1. Center for Memory and Recording Research, University of California-San Diego, La Jolla, California 92093-0401, USA
2. Department of Physics, University of California-San Diego, La Jolla, California 92093-0401, USA
3. Faculty of Polytechnics, University of Donja Gorica, 81000 Podgorica, Montenegro
4. Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439, USA