Cu-Doped SnO ₂ Nanocrystals: Tunable Magnetism, Critical Incorporation Limit, and Defect Configuration Analysis by EPR and <i>Ab Initio</i> DFT

Copper in oxide semiconductors exhibits distinct electronic and magnetic behavior depending on its oxidation state, with Cu1+ acting as a nonmagnetic impurity and Cu2+ contributing to the magnetic moment. In SnO2, Cu incorporation represents a heterovalent substitution for Sn4+, which inherently promotes formation of compensating defects, particularly oxygen vacancies, that can strongly influence electronic and magnetic properties. To elucidate these effects, we investigated Cu-doped SnO2 nanocrystals through combined experimental and theoretical approaches. Electron paramagnetic resonance (EPR) revealed that Cu incorporation of up to 3% enhances resonance intensity, consistent with isolated Cu2+ ions in the SnO2 matrix. Beyond 3%, the EPR signal intensity decreases, and hyperfine parameters stabilize due to Cu2+ clustering and spin–spin interaction. Magnetization measurements revealed a paramagnetic phase (reflecting the presence of isolated Cu2+) that coexists with a ferromagnetic phase attributed to bound magnetic polarons and magnetic clustering. Complementary first-principles calculations showed that Cu substitution modifies the electronic structure by introducing localized density of states variations and altering the spin–charge density distribution, particularly near oxygen vacancies. Deeper in-plane defects were found to stabilize magnetization, whereas surface defects promoted competing ferromagnetic and antiferromagnetic interactions. Structural characterization by X-ray diffraction and morphological analysis using high-resolution transmission electron microscopy further confirmed lattice compression and particle size reduction with increasing Cu-content. The calculated and experimental findings provide a comprehensive and interconnected understanding, not yet emphasized in the literature, of the interplay among defects, doping, and magnetism in Cu-doped SnO2.