Cobalt substitutions were investigated in Ni0.4Zn0.4Cu0.2Fe2O4 ferrites, initial complex permeability was then measured from 1 MHz to 1 GHz. It appears that cobalt substitution led to a decrease in the permeability and an increase in the \mus\timesfr factor. As well, it gave to the permeability spectrum a sharp resonance character. We also observed a spin reorientation occurring at a temperature depending on the cobalt content. Study of the complex permeability versus temperature highlighted that the most resonant character was obtained at this temperature. This shows that cobalt contribution to second order magnetocrystalline anisotropy plays a leading role at this temperature.