Thick films of superconducting YBCO as magnetic sensors

Superconducting thick films of YBCO have been prepared successfully on MgO single-crystal substrate by a screen-printing method. A prototype magnetic field sensor has been made from these thick films and characterized using x-ray diffraction, scanning electron microscopy, and a magnetoresistance method. The microstructure of the YBCO thick film sensor was prepared in such a way so as to retain a small resistance R(B = 0) at liquid nitrogen temperature and at zero magnetic field. The R-T curves of the sensor revealed that the value of the resistance R(T) is highly sensitive to the relatively high applied magnetic fields in the temperature range from 75 to 85 K. The R-B curves were plotted at liquid nitrogen temperature. At relatively low applied magnetic fields (up to 100 G), the value of the normalized resistance was well fitted to the relation R(B)/R(B = 0) 1+αB2, where α is a constant. However, at higher values of the applied magnetic fields, the normalized resistance exhibited an almost linear dependence with the applied field. The characteristics of the sensors showed considerable hysteresis at high ranges of magnetic field (from 100 to 500 G) with no significant hysteresis at low magnetic field (less than 100 G). The I-V curves at liquid nitrogen temperature were also measured after several thermal cycles for various values of applied magnetic fields. It was demonstrated that the I-V curves were independent on the number of thermal cycles and exhibited no significant change at relatively low currents and magnetic fields. That is, the I-V curves are reproducible under the above conditions. This suggests that a better performance of the sensor in the presence of relatively weak magnetic field is needed to get the ultimate sensitivity