1. Spintronics and Vacuum Nano-Electronics Laboratory, Department of Electrical Engineering and Computer Science, University of Cincinnati, Cincinnati, Ohio 45221, USA
2. Department of Electrical and Computer Engineering, Ohio State University, Columbus, Ohio 43210, USA
3. Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson Air Force Base, Ohio 45433, USA
4. Research Institute, University of Dayton, Dayton, Ohio 45469, USA