Magnetic Properties of nCoO/(1-n)ZnO Nanocomposites Obtained by Calcination

Magnetic properties of the nCoO/(1-n)ZnO (n = 0.4, 0.50, 0.60 and 0.70) nanocomposites obtained by using traditional wet chemical synthesis method followed by calcination at 600∘C were investigated by dc magnetometry and magnetic resonance spectroscopy. XRD measurements revealed the presence of only two phases: ZnO (hexagonal nanocrystals with sizes in 64–300 nm range) and spinel phase Co3O4 (spheroidal nanocrystals with sizes in 14–21 nm range). Magnetic dc susceptibility measurements in 2–300 K range revealed dominating paramagnetic behavior in the whole temperature range and the presence of a strong temperature-independent component. With exception of n = 0.70 sample, no behavior connected with the expected phase transition to antiferromagnetic phase in Co3O4 and superparamagnetism was registered. Experimental results could be consistently explained by assuming that the most of high-spin Co2+ ions are involved in formation of antiferromagnetic pairs or clusters. Low intensity electron paramagnetic resonance spectra registered at RT were attributed to two different magnetic components – one involving paramagnetic Co2+ ions at Zn2+ sites in ZnO and the other due to Co2+ in Co3O4 phase or more probably the superparamagnetic resonance of Co3O4 nanoparticles. The former component dominates in nanocomposites with small concentration of cobalt, the latter in highly Co concentrated samples.