1. Laboratory of Ferroics Physics, Physical Dept., Lviv Franko University, Kyryla i Mefodiya st. 8, 290005 Lviv-5, Ukraine
2. Institute of Experimental Physics, University of Wrocław, Pl. M. Borna 9, 50-204 Wrocław, Poland
3. Institute of Physical Optics, Dragomanova st. 23, 290005 Lviv-5, Ukraine