Giant exchange bias and ferromagnetism in the CoO shell of Co/CoO-MgO   core-shell nanoparticles
  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. 
C. N. Ge, Xiangang Wan, E. Pellegrin, Z. Hu, W. Q. Zou, Y. W. Du
cond-mat.mtrl-sci cond-mat.mes-hall