Magnetic phase transition and Fermi level tuning by alkali metal doping in a 2D magnetic material

Two-dimensional (2D) magnetic materials present exciting opportunities for probing low-dimensional spin-related phenomena and offer strong potential for next-generation spintronic technologies, owing to their tunable magnetic properties and atomically thin geometries. Among these, Cr₂Ge₂Te₆ (CGT) has emerged as a prominent van der Waals ferromagnetic semiconductor, characterized by weak interlayer interactions that enable easy exfoliation down to the monolayer limit. Recent findings suggest that its magnetic and electronic properties can be effectively engineered through external perturbations such as doping, strain, and intercalation, especially in bilayer configurations. In this work, we perform a comprehensive first-principles investigation based on density functional theory (DFT) to explore how alkali metal atoms (X = Li, Na, K) influence the structural, magnetic, and electronic properties of CGT through adsorption on monolayers (CGT/X and CGT/X₂) and intercalation in bilayers (CGT/X₂/CGT). Our results reveal that the interlayer ferromagnetic (FM) configuration is the ground state for all doped bilayer systems, although the interlayer antiferromagnetic (AFM) state remains nearly degenerate in energy. Notably, in the CGT/Na₂/CGT system with AA stacking, a magnetic phase transition occurs, stabilizing an interlayer ferrimagnetic (FiM) ground state. Additionally, the relative alignment of the Fermi level with respect to the vacuum level indicates a transition from half-metallic to metallic behavior in CGT/Na₂/CGT and CGT/K₂/CGT. These findings underscore the tunability of CGT through alkali metal doping, offering valuable insights into the coupling between structural, electronic, and magnetic degrees of freedom. Our results provide a pathway toward tailoring 2D magnetic materials for future spintronic and multifunctional device applications.