1. Department of Physics, California State University Northridge, Northridge, California 91330-8268, USA
2. College of Physics and Optoelectronics, Taiyuan University of Technology, Taiyuan 030024, China
3. Cornell Nanoscale Facility, Cornell University, Ithaca, New York 14853, USA
4. San Jose Research Center, HGST, a Western Digital company, 3403 Yerba Buena Rd., San Jose, California 95135, USA