Nitrogen-driven ferromagnetism and perpendicular magnetic anisotropy in two-dimensional transition-metal nitrides.

Data-centric technologies demand energy-efficient, densely integrable spintronic building blocks, motivating the search for two-dimensional (2D) ferromagnets combining room-temperature Curie temperatures (TC) and perpendicular magnetic anisotropy (PMA). Compared with halides and chalcogenides, transition-metal nitrides (TMN2) feature short metal-nitrogen bonds and strong p-d hybridization, which can substantially reshape crystal-field splitting and magnetic exchange pathways. Through systematic first-principles screening of hexagonal h-TMN2 (TM = 3,4,5d) monolayers, we identify h-VN2 and h-CrN2 as the only stable candidates exhibiting intrinsic PMA and half-metallicity with sizable spin-flip gaps (Δsf = 0.27 eV for h-VN2 and 0.25 eV for h-CrN2). Phonon spectra and ab initio molecular dynamics simulations confirm their dynamical and thermal stability, with h-VN2 preserving crystalline integrity up to 800 K. Monte Carlo simulations confirm out-of-plane easy axes with TC values of ≈305 K for h-VN2 and ≈134 K for h-CrN2. Notably, h-VN2 exhibits pronounced magnetoelectric tunability: its half-metallicity and PMA are robustly preserved within a broad biaxial strain window (-2%-+3%), maintaining near-room-temperature TC. In bilayer configurations, vertical stacking further strengthens exchange interactions, enhancing TC to ∼477 K. Furthermore, h-VN2 sustains its ferromagnetism and PMA at both graphene and MoS2 interfaces, underscoring its compatibility with existing 2D platforms. These results highlight h-VN2 as a versatile platform where nitrogen-mediated exchange facilitates robust, high-temperature ferromagnetism for next-generation van der Waals spintronics.