Theoretical Study of the Magnetic Mechanism of a Pca21 C4N3 Monolayer and the Regulation of Its Magnetism by Gas Adsorption

For metal-free low-dimensional ferromagnetic materials, a hopeful candidate for next-generation spintronic devices, investigating their magnetic mechanisms and exploring effective ways to regulate their magnetic properties are crucial for advancing their applications. Our work systematically investigated the origin of magnetism of a graphitic carbon nitride (Pca21 C₄N₃) monolayer based on the analysis on the partial electronic density of states. The magnetic moment of the Pca21 C₄N₃ originates from the spin-split of the 2<i>p</i>_z orbit from special carbon (C) atoms and 2<i>p</i> orbit from N atoms around the Fermi energy, which was caused by the lone pair electrons in nitrogen (N) atoms. Notably, the magnetic moment of the Pca21 C₄N₃ monolayer could be effectively adjusted by adsorbing nitric oxide (NO) or oxygen (O₂) gas molecules. The single magnetic electron from the adsorbed NO pairs with the unpaired electron in the N atom from the substrate, forming a N_sub-N_ad bond, which reduces the system's magnetic moment from 4.00 μ_B to 2.99 μ_B. Moreover, the NO adsorption decreases the both spin-down and spin-up bandgaps, causing an increase in photoelectrical response efficiency. As for the case of O₂ physisorption, it greatly enhances the magnetic moment of the Pca21 C₄N₃ monolayer from 4.00 μ_B to 6.00 μ_B through ferromagnetic coupling. This method of gas adsorption for tuning magnetic moments is reversible, simple, and cost-effective. Our findings reveal the magnetic mechanism of Pca21 C₄N₃ and its tunable magnetic performance realized by chemisorbing or physisorbing magnetic gas molecules, providing crucial theoretical foundations for the development and utilization of low-dimensional magnetic materials.