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 $H_{c2\mathrm{}}$ [namely, the generalized Ginzburg-Landau parameters ${\kappa }_1$ and ${\kappa }_2$, and the functions $h^{*}=\frac{H_{c2\mathrm{}}}{{(-\frac{d{H}_{c2\mathrm{}}}{\mathrm{dt}})}_{t=1\mathrm{}}}$] have been obtained from magnetization measurements on a series of niobiumtitanium alloys. The range of electron-transport mean free paths, from $0.1{\xi }_0$ to about $15{\xi }_0$ (where ${\xi }_0$ 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 $0.13T_c$ and $T_c$. It was found that both ${\kappa }_1$ and ${\kappa }_2$ 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 $\frac{H_{c2\mathrm{}}}{\left(\frac{d{H}_{c2\mathrm{}}}{\mathrm{dt}}\right)}$ at $T=T_c$ various with impurity roughly as expected in Ginzburg-Landau theory. Defects generated by cold work enhanced the increase of ${\kappa }_1$ at low temperatures.