1. Department of Electrical and Computer Engineering; University of California; San Diego 9500 Gilman Drive, La Jolla CA 92093-0407 USA
2. Materials Science and Engineering; University of California; San Diego 9500 Gilman Drive, La Jolla CA 92093-0418 USA
3. Center for Memory and Recording Research; University of California; San Diego 9500 Gilman Drive, La Jolla CA 92093-0401 USA