1. Universidad del País Vasco, Depto. Electricidad, P.O. Box 644, E-48080 Bilbao, Spain
2. Institute Laue Langevin, 6 Rue Jules Horowitz, F-38042-Grenoble Cdx 9, France
3. Institute of Magnetism, Kyiv 03142, Ukraine
4. Department of Appl. Physics, Tohoku Gakuin University, Tagajo 985-8537, Japan