1. NanoEngineering, University of California San Diego, La Jolla, California 92093, USA
2. Center for Memory and Recording Research, University of California San Diego, La Jolla, California 92093-0401, USA
3. Department of Physics, University of California San Diego, La Jolla, California 92093, USA
4. National Institute for Materials Science, Sengen, Tsukuba 3050047, Japan