1. Ames Laboratory—U.S. DOE and Department of Physics and Astronomy, Iowa State University 1 , Ames, Iowa 50011, USA
2. State Key Lab of Trilogy in Advanced Equipment, Department of Mechanical Engineering, Tsinghua University 2 , Beijing 100084, China
3. Seagate Research Groups, Seagate Technology 3 , Shakopee, Minnesota 55379, USA