Preparation and characterization of ultra-small, monodisperse CoxFe3-xO4 nanoparticles

•Co-doped superparamagnetic iron oxide nanoparticles can be synthesized at relatively low temperatures without the use of metal–organic precursors or rigid pH control.•This method provides good control over doping concentration.•Coercive field and blocking temperature increase with increasing Co concentration.•The saturation magnetization values are higher than for Co-doped iron oxide nanoparticles of similar size and Co content obtained from other methods.