Observation of room temperature ferromagnetism in nanocrystalline ZnO : Co system

Nanocrystalline ZnO : Co samples (having 5 at.% Co) exhibiting room temperature ferromagnetism (RTFM) have been synthesized from acetate precursors. The effect of sequential sintering of the samples at higher temperatures for prolonged duration in air ambient has been investigated for their phase-purity and magnetic characteristics. The nanocrystalline ZnO : Co obtained after calcinations at 600 °C exhibits RTFM. The study further indicates that the Co 3 O 4 phase progressively forms with increasing sintering temperature, and partially transforms into CoO at and above 800 °C. Irrespective of the sintering temperature and duration, the samples sintered at and above 800 °C exhibited identical magnetization behaviour with regard to the saturation moment and field required for technical saturation. The ferromagnetism persisting even at higher temperatures (⩾800 °C) arises intrinsically in the insulating Zn 1− x Co x O phase. The recently proposed bound magnetic polaron model appears to account for the observed ferromagnetic ordering in this carrier deficient oxide system. A paramagnetic contribution to the overall magnetization is also observed, which is attributed to the paramagnetic Co 3 O 4 and CoO and possibly to isolated polarons and/or non-interacting Co 2+ ions.