Cu-Doped SnO2 Nanocrystals: Tunable Magnetism, Critical Incorporation Limit, and Defect Configuration Analysis by EPR and Ab Initio 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.