1. Department of Applied Physics, Okayama University of Science, Ridai‐cho, Okayama 700‐0005, Japan
2. Research & Development Center, Stanley Electric Co., Ltd.,1‐3‐1 Eda‐Nishi, Aoba‐ku, Yokohama 225‐0014, Japan
3. Department of Electrical and Electronic Engineering, Miyazaki University, 1‐1 Gakuen kibanadai‐nishi, Miyazaki 889‐2192, Japan