1. Physics Dept., University of British Columbia, 6224 Agricultural Rd., Vancouver, B.C., Canada V6T 1Z1, Canada
2. Physics Dept., Lehman College, The City University of New York, Bedford Park Boulevard West, New York 10468, U.S.A.
3. Laboratoire Louis Néel, C.N.R.S., B.P. 166X, Grenoble 38042, France