Nanocrystalline Co0.8Ni0.2AlyFe2-yO4 (y = 0.00, 0.15, 0.30, 0.45, 0.60, 0.75) powders have been synthesized by the citrate precursor auto-combustion method. The effect of the nonmagnetic aluminum ion substitution on the structural and magnetic properties has been studied. X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy revealed that the obtained powders have a single phase of cubic spinel structure. The crystallite sizes estimated from XRD data have been confirmed using high-resolution transmission electron microscopy (HRTEM). Photographs showed powders consisting of nanosized grains with an average size ranging from 10 to 45 nm, depending on the Al content. Magnetic hysteresis loops were traced at room temperature using vibrating sample magnetometer (VSM). It was found that, due to the aluminum substitution, the values of magnetic losses and saturation and remanent magnetization were decreased, indicating a reduction in the ferrimagnetic behavior. This reduction of magnetization, compared to the undoped sample, was attributed to the increase of spin noncollinearity with increasing Al content. Samples with toroidal forms were used to measure both the initial permeability (mu(i)) and Curie temperature (T-C). The obtained results showed a decrease in the values of both mu(i) and T-C as the Al content increases. This behavior was explained in view of the weakness in the interaction of A and B sublattices with increasing Al3+ ion concentration. (C) 2014 Elsevier B.V. All rights reserved.