Manipulating the Magnetic Structure of Co Core/CoO Shell Nanoparticles: Implications for Controlling the Exchange Bias

We present an experimental study of the effects of oxidation on the magnetic and crystal structures of exchange biased ϵ-Co/CoO core-shell nanoparticles. Transmission electron microscopy measurements reveal that oxidation creates a Co-CoO interface which is highly directional and epitaxial in quality. Neutron diffraction measurements find that below a N\'eel temperature TN of ~235 K the magnetization of the CoO shell is modulated by two wave vectors, q₁=(1/2 1/2 1/2)2πa and q₂=(100)2πa. Oxidation affects the q₁ component of the magnetization very little, but hugely enhances the q₂ component, resulting in the magnetic decompensation of the core-shell interface. We propose that the large exchange bias effect results from the highly ordered interface between the Co core and CoO shell, and from enhanced core-shell coupling by the uncompensated interface moment.