The exchange bias field HE was much higher for Ta/Co/Co3O4/Ta than Ta/Co/C3O4, fabricated in a magnetron sputtering, system under the same experimental conditions. The XPS analysis showed that Ta atoms of cap layer for Ta/Co/Co3O4/Ta diffused into Co3O4 layer and reduced Co3O4, and introduced some nonmagnetic defects into the AFM layer. The dilution of the AFM layer led to the formation of volume domains. We believed that the higher H-E for the multilayers Ta/Co/Co3O4/Ta was primarily attributed to the formation of volume domain due to some nonmagnetic defects in AFM layer. (c) 2005 Elsevier B.V. All rights reserved.