Upper Critical Fields of Nb-Ti Alloys: Evidence for the Influence of Pauli Paramagnetism

Resistivity, flow-resistance, and ultrasonic-attenuation measurements were carried out on Nb-37 at.% Ti and Nb-56 at.% Ti in dc magnetic fields up to 150 kG. The transition temperatures and the dependence of the upper critical field on temperature were measured by the resistive method. The transition temperatures are: Tc=9.2±0.2^∘K for Nb-37 at.% Ti and Tc=9.0±0.2^∘K for Nb-56 at.% Ti. Flow-resistance measurements gave the following values for the Ginzburg-Landau-Abrikosov-Gor'kov upper critical field at T=0:111±7 kG for Nb-37 at.% Ti and 188±{}9 kG for Nb-56 at.% Ti. Changes in the ultrasonic attenuation were observed near the upper critical field. The data support the view that the Pauli paramagnetism plays a role in determining the upper critical field. However, the effect of the Pauli paramagnetism is found to be smaller than predicted by Maki.