[1] Department of Materials Science and Engineering, Yonsei University, Seoul 120-749, Korea, Republic of
[2] R and D Center, Samsung LED, 314 Maetan 3-dong, Yeongtong-gu, Suwon-si, Gyeonggi-do, 443-743, Korea, Republic of
[3] Center for Spintronics Research, Korea Institute of Science and Technology, Seoul 136-791, Korea, Republic of
[4] Nano-Science Research Division, Korea Institute of Science and Technology, Seoul 136-791, Korea, Republic of