For CoO overlayers prepared by evaporating Co atoms in an oxygen atmosphere, both the oxidation of the Co atoms at the interface and the segregation of oxygen atoms into the Co surface occur. Parts of Co atoms at the interface become nonferromagnetic and this causes the reduction of Kerr intensity at 300 K. After field cooling treatments, further reduction of the Kerr intensity is detected. The change of the Kerr intensity is an indicator of the pinned magnetic moments. In the case of forming thicker interfacial region, more pinned magnetic moments are observed and result in the larger exchange bias field for CoO on thicker Co/Ge(1 00). This tunable pinning provides a practical way of increasing exchange bias field by controlling the thickness of the interfacial region for ultrathin CoO/Co bilayers on semiconductor substrates. (C) 2015 Elsevier B.V. All rights reserved.