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

The recent discovery of two-dimensional (2D) magnetic order in monolayer CrI and bilayer FeGeTe 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 (NaN, KN, and RbN), supported by our first-principles calculations. Taking KN as a representative, three series of cation-deficient KN (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.