Hybrid magnetometers for unshielded operation: combining hall sensors with superconducting flux concentrators

Hybrid magnetometers might be interesting for upcoming space missions, as they are small, light weight and have low power consumption. Moreover, the readout of such sensors is relatively easy and straightforward. The sensors used in hybrid magnetometers are not sensitive enough for space applications but this can be enhanced with a flux concentrator. Hybrid magnetometers consisting of a normal conducting Hall sensor and a high-Tc superconducting flux concentrator have been investigated. The operation of such sensors in an unshielded environment gives rise to 1/f noise due to thermally activated movement of trapped flux vortices in the superconducting body. Vortex trapping can be prevented by dividing the superconducting body into narrow strips. The strips have a certain critical field below which no vortex trapping occurs. A new model for the critical field of vortex trapping in thin film strips has been derived and experimentally verified by scanning SQUID microscope measurements. The results show an excellent agreement between the critical field and the model values. Hybrid magnetometers with bismuth and doped silicon Hall sensors have been produced. The results of the bismuth Hall device hybrid magnetometers showed moderate field sensitivities that can be attributed to rather weak coupling between the flux concentrator and the sensor and a low Hall coefficient. Doped Si can exhibit much higher Hall coefficients which moreover can be tuned by varying the doping density. The Hall sensor is constructed by implanting doping in “silicon on insulator” (SOI) wafers with an ion-implanter. The flux concentrator is constructed from YBa2Cu3O7-δ (YBCO) which is deposited by pulsed laser deposition. YBCO cannot directly be deposited on Si and yttrium stabilized Zirconia (YSZ) and CeO2 buffer layers are used. Doping implantation in SOI wafers and YBCO growth on SOI are discussed in this thesis.