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 epsilon-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 Neel temperature T-N of similar to 235 K the magnetization of the CoO shell is modulated by two wave vectors, q(1)= (1/2 1/2 1/2)2 pi/alpha and q(2) = (100)2 pi/alpha. Oxidation affects the q(1) component of the magnetization very little, but hugely enhances the q(2) 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.