In this work cobalt substituted magnesium zinc nanocrystalline spinel ferrites having general formula Mg(0.5)CoxZn(0.5-x)Fe(2)O(4) where x 0.1, 0.2, 0.3, 0.4, 0.5 were synthesized using micro-emulsion technique. The Co substituted samples annealed at 700 degrees C and characterized by various characterization techniques, such as X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), dielectric measurements and vibrating sample magnetometer (VSM). XRD analysis confirmed single phase spinel structure and the crystalline size calculated by Scherrer's formula found to be in 21.38-45.5 nm range. The lattice constant decreases as substitution of Co is increased. The decrease in lattice constant is attributed to the smaller ionic radius of cobalt as compared to zinc ion. The FTIR spectra reveled two prominent frequency bands in the wave number range 400-600 cm(-1) which confirm the cubic spinel structure and completion of chemical reaction. The dielectric parameters were observed to decrease with the increased Co contents. The peaking behavior was observed beyond 1.8 GHz. The frequency dependent dielectric properties of all these nanomaterials have been explained qualitatively in accordance with Koop's phenomenological theory. Magnetic studies revealed that the coercivity (H-c) attains maximum value of 818 Oe at similar to 21 nm. The increasing trend of magnetic parameters (coercivity and retentivity) is consistent with crystallinity. The crystallite size is small enough to attain considerable signal to noise ratio in high density recording media. The optimized magnetic parameters suggest that the material with composition Mg0.5Co0.5Fe2O4 may have potential applications in high density recording media. (C) 2014 Elsevier B.V. All rights reserved.