1. Department of Electrical & Computer Engineering and Carbon Nanotechnology Laboratory, Rice University, Houston, Texas 77005, Laboratoire National des Champs Magnétiques Pulsés, CNRS-INSA-UPS, 31400 Toulouse, France, Department of Mechanical Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan, and IBM Research Division, T. J. Watson Research Center, Yorktown Heights, New York 10598