Engineering 2D high-temperature ferromagnets with large in-plane anisotropy via alkali-metal decoration in a tetragonal CoSe monolayer

Two-dimensional (2D) ferromagnetic materials with high Curie temperature (Tc) and large magnetic anisotropy energies (MAEs) are critical for nanoscale spintronics but remain rare. Using first-principles calculations, that adsorbing alkali atoms (A = Li, Na, K, Rb, Cs) onto a tetragonal CoSe monolayer transforms it into a series of stable 2D ferromagnetic metals, ACoSe, with an in-plane easy axis. Notably, LiCoSe is a half-metal. Compared with the pristine layer, these functionalized monolayers exhibit dramatically enhanced ferromagnetism, with Tc > 300 K and MAE > 800 μeV/Co. The coupled alkali atoms amplify the local magnetic moment of Co ions, strengthen ferromagnetic Ruderman–Kittel–Kasuya–Yoshida (RKKY) and superexchange couplings, and simultaneously weaken the direct antiferromagnetic exchange between Co ions. Tensile strain can further increase both the MAE (via band shifting) and Tc (by strengthening the nearest-neighbor exchange J1). Among the series, NaCoSe exhibits the highest MAE and excellent strain-modulated Tc, rendering it the most promising candidate material. These results establish alkali-metal decoration as an effective strategy for realizing 2D ferromagnets with high Tc and large MAE in tetragonal lattices.