Effect of annealing on the exchange coupling between magnetic phases and the exchange bias effect in Co/CoO nanocomposite

In this study, Co particles were synthesized by a polyol method, and the subsequent annealing in air was performed to prepare Co/CoO nanocomposites. Samples were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and magnetic measurements. SEM images of a typical nanocomposite sample pointed out the agglomeration of about 20 nm particles into micrometer-sized spheres. XRD analysis showed an increased content of the CoO phase with annealing time. The main focus was given to the effect of annealing on the exchange coupling between the Co ferromagnetic phase and the CoO antiferromagnetic phase, which in turn affects the exchange bias effect (EB) in this material. It was shown that annealing enhanced the exchange coupling between two phases leading to an increase of the exchange bias field (Heb) as compared to the unannealed sample. A value of Heb = 155 Oe was attained with annealing at a temperature of 300 degrees C for 90 min. The dependence of Heb on the exchange coupling between phases was discussed using a 1D-Heisenberg-model Monte Carlo simulation. Besides, the simulation also underpined the experimental data on temperature dependence of the EB and coercivity.