Anisotropy of pinning forces in NbTi

For wire samples of cold worked Nb- 40wt%Ti and Nb-50wt%Ti a strong anisotropy of the volume pinning force F <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</inf> is observed, when the angle between transport current and external magnetic field is varied. Good agreement with the experimental results is found, if F <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</inf> depends on f <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</inf> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , where f <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</inf> is the pinning force per pinning centre, and if one takes into account the finite dimensions of the subbands that act as pinning centres. For the current-optimized samples, a smaller anisotropy is seen, and for recrystallized samples the pinning force is practically independent of the orientation of current and field, as it was expected.