1. Advanced Photon Source, Argonne National Laboratory, Lemont, Illinois 60439, United States
2. Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53706, United States
3. Materials Science Division, Argonne National Laboratory, Lemont, Illinois 60439, United States
4. Center for Memory and Recording Research, University of California at San Diego, La Jolla, California 92903, United States