1. Department of Physics, Toyama University, Toyama 930
2. Faculty of Engineering, Toyama University, Toyama 930
3. CEA/Departement de Recherche Fondamentale sur la Matiére Condensée/SPSMS/MDN, 85X-38041 Grenoble Cedex, France
4. Laboratoire Louis Néel, CNRS, 166X-38042 Grenoble Cedex, France
5. Clarendon Laboratory, University of Oxford, Oxford OX1 3PU, U.K.