Magnetic Properties of Some Type-II Alloy Superconductors near the Upper Critical Field

Parameters pertinent to the magnetic properties of type-II superconductors near the upper critical field Hc2 [namely, the generalized Ginzburg-Landau parameters κ₁ and κ₂, and the functions h*=Hc2(-dHc2dt)ₜ₌₁] have been obtained from magnetization measurements on a series of niobiumtitanium alloys. The range of electron-transport mean free paths, from 0.1ξ₀ to about 15ξ₀ (where ξ₀ is the coherence length in pure Nb), effectively spans the range from the clean to the dirty limit, with annealed and cold-worked specimens at temperatures between 0.13Tc and Tc. It was found that both κ₁ and κ₂ increased with decreasing temperature in all alloys and that the magnitude of the increase was 20-50% higher than expected from existing theory. The experimental value of the parameter Hc2(dHc2dt) at T=Tc various with impurity roughly as expected in Ginzburg-Landau theory. Defects generated by cold work enhanced the increase of κ₁ at low temperatures.