1. Division of Advanced Materials Science, Pohang University of Science and Technology (POSTECH), Pohang 790-784, South Korea
2. Department of Physics, University of Central Florida, Orlando, Florida 32816, USA
3. Institut Néel, CNRS, départment MCBT, 25 avenue des Martyrs, BP 166, 38042 Grenoble Cedex 09, France
4. School of Engineering and Science, Jacobs University Bremen, Bremen 28759, Germany