Evolution of the crystal structure, morphology, and magnetic properties of cobalt-based nanoparticles during heat treatment

Carbon-encapsulated cobalt nanoparticles (NPs) were synthesized by thermal decomposition of a solution of organic components and cobalt acetate followed by heat treatment in an argon atmosphere at different temperatures Theat from 400 to 1000 °C. The crystal structure, morphology, magnetic properties, and magnetic circular dichroism (MCD) of NPs formed at each Theat were studied. The decomposition resulted in the formation of Co3O4 NPs in a carbon shell with an average diameter of ∼10 nm. Heat treatment ensured the reduction of Co3O4 first to CoO monoxide and then to metallic cobalt with an increasingly perfect crystalline structure and a higher degree of carbon shell graphitization as Theat increased. The as-synthesized (initial) Co3O4 NPs were paramagnetic in the temperature range used in magnetization measurements. Starting with NPs treated at Theat = 500 °C, a hysteresis loop was observed. The shape of the hysteresis loop was identical for all the samples heat-treated at Theat ≥ 500 °C. The saturation magnetization (Ms) increased significantly with increasing Theat and reached 110 emu/g at Theat = 1000 °C, with a low coercivity Hc (75 Oe). The value of the MCD signal and its spectrum varied significantly with Theat.