1. Graduate School of Science and Technology, Hirosaki University, Hirosaki, Aomori 036-8561, Japan
2. Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
3. Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan
4. University of Tokyo, Cryogenic Research Center, Bunkyo, Tokyo 113-0032, Japan