Investigation of the factors affecting 2nd harmonic generation in bulk high TC superconductors

The researches on granular high-TC cuprates have shown that these materials make quite low flux entries possible through grain boundaries and show nonlinear magnetization which causes the generation of harmonics of excitation field. Especially, perfect linear dependence of second harmonic signal to applied DC fields, where the second harmonic signal is generated by co-application of AC and DC fields, has attracted interests to magnetic sensor technology. The aim of this thesis was to inspect the factors affecting second harmonic signal, which could be done by developing a DC magnetometer with a sensitivity of the order of 10-9 T that uses polycrystalline superconductor as a sensor element. The key conclusion is that the field of 0.5 nT can be easily measured. In order to attain this result, many different parameters have been improved and the affects of those parameters on the sensor's sensitivity have been studied broadly. In order to study the affect of the inter-grain connectivity on the sensor's sensitivity, yttrium superconductor was doped with different elements, like boron, bismuth and zinc which, according to the literature, are known to either ascend or descend the inter-grain connectivity in this material. The superconducting properties and the performance as a sensor element for superconductors were studied by annealing at different temperatures and thus having a different crystalline structure. In order to get the maximum signal to noise (S/N) ratio, the environmental conditions have been optimized. The environmental noise was suppressed from the working frequency by active and passive band-pass filters. By optimizing the configuration of the superconducting sample and the coils the best sensor geometry has been found. Furthermore, the optimal excitation frequency and amplitude have been determined.