Structural And Magnetic Implications Of Transition Metal Migration Within Octahedral Core-Shell Nanocrystals

Octahedron-shaped cobalt oxide nanocrystals undergo a structural evolution once coated with thin shells of manganese or cobalt ferrite, by means of an asymmetric solid-solid diffusion occurring at the interface established between the oxides. The resultant mixed ferrites in the final nanostructures stem from the phase progression associated with a nonequilibrium kinetic product that evolves to reach the thermodynamic equilibrium. In this process, the initially strained crystalline lattice closer to the interface influences the progressive redistribution of Co2+ cations diffusing out of the initial cobalt oxide core, dictating the final magnetic properties. When starting with a nonstoichiometric manganese ferrite shell, the preferential occupation of tetrahedral sites by Mn2+ cations forces the Co2+ to occupy octahedral sites, offering a Mn- and Co-doped magnetite shell onto the CoO core. However, when starting with a cobalt ferrite shell, an extra doping of Co2+ cations in the strained layers close to the interface forces this ferrite to transition from the inverse to the normal spinel structure, leading to core-shell nanocrystals of CoO and Co-rich cobalt ferrite with an enhanced magnetic moment.