1. Institute for Physics, Martin-Luther-University Halle-Wittenberg, 06099 Halle, Germany
2. Institute for Metallic Materials, IFW Dresden, 01069 Dresden, Germany
3. Institute for Physics, Johannes-Gutenberg-University Mainz, 55128 Mainz, Germany
4. Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA