[1] INM Leibniz Inst New Mat, Saarbrucken, Germany
[2] Saarland Univ, Dept Phys, Saarbrucken, Germany
[3] Forschungszentrum Julich, Ernst Ruska Ctr Microscopy & Spect Electrons, Julich, Germany
[4] Forschungszentrum Julich, Peter Grunberg Inst, Julich, Germany
[5] Weizmann Inst Sci, Dept Chem Res Support, Rehovot, Israel
[6] IBM TJ Watson Res Ctr, Yorktown Hts, NY 10598 USA
[7] MIT, Dept Mat Sci & Engn, Cambridge, MA 02139 USA