Exchange bias effect at the irregular interfaces between Co and CoO nanostructures

Nanostructures of Co and CoO particles were synthesized by using a modified sol-gel technique. Coexistence of Co and CoO with fcc structure is identified by powder X-ray diffraction and electron diffraction studies. Particle sizes are found to be in between 4 nm and 13 nm obtained from Transmission Electron Microscopy where high resolution images do not reveal any regular core-shell structure; rather it shows a coexistence of Co and CoO nanostructure having irregular interfaces between them. A considerable shift in the magnetic hysteresis loop associated with the substantial enhancement of coercivity (from ∼1.0 kOe to ∼3.7 kOe) is observed at 5 K when the composite was cooled in field-cooled mode with a static cooling field, Hcool = 8 kOe. The shift in the hysteresis loop is the typical manifestation of exchange bias effect which is strongly dependent on the cooling field. The results reveal that substantial enhancement of coercivity is attributed to the strong exchange coupling at the irregular Co/CoO interfaces where strong exchange bias effect does not require any regular core-shell structure.