1. Institute of Physics, Polish Academy of Sciences, Al. Lotników 32/46, 02-668 Warszawa, Poland
2. Laboratoire Louis Néel, CNRS, 166X, 38042 Grenoble, France
3. Departamento de Physica, Universidad de Oviedo, Oviedo, Spain
4. CRTBT, CNRS, 166X, 38042 Grenoble, France