1. International Center for Materials Nanoarchitectonics, National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan
2. PRESTO and CREST, JST, Kawaguchi, Saitama 332-0012, Japan
3. Department of Physics and Astronomy, University of Basel, 4056 Basel, Switzerland
4. Peter Grünberg Institut, Forschungszentrum Jülich and JARA, D-52425 Jülich, Germany
5. California NanoSystems Institute at UCLA, Los Angeles, California 90095, United States