Defect induced, layer-modulated magnetism in ultrathin metallic PtSe(2)

Defects are ubiquitous in solids, often introducing new properties that are absent in pristine materials. One of the opportunities offered by these crystal imperfections is an extrinsically induced long-range magnetic ordering,1 a long-time subject of theoretical investigations.1–3 Intrinsic, two-dimensional (2D) magnetic materials4–7 are attracting increasing attention for their unique properties including layer-dependent magnetism4 and electric field modulation6. Yet, inducing magnetism into otherwise non-magnetic 2D materials remains a challenge. Here, we investigate magneto-transport properties of ultrathin PtSe(2) crystals and demonstrate unexpected magnetism. Our electrical measurements show the existence of either ferromagnetic or anti-ferromagnetic ground state orderings depending on the number of layers in this ultra-thin material. The change in the device resistance upon application of a ~ 25 mT magnetic field is as high as 400 Ω with a magnetoresistance (MR) value of 5%. Our first-principles calculations suggest that surface magnetism induced by the presence of Pt vacancies and the Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange couplings across ultra-thin films of PtSe(2) are responsible for the observed layer-dependent magnetism. Considering the existence of such unavoidable growth-related vacancies in 2D materials, 8,9 these findings can expand the range of 2D ferromagnets into materials that would otherwise be overlooked.