HTS SQUID Magnetometer Using Resonant Coupling of Cooled Cu Pickup Coil

We have designed and tested an HTS SQUID magnetometer using the resonant coupling of a copper pickup coil cooled at T= 77 K. The coil was made of a twisted multi-filamentary wire so as to reduce the ac resistance. We first showed that the ac resistance of the coil became higher than the dc resistance because of the eddy current loss of the coil. We clarified the dependence of the ac resistance on the parameters of the coil and obtained an empirical expression for the ac resistance. Next, we constructed a magnetometer using a pickup coil with the average diameter D= 50 mm and number of turns N=150 . The pickup coil with an inductance of Lp= 1.05 mH was connected to an input coil through a resonant capacitance of C= 0.29 μF. The HTS SQUID was magnetically coupled to the input coil with a mutual inductance of M= 500 pH. The estimated magnetic field noise of the magnetometer was approximately 2.5 fT/Hz ^1/2 at the resonant frequency of f= 9.05 kHz. The Q value of the resonant circuit was 180. The experimental results agreed well with the designed values. The obtained high sensitivity of the magnetometer will be useful for low frequency applications such as low-field NMR.