1. Center for Magnetic Recording Research, University of California San Diego, La Jolla, CA 92093-0401, USA.
2. Institut Jean Lamour, UMR CNRS 7198–Université de Lorraine–BP 70239, F-54506 Vandoeuvre, France.
3. Magnetic Materials Unit, National Institute for Materials Science, Tsukuba 305-0047, Japan.
4. Department of Physics and Research Center OPTIMAS, University of Kaiserslautern, Erwin Schroedinger Strasse 46, 67663 Kaiserslautern, Germany.
5. Department of Electrical and Computer Engineering, University of California San Diego, La Jolla, CA 92093-0401, USA.