1. Department of Radiophysics, Taras Shevchenko University, Glushkov str. 2, build. 5, 03127, Kiev-127, Ukraine
2. Institute for Metal Physics, Ukrainian Academy of Sciences, Vernadskii str. 36, 03142, Kiev-142, Ukraine
3. Helsinki University of Technology, Laboratory of Materials Science, P.O. Box 6200, Espoo FI-02015 TKK, Finland