Magnetic properties and atomic structure of Finemet type Fe75Sil5(NbBCu) ribbons were investigated during and after special heat treatments. Materials with the highest standard technical magnetic parameters were prepared. Extremely large and stable permeability (over 750000 at 50Hz and over 100000 at 10kHz exciting field) was reached. The same heat treatments resulted in the most stable product for 10kHz use. Wide domains (of 0,01mm characteristic size), deeply piercing into ribbons, were observed. On the other hand, near the surface, closure domains and branches of domains were also detected. Shapes of domain walls in this surface region were smoothly curved and zigzag type. All of the observed types of domain structure were weakly sensitive to the surface roughness. The distribution of the above three types of domain areas substantially changed after special heat treatments during which the initial industrial magnetic parameters have been changed. From investigations of atomic structure, structural relaxation in amorphous state was confirmed, and also additional atomic-rearrangements, without detectable increase of the crystallite size, were suggested after the amorphous-->nanocrystalline transformation.