Interplay of dopants and defects in making Cu doped TiO₂ nanoparticle a ferromagnetic semiconductor

Here we have studied the role of oxygen defects and Cu dopants on ferromagnetism in Cu doped TiO₂ nanoparticles with nominal Cu concentration of 2%, 4% and 6 mol%. Electron paramagnetic resonance (EPR) spectra analysis reveals the presence of Cu²⁺ in the distorted octahedral coordination of TiO₂. Cu d-states undergo strong p-d coupling with the valence band O 2p state of TiO₂ resulting the extended absorption hump in the visible region. Photoluminescence results reveal the presence of oxygen defect related emission peaks in Cu doped TiO₂. Room temperature ferromagnetism is observed in all the Cu doped TiO₂ nanoparticles. Saturation magnetization is the highest at 4 mol% and then there is a decrease in magnetization at 6 mol%. Ferromagnetism completely disappears on calcinations of 4% Cu doped TiO₂ in air at 450 °C for 8 h. It is speculated that both oxygen vacancies and Cu d-states are involved in the room temperature ferromagnetism. Spin polarization occurs by the formation of bound magnetic polaron between electrons in Cu²⁺d-states and the unpaired spins in oxygen vacancies. Presence of Cu²⁺-Cu²⁺d-d exchange interaction and Cu²⁺-O²⁻-Cu²⁺ antiferromagnetic superexchange interactions might have resulted in the reduction in magnetization at 6 mol% Cu. - Graphical abstract: Ferromagnetism in Cu doped TiO₂ requires presence of both Cu dopant and oxygen vacancies. - Highlights: • Cu doped TiO₂ nanoparticle displays room temperature ferromagnetism. • Ferromagnetism requires presence of both Cu and oxygen vacancies. • Antiferromagnetic interaction persists at high Cu dopant concentration. • Paramagnetism appears on air annealing of the doped system for longer period