We report on the preparation of cobalt-substituted nickel–zinc (Ni–Zn) ferrite aluminate nanoparticles (Ni0·3Zn0.7−xCoxFeAlO4) with x = 0.0–0.5 using sol–gel chemical synthesis method. The combination of metal ions forms the structure of a spinel cubic lattice for which the lattice parameter decreases and the crystallite size increases with increasing Co2+ substitution. We have determined the cation distribution of the ferrite aluminate with the help of X-ray diffraction analysis. Analysis of Fourier-transform infrared spectra demonstrates the formation of the spinel cubic structure. The nanocrystalline nature of the product with 28−34 nm sizes is determined from the transmission scanning-electron-microscope images. The saturation magnetization of the ferrite aluminates increases with increasing Co2+ ion substitution. The magnetocrystalline anisotropy also increases with increasing Co2+ substitution, influencing the coercivity of the ferrite aluminates. Analysis shows that the frequency-dependent complex permeability of the ferrite aluminates is controlled by domain-wall motion at lower frequencies and by spin rotations at higher frequencies.