Oxygen-vacancy-induced half-metallicity and enhanced ferromagnetism in Sr₂NiMnO₆: a DFT and Monte Carlo approach

Realizing room-temperature spintronic functionality requires materials that maintain complete spin polarization at the Fermi level and robust ferromagnetic stability against thermal fluctuations. Here, Sr₂NiMnO₆ (SNMO) is introduced as a novel, high-performance double-perovskite candidate, and a mechanism to unlock its half-metallic potential via oxygen-vacancy (V_O) engineering is uncovered. Using a combined DFT, lattice dynamics, and Monte Carlo approach, it is revealed that V_O induction triggers a symmetry reduction (from I4/m to P1), driving a Ni-centered redox transition from a formal Ni⁴⁺ toward a Ni²⁺-like character, thereby evolving the electronic state from a spin-asymmetric semiconductor to a half-metal. The transition is further characterized by a substantial increase in the carrier effective mass (up to 1.235 m ₀), consistent with the emergence of localized polaronic character supporting the material's high-temperature stability and spin-selective transport. This structural-electronic coupling reconfigures the magnetic interaction from a weak superexchange-dominated regime toward an enhanced carrier-mediated exchange, consistent with, but not directly demonstrated by, the localized in-gap Ni states reported here, elevating the saturated magnetic moment to 6.00 µ _B (from -2.97 µ _B) and the Curie temperature to 480 K. These analyses establish a direct relationship between oxygen-vacancy formation, structural distortion, electronic reconstruction, and magnetic ordering in SNMO, highlighting oxygen-vacancy engineering as a strategy for tuning the spin-dependent properties of double perovskites for high-temperature spintronic applications.