Magnetic domain structure of amorphous Fe_73.5Si_13.5B₉Nb₃Cu₁ wires under torsional stress

The evolution of the magnetic domain structure with the applied torsion has been studied by means of the Bitter technique on high magnetostrictive Fe_73.5Si_13.5B₉Nb₃Cu₁ amorphous wires. A drastic change in domain patterns is observed by applying a very low torsional strain value as small as 2.4 (2π rad/m) in the wire. The domain structure on the wire surface changes from a maze configuration, with zigzag walls, to a helical one. When the magnitude of the applied torsion is increased, both in clockwise or counterclockwise senses, a change in the values for the domain width as well as the wall angle tilting with respect to the wire axis are clearly observed, showing a saturation value of 45° for an applied torsional strain equal or higher than 10 (2π rad/m). Also, the coercive field variation with the applied torsion shows a minimum value of 26 A m^-1 just for a torsional strain of 2.4 (2π rad/m), suggesting that the internal and the applied helical anisotropies are then balanced, at least partially, resulting in the softest magnetic state for the amorphous wire with respect to the coercive field of 40 A m^-1 for the untwisted wire.