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

Abstract

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.