1. Center for Spintronics Research, Korea Institute of Science and Technology (KIST), 39-1 Hawolgok-dong, Seongbuk-gu, Seoul, 136-791, Korea.
2. Department of Physics, Sejong University, 98 Gunja-dong, Gwangjin-gu, Seoul, 143-747, Korea.
3. Naval Research Laboratory, 4555 Overlook Avenue SW, Washington, DC 20375, USA.