Miniature Ultrasensitive Superconducting Magnetic Gradiometer and Its Use in Cardiography and Other Applications

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Abstract
A portable magnetic gradiometer was built using an rf‐biased point contact device as a sensor and a superconducting flux transformer 2.5 cm in diameter by 10 cm long to couple to the gradient of the external field. The instrument is limited by fluctuations which are probably associated with an uneven boiling of the cryogenic liquids and by environmental fluctuations in some locations. It is shown that the inherent sensitivity can be increased by an order of magnitude by increasing the bias frequency from 30 to 300 MHz. A possible use of the instrument as a magnetocardiograph in an unshielded environment was demonstrated.