1. Department of Chemistry and Biochemistry, University of California at San Diego, La Jolla, California 92093-0358, Laboratoire Louis Néel-CNRS, 25 Avenue des Martyrs, 38042 Grenoble Cedex 9, France, and Institute of Solid State Physics, The University of Tokyo, Kashiwanoha 5-1-5, Kashiwa, Chiba 277-8581, Japan