1. Spintronics and Vacuum Nanoelectronics Laboratory, University of Cincinnati, Cincinnati, Ohio 45221, USA
2. Materials and Manufacturing Directorate, Air Force Research Laboratory, WPAFB, Ohio 45433, USA
3. Research Institute, University of Dayton, Dayton, Ohio 45469-0170, USA
4. Center of Excellence for Thin Film Research and Surface Engineering, University of Dayton, Dayton, Ohio 45469-0170, USA