1. Department of Physics, Umeå University, SE-901 87 Umeå, Sweden, Department of Colloid Chemistry and Nanostructured Materials, University of Szeged, H-6720 Szeged, Hungary, Bayerisches Geoinstitut, Universität Bayreuth, D-95440 Bayreuth, Germany, and LPMTM-CNRS, Université Paris Nord, F-93430 Villetaneuse, France