Anomalous displacement reaction for synthesizing above-room-temperature and air-stable vdW ferromagnet PtTe₂Ge_1/3

A novel van der Waals ferromagnet PtTe2Ge1/3 with above room-temperature Curie temperature and high air stability was synthesized through an anti-metal-activity-order displacement reaction in Pt/Cr2Ge2Te6 heterostructure. Emerging van der Waals (vdW) magnets provide a paradise for the exploration of magnetism in the ultimate two-dimensional (2D) limit, and the construction of integrated spintronic devices, and have become a research frontier in the field of low-dimensional materials. To date, prototypical vdW magnets based on metals of the first transition series (e.g. V, Cr, Mn and Fe) and chalcogen elements suffer from rapid oxidation restricted by the Hard-Soft-Acid-Base principle, as well as low Curie temperatures (T-C), which has become a generally admitted challenge in 2D spintronics. Here, starting from air-unstable Cr2Ge2Te6 vdW thin flakes, we synthesize Ge-embedded PtTe2 (namely PtTe2Ge1/3) with superior air stability, through the displacement reaction in the Cr2Ge2Te6/Pt bilayer. In this process, the anomalous substitution of Cr with Pt in the thermal diffusion is inverse to the metal activity order, which can be attributed to the compatibility between soft-acid (Pt) and soft-base (Te) elements. Meanwhile, the layered uniform insertion of Ge unbalances Pt-Te bonds and introduces long-range ordered ferromagnetism with perpendicular magnetic anisotropy and a Curie temperature above room temperature. Our work demonstrates the anti-metal-activity-order reaction tendency unique in 2D transition-metal magnets and boosts progress towards practical 2D spintronics.