Neutron diffraction studies of exchange-biased Co/CoO core-shell nanoparticles

We present neutron diffraction data on exchange-biased Co/CoO core-shell nanoparticles. Surfactant-coated Co particles were prepared by thermal decomposition of Co$_{2}$(CO)$_{8}$, then oxidized to form core/shell particles with a poly-crytalline CoO shell. Magnetization data display strong exchange bias below the 200K blocking temperature (T$_{B})$, well below the Neel temperature of bulk CoO (293 K). Neutron diffraction data show a broad peak at the ($\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $ $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $ $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} )$ antiferromagnetic CoO ordering wave vector. The anti-ferromagnetic order parameter is constant below T$_{B}$, and decreases with increasing temperature above T$_{B}$. These results demonstrate that the diminished T$_{B}$ in small core/shell particles is not due to a reduction of T$_{N}$ of the antiferromagnetic shell.