The magnetic properties of some superconducting mercury colloids

Magnetization curves have been measured for fourteen colloidal mercury specimens whose average radius varied from 3 x 10-6 to 15 x 10-6 cm. For small applied fields the magnetic moment is proportional to the field and the observed temperature dependence of the slope is as predicted by the London theory. However, the dependence of these initial slopes on the average particle size cannot be explained by the London theory; the values found experi­mentally, extrapolated to 0°K, are numerically in better agreement with Pippard’s modification of the London equation, but this does not correctly describe the temperature dependence. Measurements of the area of the magnetization curves give a mean value of ─5 ± 12 Å for the surface-energy parameter β, which means that the influence of this surface energy is negligible for most specimens. It was found that them magnetization curves of drops small compared with the penetration depth can be scaled to have the same shape at all temperatures. This property has been explained quantitatively by a suitable modification of the two-fluid model of the superconducting state, but the theory does not seem adequate to explain some of the hysteresis features in the magnetization curves of larger drops.