Microwave assisted combustion method was used to produce nanocrystalline cobalt doped zinc ferrite, CoxZn1-xFe2O4, from stoichiometric mixture of (Co(NO3)(2)center dot 6H(2)O), (Fe(NO3)(3)center dot 9H(2)O), (Zn(NO3)(2)center dot 6H(2)O), and urea (CO(NH2)(2)) as a fuel. The structural, morphological and magnetic properties of the products were determined by X-ray powder diffractometry (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), vibrating sample magnetometer (VSM) respectively. The average crystallite sizes obtained from XRD were between 35 and 39 nm. Magnetization measurements indicate that samples with less Co content have superparamagnetic behavior at room temperature. When the Co substitution increases the saturation magnetization due to the magnetic character of the Co cations substituting the non-magnetic Zn and coercivity also increase due to anisotropic nature of cobalt. The CoxZn1-xFe2O4 nanocrystals exhibit typical features of an assembly of magnetic particles with a distribution of blocking temperatures and indicate the spin-glass behavior. (C) 2009 Elsevier Ltd. All rights reserved.