Two-Dimensional AXenes: A New Family of Room-Temperature d0 Ferromagnets and Their Structural Phase Transitions.

The recent discovery of two-dimensional (2D) magnetic order in monolayer CrI3 and bilayer Fe3GeTe2 has stimulated intense experimental and theoretical activities to expand the family of 2D magnets. Most 2D magnets reported to date are transition metal compounds with unpaired d electrons. Novel 2D intrinsic magnets with long-range p state coupling are also highly desirable. Here, we propose that nonstoichiometry is a feasible and universal strategy to realize long-range p electron magnetic order in 2D metal-shrouded AXenes (Na2N, K2N, and Rb2N), supported by our first-principles calculations. Taking K2N as a representative, three series of cation-deficient K2N (T, H, and I phases) have been predicted as stable ferromagnetic half-metal/metal with a Curie temperature of 480-1180 K. Their robust ferromagnetism is ascribed to the coexistence of carrier-mediated exchange and N-K-N superexchange interaction. Moreover, mechanical deformation can trigger reversible phase transformation by choosing their 3D layered counterpart as the intermediate phase.