Manipulating the magnetism in Fe₅GeTe₂ by cobalt substitution

The van der Waals metal Fe_5-xGeTe₂ has one of the highest reported Curie temperatures (270-310K) for cleavable materials. The crystal structure contains three Fe sublattices, one of which possesses intrinsic disorder and greatly impacts the magnetism. Here, the impact of cobalt substitution on the lattice and magnetic properties will be discussed. Ferromagnetism persists with modest cobalt concentration and a change in the magnetic anisotropy occurs near 20% cobalt for iron. Near 50% cobalt, the layer stacking changes from rhombohedral to primitive and a decrease in stacking fault density occurs. A concomitant change to antiferromagnetic order is demonstrated through magnetization measurements that reveal a Néel temperature near 340K. These experimental findings are supported by first principles calculations that suggest site preference exists for cobalt substitution, and the calculations also demonstrate that both a relatively high cobalt content and the primitive layer stacking are required to produce the antiferromagnet ground state. These results demonstrate a strong coupling of atomic configurations and interlayer stacking to the magnetism in a metallic, cleavable system.