Direct Imaging of an Order-to-disorder Transition in an Exchange-coupled CoO/Co Bilayer

Exchange bias was discovered in surface-oxidized cobalt particles six decade ago, yet its microscopic mechanism is still being debated because of the lack of atomic-scale measurements at the interfacial region between the CoO and the Co layers. Here, we report our atom-resolved measurements by using scanning tunneling microscopy/spectroscopy of an ultrathin CoO layer grown on a Co(0001) single crystal. We have found that the layer undergoes an order-to-disorder transition upon cooling from a crystalline structure at room temperature to a disordered, glassy state at low temperatures. Accordingly, its electronic state changes from a metallic character in the crystalline phase to an Efros-Shklovskii Coulomb gap state in the disordered one. These findings provide direct evidence that exchange bias in the CoO/Co heterostructure is mediated by the atomic-scale disorder in or the spin-glass-like phase of the CoO layer.