Superconductivity and small magnetic fields

Abstract

The use of the flux-gate,magnetometer as a sensitive measuring inastrument for investigating the magnetic properties of superconductors is discussed. The construction of a magnetometer is described and an analysis of the system is included. The magnetic properties considered are the Meissner effeat, the "London field", and quantized trapped magnetic flux. The reasults of a Meissner effect experiment are given. In this case, an Indium sample is placed inside a hollow Tin cylinder in a uniform external magnetic field. The temperature is lowered until the cylinder becomes superconducting. The magnetic flux within the cylinder is then a constant, and as the sample becomes superconducting, the field inside the cylinder will rise. This transition can be detected with the magnetometer. Three critical points were obtained, and compared to recent measurements by Shaw, Mapother, and Hopkins. The three points were Tc = 3.394°K @ H=1.174 Gauss, Tc=3.367°K @H=4.859 Gauss, and Tc = 3.330°K @ H=11.92 Gauss. The comparison gave errors of -10, -8, and 0 millidegrees, respectively. To make the comparison to the data by Shaw, et. al., a least squares fit to their data was made using a standard computer program. The critical curve can be represented by, Hc = 284.39 -26.751 Tc2 + 0.096352 Tc4