Field line distribution in a mixed sensor

We have demonstrated that a very low magnetic field can be detected by a sensor combining a superconducting pick-up loop and a GMR element [1] and [2]. In this device, the excitation field applied to the superconductor generates a supercurrent running through the loop. This current is driven through a constriction, where it generates high density magnetic field lines that can be detected by the GMR sensor.We have performed magneto-optical imaging on a YBCO ring with a constriction to estimate the magnetic field—enhancement and the localization of the field lines in this design for applied fields down to 1 μT. The obtained measurements give the vertical component of the local magnetic field. Locally the field lines are enhanced by a factor larger than 200 for fields small enough to induce a current lower than the critical current. A decrease of the gain for higher applied fields is observed, when the critical current is reached.We present also a calculation of the gain, taking into account the precise distribution of current in the rectangular section of the superconductor [3] and the inductance of the loop. These results are in agreement with finite element modeling. The gain for various dimensions and aspect ratio's is given and compared to the experimental results. An optimized design is given for magnetoencephalography applications, in order to replace low-Tc SQUIDs used in the present apparatus. The expected sensitivity of this system is calculated to be in the range of a few fT/Hz.