Magnetic Reconstruction at Grain Boundaries in Ni-dihalides

Grain boundaries (GBs) naturally form in and dominate the physical properties of polycrystals. However, the associated atomic reconstruction and magnetism have been scarcely studied in vdW magnets. Here, we systematically investigate the thermodynamically stable GBs in magnetically frustrated Ni-dihalides, where the reconstructed atomic configurations not only give rise to dangling bounds that suppress the local magnetic moment at the GBs but also induce strongly antiferromagnetic coupled four-coordinate Ni atoms, leading to two segmented helical domains that are antiferromagnetically aligned. Moreover, the defective energy can stabilize ultrasmall metastable skyrmions/antiskyrmions (∼1.4 nm) in NiCl₂ and NiBr₂. They are pinned at the GB and remain stable even when B_z > 18 T. Additionally, lower magnetic fields can also nucleate nanometer-sized skyrmions/antiskyrmions within the domain regions, which can move along the GB direction under a spin-transfer-torque effective field. These results provide a detailed description of magnetism in vdW GBs and provide a new strategy to eliminate the skyrmion Hall effect.