Experimental Study and Monte-Carlo Simulation of Exchange Bias Effect in Co-CoO Composite Powder Fabricated by High-Energy Ball Milling

Co-CoO composite powders were prepared by high-energy ball milling and subsequent annealing with the aim to probe an exchange bias (EB) effect. A microstructure consisting of flakes with a thickness of about 100 nm was revealed from scanning electron microscopy images. X-ray diffraction phase identification indicated that the optimal annealing temperature of as-milled Co for the formation of Co-CoO composite structure is 300°C. Magnetic measurements showed that saturation magnetization, Ms, of annealed Co-CoO decreased as compared to that of as-milled Co. This implies that a fraction of the oxide phase was formed after heat treatment. Furthermore, the hysteresis loop measured at 5 K after cooling in a magnetic field of 50 kOe from 350 K showed a presence of the EB effect, which reached the value of 120 Oe. It is closely related to the formation of an antiferromagnetic (AFM) CoO phase, which interacts with the adjacent Co ferromagnetic (FM) phase. A Monte-Carlo simulation was also performed to demonstrate the EB effect in FM–AFM structured materials. A better agreement between simulated and experimental hysteresis loops was obtained when averaged characteristics of the randomly-oriented individual powders were taken into account. Simulation results also showed that EB is largest when easy axes of FM and AFM phases are parallel to the magnetic field, and a critical fraction of AFM phase was suggested to be necessary for appearance of the EB effect.