transition-Metal Trihalides.

Transition-metal trihalides MX 3 (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 MX 3 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 MX 3 makes their direct atomic-scale analysis challenging. Therefore, information on the stacking-registry-dependent magnetism for MX 3 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 MX 3 , as evidenced by scanning transmission electron microscopy (STEM). After surveying hundreds of MX 3 thin flakes, we provide a full spectrum of stacking orders in MX 3 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 C 2/ c phase in the antiferromagnetic (AFM) structure widely existing in MX 3 . Rich stacking polytypes, including C 2/ c , C 2/ m , R 3̅, P 3 1 12, etc. , provide rich and distinct magnetic ground states in MX 3 . Besides, a high density of strain soliton boundaries is consistently found in all MX 3 , combined with likely inverted structures, allowing AFM to ferromagnetic (FM) transitions in most MX 3 . Therefore, our study sheds light on the structural basis of diverse magnetic orders in MX 3 , paving the way for modulating magnetic couplings via stacking engineering.