order in the van der Waals metal Fe5GeTe2 cobalt substitution

Fe5-xGeTe2 is a van der Waals material with one of the highest reported bulk Curie temperatures, T-C approximate to 310 K. In this study, theoretical calculations and experiments are utilized to demonstrate that the magnetic ground state is highly sensitive to local atomic arrangements and the interlayer stacking. Cobalt substitution is found to be an effective way to manipulate the magnetic properties while also increasing the ordering temperature. In particular, cobalt substitution up to approximate to 30% enhances T-C and changes the magnetic anisotropy, while approximate to 50% cobalt substitution yields an antiferromagnetic state. Single crystal x-ray diffraction evidences a structural change upon increasing the cobalt concentration, with a rhombohedral cell observed in the parent material and a primitive cell observed for approximate to 46% cobalt content relative to iron. First-principles calculations demonstrate that it is a combination of high cobalt content and the concomitant change to primitive layer stacking that produces antiferromagnetic order. These results illustrate the sensitivity of magnetism in Fe5-xGeTe2 to composition and structure, and emphasize the important role of local structural order-disorder and layer stacking in cleavable magnetic materials.