Positive and negative exchange bias in maghemite nanoparticles

Maghemite nanoparticles were synthesized using sol-gel method. The structural study of the samples was done using XRD technique. Zero-field-cooled (ZFC) and field-cooled (FC) magnetization versus applied field (M-H) measurements were conducted between-5 T and +5 T at several temperatures and several field cooled values. The XRD study revealed that the average diameter of the nanoparticles to be around 20 nm. XRD patterns show mainly maghemite phase with very slight hematite phase. The coercively was found to decrease sharply with temperature up to 200 K above which it remains almost constant. Negative and positive horizontal exchange bias was obtained in the ZFC state and only negative exchange bias was obtained at all field cooled values. The magnitudes of the exchange bias were found to decrease sharply with temperature and disappearing above 100 K. Negative and positive vertical loop shifts were obtained in the ZFC state and only positive loop shifts were obtained at all field cooled values. The magnitudes of the loop shifts were found to decrease sharply with temperature and disappearing above 50 K. These results are discussed and attributed mainly to the exchange coupling between the maghemite (ferrimagnetic) and hematite (antiferromagnetic) phases in the nanoparticle system.