1. Department of Physics, University of Oregon, Eugene, Oregon 97401, USA
2. Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
3. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
4. Center for Memory and Recording Research, University of California, San Diego, California 92093, USA
5. Physics Department, University of California, Santa Cruz, California 94056, USA
6. Department of Physics, University of California, San Diego, California 92093, USA