Topotaxial Phase Transformation in Cobalt Doped Iron Oxide Core/Shell Hard Magnetic Nanoparticles

Abstract

Core–shell nanoparticles based on a CoxFe1–xO rock-salt core, and on a shell corresponding to cubic spinel CoxFe3–xO4, have been systematically annealed to completely oxidize and generate the fully ferrimagnetic cobalt ferrite structure. The annealing has been performed through a solvent-mediated process at high temperatures to avoid interparticle aggregation, usually observed in classical annealing methods. We carefully describe how the oxidative process occurred during the initial shell passivation and in the following O2 mediated oxidation. It has been found that the rock-salt to spinel transformation occurs via topotaxial growth over the (200)RS//(400)S and (220)RS//(440)S planes shared between the two structures. This chemical transformation depends on the amount of divalent cobalt atoms present in the oxide structures. Within this respect, the solvent-mediated annealing process permits the release of a small amount of divalent cations, which allows the stoichiometry rearrangement required to form th...