1. Department of Materials Science and Engineering, Cornell University, Ithaca, New York 14853, USA
2. Department of Materials Science and Engineering, University of California, Berkeley, Berkeley, California 94720, USA
3. Laboratory of Atomic and Solid State Physics, Cornell University, Ithaca, New York 14853, USA
4. Physics Department, Cornell University, Ithaca, New York 14853, USA
5. Peter Grünberg Institute (PGI-9) and JARA-Fundamentals of Future Information Technology, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany
6. Kavli Institute at Cornell for Nanoscale Science, Ithaca, New York 14853, USA
7. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA