Hard ferromagnetic van-der-Waals metal (Fe,Co)GeTe: a new platform for the study of low-dimensional magnetic quantum criticality.

The widely-studied ferromagnetic van-der-Waals (vdW) metal FeGeTe has great promise for studies of quantum criticality in the 2D limit, but is limited by a relatively high Curie temperature in excess of 200 K. To help render the quantum critical point achievable in such a system within the reach of practically possible tuning methods, we have grown single crystals of a variant of (Fe,Co)GeTe with useful physical properties for both this purpose and the wider study of low-dimensional magnetism and spin transport. (Fe,Co)GeTe is found through x-ray diffraction and electron microscopy to have an equivalent crystal structure to FeGeTe, with a random distribution of the cobalt dopant sites. It exhibits a sharp ferromagnetic transition at a value below 40 K, a stronger anisotropy and a coercive field ten times larger than that of FeGeTe. The transport properties and specific heat show the electronic properties and strong correlations of FeGeTe to be near-unchanged in this doped material. We demonstrate that (Fe,Co)GeTe can be cleanly exfoliated down to monolayer thickness. This unprecedented hard metallic vdW ferromagnet is a valuable new addition to the limited range of materials available for the study of 2D magnetism.