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

Transition-metal trihalides MX (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 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 makes their direct atomic-scale analysis challenging. Therefore, information on the stacking-registry-dependent magnetism for MX 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, as evidenced by scanning transmission electron microscopy (STEM). After surveying hundreds of MX thin flakes, we provide a full spectrum of stacking orders in MX 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 2/ phase in the antiferromagnetic (AFM) structure widely existing in MX. Rich stacking polytypes, including 2/, 2/, 3̅, 312, , provide rich and distinct magnetic ground states in MX. Besides, a high density of strain soliton boundaries is consistently found in all MX, combined with likely inverted structures, allowing AFM to ferromagnetic (FM) transitions in most MX. Therefore, our study sheds light on the structural basis of diverse magnetic orders in MX, paving the way for modulating magnetic couplings stacking engineering.