Room temperature ferromagnetism and Cu doping level in ZnO nanocrystals

Cu-doped ZnO nanocrystals of size 6.3±1.1 nm reveal room temperature ferromagnetic ordering while Cu-related electron paramagnetic resonance (EPR) signal is absent at low doping level. On the other hand, at relatively higher doping level of copper (0.23%), EPR measurements confirm presence of isolated Cu2+ ions in ZnO nanocrystals while the saturation magnetization increases. Associated with these findings, considerably sharp excitonic feature is observed near band edge region of the photoluminescence emission spectra. This feature is attributed to (Cu1+, h) - (Cu2+) excitonic recombination. An exchange interaction appears to originate from this defect-Cu ion complex formation in nanocrystals. Magnetization as a function of temperature curve indicates carrier mediated ferromagnetic ordering in contrast to most of the reports published on diluted magnetic oxide nanocrystals.