Superconducting Transition in Aluminum

Further results on the superconducting properties of aluminum are given which supplement those previously reported. The critical field curve for pure Al has been measured by using a paramagnetic salt as a thermometer. Except for a region very near $T_c$, our data are accurately represented by the parabolic relation, $H_c=H_0[1-{\left(\frac{T}{T_c}\right)}^2]$, where $T_c=1.196\mathrm{}\pm 0.005°$K and $H_c$ is about 99±1 gauss. The excellent reproducibility of the $S-N$ transition in Al suggests that it may serve as a useful secondary thermometric standard near 1°K. Further observations are reported on the broadening tendency of magnetically measured transitions in single-crystal specimens near $T_c$. It is also found that polycrystalline specimens of the same material do not show the broadening effect. A semiquantitative theory of the intermediate state is applied to explain the observation that the magnetic transition is sharper than predicted on the basis of specimen geometry at temperatures well below $T_c$. The difference in sharpness can be explained as a contribution of the interphase free energy whose order of magnitude has been estimated with fair success from the observed shape of the magnetic transition.