Affiliation: | a Departamento de Física, Instituto Superior de Engenharia do Porto, Rua Dr. António Bernardino de Almeida, 431, 4200-072 Porto, Portugalb Centro de Física, Universidade do Minho, 4800-058 Guimarães, Portugalc Shenyang National Laboratory for Material Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, People's Republic of China |