1. Department of Physics and Research Center OPTIMAS, University of Kaiserslautern, D-67663 Kaiserslautern, Germany
2. Institut Jean Lamour, UMR CNRS 7198, Université de Lorraine, F-54505 Vandoeuvre-lès-Nancy, France
3. Center of Magnetic Recording Research, University of California, San Diego, California 92093-0401, USA