  Using magnetron sputtering, we produced a series of Co/CoO-MgO nanoparticles
on Si(100) substrates. High-resolution transmission electron microscopy (HRTEM)
image shows that small isolated Co-clusters (core) covered with CoO (shells)
with a size of a few nm embedded in a MgO matrix. Resistivity as a function of
Co atomic ratio exhibits a distinct percolation threshold with a sharp decrease
around 69% Co content. Across the threshold, the resistivity drops about 7
orders of magnitude. For a sample at this percolation critical threshold, we
have observed a giant exchange bias field HE=2460 Oe at T= 2K, and using soft
x-ray magnetic circular dichroism at the Co-L2,3 edge, we have detected a
ferromagnetic (FM) signal originating from the antiferromagnetic CoO shell.
Moreover, decreasing the Mg-impurities will reduce the FM signal from CoO shell
(namely the uncompensated spin density) and the size of HE, thus directly
support the uncompensated spin model.
