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

Abstract

Parameters pertinent to the magnetic properties of type-II superconductors near the upper critical field ${H}_{c2}$ [namely, the generalized Ginzburg-Landau parameters ${\ensuremath{\kappa}}_{1}$ and ${\ensuremath{\kappa}}_{2}$, and the functions ${h}^{*}=\frac{{H}_{c2}}{{(\ensuremath{-}\frac{d{H}_{c2}}{\mathrm{dt}})}_{t=1}}$] have been obtained from magnetization measurements on a series of niobiumtitanium alloys. The range of electron-transport mean free paths, from $0.1{\ensuremath{\xi}}_{0}$ to about $15{\ensuremath{\xi}}_{0}$ (where ${\ensuremath{\xi}}_{0}$ 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 $0.13{T}_{c}$ and ${T}_{c}$. It was found that both ${\ensuremath{\kappa}}_{1}$ and ${\ensuremath{\kappa}}_{2}$ 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 $\frac{{H}_{c2}}{(\frac{d{H}_{c2}}{\mathrm{dt}})}$ at $T={T}_{c}$ various with impurity roughly as expected in Ginzburg-Landau theory. Defects generated by cold work enhanced the increase of ${\ensuremath{\kappa}}_{1}$ at low temperatures.