[1] Univ Calif Berkeley, Dept Elect Engn & Comp Sci, Berkeley, CA 94720 USA
[2] Univ Calif Berkeley, Lawrence Berkeley Lab, Div Mat Sci, Berkeley, CA 94720 USA
[3] Univ Calif Berkeley, Berkeley Sensor & Actuator Ctr, Berkeley, CA 94720 USA
[4] Lanzhou Univ, Minist Educ, Key Lab Magnetism & Magnet Mat, Lanzhou 730000, Gansu, Peoples R China