Reduction of losses at low frequencies and large amplitudes in NbTi ribbons

The critical current density in NbTi ribbons increases considerably and hysteretic (A.C.) losses decrease by an order of magnitude when a magnetic field is present along the length of the ribbon. In our work, rather than feeding a current to the sample through leads connected to an external supply, we cause persistent transport currents to flow along the length of the ribbon by induction (by applying a magnetic field transverse to the ribbon axis and along its broad faces). The magnitude of the induced currents, their pattern of circulation and the hysteretic losses are determined by monitoring the evolution of the magnetic moment of the ribbon both along its length and along the wide faces as the transverse field is cycled. We observe that when a longitudinal field is present, the induced currents adopt trajectories which are "tilted" and tend to lie along the total magnetic induction along the flat faces of the ribbon, hence tend towards force-free configurations where .