Nature of room-temperature ferromagnetism from undoped ZnO nanoparticles

ZnO nanoparticles were synthesized by a sol-gel calcination process following being annealed in air at 400, 600, 800 and 1000A degrees C. X-ray diffraction pattern and X-ray photoelectron spectroscopy show that all the samples present a typical wurtzite structure and no other impurity phases are observed. Room-temperature ferromagnetism from all the samples is confirmed by the vibrating sample magnetometer measurements, which shows that the RTFM decreases with the annealing temperature increasing from 400 to 800A degrees C, and then became larger for the sample annealed at 1000A degrees C. According to the Raman scattering spectra and electron paramagnetic resonance, the RTFM of samples annealed at 600, 800 and 1000A degrees C might be related to the oxygen vacancy related defects. However, the RTFM from the sample annealed at 400A degrees C, presenting nearly the same value as that of the sample annealed at 1000A degrees C, could originate with the interstitial zinc defects associated with XPS and photoluminescence analyses.