Thermite synthesis, structural and magnetic properties of Co-Al2O3 nanocomposite films

We have synthesized ferromagnetic nanocomposite Co-Al₂O₃ thin films via a thermite reaction between Al and Co₃O₄ with layer geometry. The starting Al/Co₃O₄ bilayers were obtained by the deposition of Al layers onto Co₃O₄ films at room temperature and were annealed at temperatures between 50 and 700 °C at 50 °C intervals. Above ∼450 °C the Co₃O₄ partially transformed into the CoO phase. The simultaneous solid-state reactions of Al with Co₃O₄ and Al with CoO started above the initiation temperature T_in ∼500 °C, which did not depend on the bilayer thickness. After annealing at 700 °C about 60% of the Co was reduced by the Al and the rest of the Co was contained in intermediate CoAl₂O₄ shells, which separated the Co nanoparticles from the Al₂O₃ matrix. Above 700 °C the reaction was complete and the final products were noninteracting Co nanoparticles with an average size of ∼40 nm enveloped by CoAl₂O₄ shells, which embedded into a dielectric Al₂O₃ matrix. The synthesized Co-Al₂O₃ nanocomposite films possessed soft magnetic behavior and good chemical stability.