1. Department of Materials Science and Engineering, Cornell University, Ithaca, New York 14853, USA
2. Physics Department, Cornell University, Ithaca, New York 14853, USA
3. Peter Grünberg Institute (PGI-9) and JARA-Fundamentals of Future Information Technology, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany
4. Kavli Institute at Cornell for Nanoscale Science, Ithaca, New York 14853, USA