1210.2510 (C. N. Ge et al.)

Giant exchange bias and ferromagnetism in the CoO shell of Co/CoO-MgO
core-shell nanoparticles    [PDF]

C. N. Ge, Xiangang Wan, E. Pellegrin, Z. Hu, W. Q. Zou, Y. W. Du

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.

View original: http://arxiv.org/abs/1210.2510