Ultra-sensitive mixed sensors—Design and performance

Low-Tc SQUIDs are currently the only devices with a sufficiently low level of noise for magnetic field measurements in the femtotesla range. An alternative to SQUIDs is to combine a superconducting flux-to-field transformer with a high sensitivity magnetoresistive sensor. With a correct design, the excitation field can be sufficiently enhanced to be measured by a magnetoresistance (MR).We have fabricated and tested a micron-sized integrated YBCO-based mixed sensor of small size. It is comprised of a GMR stack separated from a YBCO thin film by an insulating layer. A constriction in the superconducting loop drives the induced current through a small section and therefore creates field lines large enough to be detected by the GMR down to 30 fT/Hz1/2, that is in the range of high-Tc SQUIDs. The device characteristics at 4.2 and 77 K are shown.Experimental results are compared to the theoretical gain of the structure.Applications of this sensor are for instance in the biomagnetism area, especially for magnetoencephalography, where neural signals emitted by the brain are of the order of few fT/Hz1/2. Various other fields are concerned, from earth mapping to fundamental physics (vortex motion, magnetic interactions, etc.).