Tuning magnetic order in the van der Waals metal Fe5GeTe2 by cobalt substitution

This paper is a contribution to the joint Physical Review Applied and Physical Review Materials collection titled Two-Dimensional Materials and Devices. Fe5-xGeTe2 is a van der Waals material with one of the highest reported bulk Curie temperatures, TC≈310K. 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 ≈30% enhances TC and changes the magnetic anisotropy, while ≈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 ≈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.