Ultra-fine nanocrystals of ZnxFe1-xFe2O4 (x = 0.1, 0.3, 0.5 and 0.7) have been synthesized by chemical coprecipitation method at low temperature (similar to 60 degrees C). Rietveld fitted X-ray diffraction patterns confirm the formation of cubic spinel structure with Fd-3m space group. High resolution transmission electron microscopy (HRTEM) analysis reveals that the average particle size reduces with the substitution of higher Zn concentration from 12 to 3 nm for sample x = 0.1 to 0.7. Vibrating sample magnetometer (VSM) results indicate that the blocking temperature (T-B) for sample x = 0.1 is 120 K and significantly decreases up to less than 5 K as we move towards the sample x = 0.7. Moreover, the observed irreversible magnetization (Delta M = M-FC - M-ZFC), coercivity and remanence are indistinguishable for sample x = 0.7 down to 5 K, which infers the superparamagnetic behaviour in this sample in a wide range of temperature from room temperature down to 5 K. The reduction in peak to peak line width and enhanced magnetic field asymmetry observed in electron paramagnetic resonance (EPR) spectra also supports the presence of superparamagnetism in sample x = 0.7. (c) 2012 Elsevier B.V. All rights reserved.