  We report the magnetotransport properties of self-assembled Co@CoO
nanoparticle arrays at temperatures below 100 K. Resistance shows thermally
activated behavior that can be fitted by the general expression of R
exp{(T/T0)^v}. Efros-Shklovskii variable range hopping (v=1/2) and simple
activation (hard gap, v=1) dominate the high and low temperature region,
respectively, with a strongly temperature-dependent transition regime in
between. A giant positive magnetoresistance of >1,400% is observed at 10K,
which decreases with increasing temperature. The positive MR and most of its
features can be explained by the Zeeman splitting of the localized states that
suppresses the spin dependent hopping paths in the presence of on-site Coulomb
repulsion.
