Design of a Large Bandwidth Scanning SQUID Microscope using a Cryocooled Hysteretic dc SQUID

Abstract

I present the design and analysis of a large bandwidth scanning Superconducting Quantum Interference Device (SQUID) microscope. Currently available SQUID microscopes are limited to detecting magnetic fields with frequencies less than 1 MHz. However, for observing nanosecond time scale phenomena such as logic operations in today's computer chips, SQUID microscopes with 1 GHz bandwidth and larger are required. The major limitation in SQUID microscope bandwidth is not the SQUID itself but the electronics and readout technique. To increase bandwidth, the fast transition of a hysteretic dc SQUID from the zero voltage state to the resistive state can be used as the detection element in a new SQUID readout technique, referred to as pulsed SQUID sampling. The technique involves pulsing the bias current to the dc SQUID while monitoring the voltage across it. As the pulse length shortens, the SQUID measures the applied external magnetic flux with shorter sampling time, which increases the bandwidth. Experimental tests of the technique have demonstrated the possibility of following signals with frequencies up to 1 GHz using a dc SQUID with Nb-AlOx-Nb Josephson junctions at around 4 K. Ringing in the pulse profile permitted the effective bandwidth of the sampling technique to be much greater than the nominal value suggested by the pulse length setting on the generator. I identify additional means of increasing bandwidth: redesigning the dc SQUID, implementing transmission line wiring, adding high speed superconducting circuits, etc. which should allow bandwidths to reach 40 GHz and higher. Towards creating a large bandwidth SQUID microscope, I also assembled and tested with collaborators a fully functional 4 K scanning SQUID microscope. With the microscope, which used a nonhysteretic niobium dc SQUID with conventional flux-locked-loop SQUID electronics, I was able to obtain the magnetic field image of a current carrying circuit.