Exchange biasing and magnetic properties of partially and fully oxidized colloidal cobalt nanoparticles

Colloidal magnetic nanoparticles (NPs) have been applied in magnetic separations, in medicine and in biochemistry. They are also potentially applicable in magnetic recording media. In this paper, we report a systematic investigation of the magnetic properties of colloidal Co NPs after three extents of oxidation. The native sample has a thin (1.0 nm) CoO shell and exhibits no exchange biasing. The purposefully partially oxidized sample has a thicker CoO shell (3.2 nm), and is exchange biased. The sample fully oxidized to CoO loses exchange biasing. We observe three distinct magnetic properties that result from the finite-thickness antiferromagnet shell exchange coupled to a finite-size ferromagnet core, and from crystal and stoichiometric defects: (1) an enhancement of the thermal stability of the orientation of the magnetic moment due to exchange biasing in the partially oxidized sample, (2) a low-temperature paramagnetic response in the partially and fully oxidized samples due to defects in the CoO shell, and (3) an asymmetry in the field-dependent magnetization for the partially oxidized sample at low temperature due to small clusters of Co in a diffusion layer around the Co core. We propose a simple model to interpret these effects.