1. Department of Physics, Umeå University, Umeå, 901 87 Sweden
2. Swedish Defence Research Agency (FOI), Umeå, 906 21 Sweden
3. Department of Engineering Sciences, Uppsala University, Box 35 751 03, Uppsala, Sweden
4. Umeå Core Facility for Electron Microscopy, Umeå University, Umeå, 901 87 Sweden