First-Principles Investigation of Oxygen Vacancy Defects and Strain Engineering on the Electronic Structure and Magnetic Ordering in Perovskite SrTiO3: A DFT+U Framework

Abstract

Perovskite strontium titanate (SrTiO3) is a model material system for the investigation of correlated electron effects, emergent interface properties, and functional spintronics. A prototypical non-magnetic band insulator SrTiO3, on the other hand, is often associated with defect-induced d0 ferromagnetism through intrinsic oxygen vacancies. However, an in-depth quantitative understanding combining defect thermodynamics, continuous mechanical strain, and long-range magnetic ordering in a unified ab initio framework has been lacking.  This first-principles study combines density functional theory with Hubbard corrections (GGA+U) to address couplings between neutral oxygen vacancies and continuous biaxial strain from -4% compressive to +4% tensile. Methodological convergence benchmarks ensure numerical stability within +/-0.01 eV for bandgaps and +/-0.02 muB for magnetic moments. Phonon dispersion spectra and elastic constants prove mechanical and dynamical lattice stability without imaginary vibrations across all strains.  Under reducing conditions, in-plane oxygen vacancies are selectively stabilized by tensile strain, with formation energy dropping linearly (R2 = 0.996) from 5.74 eV to 4.21 eV at +4% strain. Oxygen removal releases two electrons, transforming neighbouring Ti4+ (3d0) into magnetic Ti3+ (3d1) polarons, confirmed by quantitative Bader charge transfer (0.48 e- depletion) and orbital occupation matrices. The C4v crystal field splits t2g states, sinking Ti-3d_xy below the Fermi level at +4% tensile strain to achieve a 100% spin-polarized half-metallic state. Classical Monte Carlo simulations reveal long-range ferromagnetic ordering with a Curie temperature of TC = 342 K, overcoming longstanding mean-field overestimation while agreeing well with room-temperature magnetometry measurements.