1. Center for Spintronics, Korea Institute of Science and Technology, Seoul 136-791, South Korea
2. Department of Materials Science and Engineering, University of California, Berkeley, California 94720, USA
3. Department of Materials Science and Engineering, Yonsei University, Seoul 120-749, South Korea
4. Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, USA
5. Department of Electronics Engineering, National Changhua University of Education, Changhua 50007, Taiwan
6. Department of Electronics Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan
7. Department of Nanomaterials Science and Engineering, Korea University of Science and Technology, Daejeon 305-350, South Korea