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.

• Received 24 October 2004

DOI:https://doi.org/10.1103/PhysRevB.72.064404