1. The Center for Micromagnetics and Information Technologies (MINT) Department of Electrical and Computer Engineering University of Minnesota Minneapolis MN 55455 USA
2. Department of Chemical Engineering and Material Science University of Minnesota Minneapolis MN 55455 USA
3. School of Physics and Astronomy University of Minnesota Minneapolis MN 55455 USA
4. Department of Optical Science and Engineering Fudan University Shanghai 200433 P. R. China