1. Information Storage Materials Laboratory, Department of Electrical and Computer Engineering, National University of Singapore, Singapore 117576, Singapore
2. IFIMUP-IN/Departamento de Fisica e Astronomia, Universidade do Porto, Rua Campo Alegre 687, 4169-007 Porto, Portugal
3. Institute of Magnetism, National Academy of Sciences of Ukraine, 36-b Vernadskogo Blvd., 03142 Kyiv, Ukraine