Sketch of the device including the signal coil (yellow/light gray color), the intermediary flux transformer (orange/dark gray color), and the SQUID in a washer configuration. The signal is directly applied to the innermost and outmost turns by ultrasonic bonded aluminium wires.
Picture of the SQUID current sensor and its particulars. (a) Picture of whole device; the integrated feedback coil for flux locked loop operations is located in the pick-up coil hole. In order to reduce the connecting wire length, the feedback coil is connected to intermediate contact pads (left-hand structure close to the washer). (b) Particular of the SQUID in a washer configuration (white color) including the 12-turn input coil located under the washer. Some turns of the signal coils are also shown in the lower side of the picture. (c) Particular showing the integrated test resistors and the signal input pads.
Voltage-flux characteristics (V–Φ) of a SQUID current sensor measured at T = 4.2 K before (lower curve) and after (upper curve) a thermal annealing, performed by using a controllable hot plate at T = 180 °C for 40 min.
Sensor magnetic flux spectra measured at T = 4.2 K in flux locked loop configuration. The lower curve refers to the sensor noise when the signal coil is open. The other curves correspond to the Nyquist noises induced by three different test resistors (50, 8, and 4 Ω) connected to the signal coil.
Sensor magnetic flux spectral density measured at T = 4.2 K as a function of 1/R t 1/2 (R t is resistor value connected to the signal coil). From the slope of the linear best fit curve (straight line) it is possible to obtain a current-to-magnetic flux transfer factor (current sensitivity) of I Φ = 62 nA/Φ0.
Main SQUID amplifier design parameters.
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