We have designed and fabricated a flip-chip-type gradiometer as a sensor for measurement of weak magnetic signals. The gradiometer consists of a directly coupled direct current superconducting quantum interference device (SQUID) gradiometer coupled to a flux transformer made from a YBa2Cu3O7*/YBa2Cu3O7/CeO2/YSZ multilayer on 2 in. Si wafer, where YBa2Cu3O7*(YBCO*) is a nonsuperconducting crystalline film. The coupling coefficient of 0.18 is obtained for the flip-chip gradiometer. The transformer increases effectively the resolutions of the gradiometer. We obtain a magnetic field gradient resolution of 73 fT cm−1 Hz−1/2 in the white region, and 596 fT cm−1 Hz−1/2 at 1 Hz. The magnetocardiograms were successfully measured at a distance of nearly 100 mm from the gradiometer center to the human heart in a shielded room.
We have developed one-channel high- d.c. SQUID heart monitoring system for non-invasive measurements in an unshielded environment. This system gives fast and reliable information about the heart condition of a patient, for example in an emergency case. Planar galvanically coupled thin film gradiometers and magnetometers with step-edge and bicrystal junctions have been prepared. These sensors were used to carry out magneto-cardiogram measurements in a magnetically shielded room with open doors as well as in an unshielded environment. The results of these measurements are presented and different filtering and averaging methods of the data discussed.
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