Thermite synthesis and characterization of Co–ZrO2ferromagnetic nanocomposite thin films

Abstract

Co–ZrO2 ferromagnetic nanocomposite thin films were successfully synthesized using a new thermite reaction between Zr and Co3O4 in layer geometry. The initial Zr/Co3O4 bilayers were obtained by the deposition of Zr layers onto Co3O4 films at room temperature. The process of mixing at the Zr/Co3O4 interface and synthesis of fine-crystalline Co and amorphous ZrO2 phases started at a temperature above the initiation temperature Tin ∼250 °С which did not depend on the bilayer thickness. For the bilayer thickness more than 300 nm high-temperature fcc-Co and cubic c-ZrO2 (or tetragonal t-ZrO2) phases were formed. For the bilayer thickness less than ∼50 nm stable low-temperature hexagonal hcp-Co and monoclinic m-ZrO2 phases were also present in the reaction product. A partial transformation from Co3O4 to CoO occurred after annealing at 300 °С. The secondary reaction between CoO and Zr started soon after Co3O4 had been converted to the CoO phase. After annealing at 500 °С more than 80% of Co was reduced and the final product contained Co nanoparticles above and below the superparamagnetic critical size embedded into a dielectric ZrO2 matrix. The synthesized Co–ZrO2 nanocomposite films possessed soft magnetic behavior, high magnetization and good chemical stability.