1. a Department of Electrical Engineering , Nagaoka University of Technology , 1603‐1 Kamitomioka, Nagaoka 940‐2188, Japan
2. b Department of Material Science and Technology , University of Hyogo , 2167 Shosha, Himeji 671‐2201, Japan
3. c Department of Electrical and Computer Engineering , Takamatsu National College of Technology , 355 Chokusi, Takamatsu, 761‐8058, Japan