Structural asymmetry induced high Curie temperature and large out-of-plane piezoelectricity in Gd₂COF MXenes and Gd₂COF/MoS₂ heterostructure

Two-dimensional (2D) rare-earth (RE) ferromagnetic (FM) materials exhibit significant potential for next-generation spintronic devices. In this work, first-principles calculations are performed to study the MXene Gd₂COF monolayer. We demonstrate that Gd₂COF is a ferromagnetic half-metal with a large magnetic moment of 13 μ_B per formula unit and a Curie temperature (T_C) of 750 K, the highest one among the reported RE monolayers. The high T_C is attributed to both the out-of-plane structural asymmetry induced interlayer FM double-exchange interaction and the dual intralayer FM super-exchange pathways. Additionally, the system possesses a wide band gap of 3.09 eV and a magnetic anisotropy energy (MAE) of −0.25 meV, features consistent with half-metallicity and ferromagnetism. The monolayer Gd₂COF is metallic with −5% ∼ −4% biaxial strain, transforms into a half-metal with −3% ∼3% biaxial strain, and then becomes a semiconductor with 4% ∼5% biaxial strain. It has a high Curie temperature of 550 K ∼ 900 K throughout the entire strain range. The Curie temperature decreases with increasing biaxial strain due to the weakened Gd–Gd and Gd–C/O/F bonds. Furthermore, the monolayer exhibits significant out-of-plane piezoelectricity at tensile strains of 4% and 5%. Furthermore, the planar Gd₂COF can be stabilized when supported on monolayer MoS₂ substrate to avoid bending. The Gd₂COF/MoS₂ heterostructure also has a high Curie temperature of 550 K ∼ 900 K with −5% ∼ 5% biaxial strain. The metal–half-metal–semiconductor transition and large piezoelectricity with strain can be similarly observed in this heterostructure.