Transition of flux pinning mechanism in mesoscopic NbTi/Nb/Cu superconductor

Abstract

The flux pinning mechanism in NbTi alloy including artificial pinning centres is studied by a NbTi/Nb/Cu multilayer composite. The α-Ti phase in NbTi alloy is the dominant flux pinning site when a period of superconducting plus normal layer thickness is higher than 1000 nm, while the artificial pinning centres of superconducting/normal boundaries are dominant when a period of the superconducting plus normal layer thickness is less than 100 nm. The maximum global pinning force ( F pmax) shows 14.8 GN m −3 at 4 T when the α-Ti phase is the pinning site. Alternatively, F pmax shows 23.7 GN m −3 at 1 T when an artificial normal Cu layer is the pinning site and the peak shifted to lower magnetic field. The critical temperature ( T c) reduces due to the proximity effect of both α-Ti phase and normal Cu layer. However, the layer thickness where T c starts to drop due to the proximity effect of α-Ti, is different from that for the normal Cu layer.