1. Department of Materials Science and Engineering, Cornell University, Ithaca, New York 14853-1501, USA
2. Department of Materials Science and Engineering, Pennsylvania State University, University Park, Pennsylvania 16802, USA
3. Department of Materials Science and Engineering and Materials Research Laboratory, University of Illinois, Urbana, Illinois 61801, USA
4. Peter Grünberg Institute (PGI9-IT), JARA-Fundamentals of Future Information Technology, Research Centre Jülich, D-52425 Jülich, Germany
5. School of Applied and Engineering Physics, Cornell University, Ithaca, New York 14853, USA
6. Kavli Institute at Cornell for Nanoscale Science, Ithaca, New York 14853, USA