The rational design of complex nanostructures is of paramount importance to gain control over their chemical and physical properties. Recently, magnetic-plasmonic heterostructured nanocrystals have been recognized as key players in nanomedicine as multifunctional therapeutic-diagnostic tools and in catalysis. Here we show how the properties of gold-iron oxide heterostructured nanocrystals can be tuned by chemical doping of the magnetic subunit. The divalent cations in the iron oxide were substituted with cobalt and manganese to obtain a general formula Au-MFe₂O₄ (M = Fe, Co, Mn). Magnetic properties of the heterostructures could be tuned, while maintaining well-defined plasmon resonance signatures, confirming the dual magnetic-plasmonic functional capability of these nanostructures.