Magnetic Flux Transfer in Needle Probe of Scanning SQUID Probe Microscope

A scanning SQUID probe microscope, in which a superconducting quantum interference device (SQUID) is combined with a scanning probe microscope, has been developed using a needle probe with high magnetic permeability. The probe played a key role of a flux guide from the sample to the SQUID. An rf-SQUID magnetometer used with a substrate resonator had been adopted in the initial setup. The magnetic sensitivity was not so high though the spatial resolution was good. In this paper, the flux transfer from the sample to the SQUID through the probe was investigated by using the magnetic field simulations. The result showed that the flux was decreased by a factor of more than 1,000 from the probe tip to the SQUID. The critical drop was occurred around the end of the probe. It suggested the magnetic coupling between the SQUID and the probe should be improved. The additional gap distance was needed in the initial rf-SQUID because of the thickness of the substrate resonator used with the rf-SQUID. Therefore, new rf-SQUIDs, which could be operated without a resonator, were fabricated to reduce the gap distance between the SQUID and the probe. The gap distance affected the quality of the magnetic images.