1. BCMaterials, Ibaizabal Bidea, Edificio 500, Parque Tecnológico de Bizkaia, Derio 48160, Spain
2. Dept. of Electricity and Electronics, Universidad del País Vasco/Euskal Herriko Unibertsitatea, Faculty of Science and Technology, P. Box 644, 48080 Bilbao, Spain
3. Dept. of Inorganic Chemistry, Universidad del País Vasco/Euskal Herriko Unibertsitatea, Faculty of Science and Technology, P. Box 644, 48080 Bilbao, Spain
4. Dept. of Physics, Universidad de Oviedo, Calvo Sotelo s/n, 33007 Oviedo, Spain