This work focuses on the synthetic control of magnetic properties of mixed oxide magnetic nanoparticles of the general formula Fe3-xCoxO4 (x <= 0.33) in the protein cage ferritin. In this biomimetic approach, variations in the chemical synthesis result in the formation of single-phase Fe3-xCoxO4 alloys or intimately mixed binary phase Fe/Co oxides, modifying the chemical structure and magnetic behavior of these particles, as characterized by static and dynamic magnetization measurements and X-ray absorption spectroscopy.