Structural and Magnetic Implications of Transition Metal Migration within Octahedral Core–Shell Nanocrystals

Abstract

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.