Magnetic Coupling between SQUID and Probe of STM-SQUID

An STM-SQUID microscope has been developed by combining a scanning tunneling microscope (STM) and a high-Tc superconducting quantum interference device (SQUID) to simultaneously observe the surface morphology and the local magnetic field. In our microscope, a permalloy probe with high permeability, whose tip is electrochemically polished, is placed between the rf-SQUID and the sample. The probe plays a flux guide role of transferring the local magnetic field of the sample to the SQUID. The probe tip could be approached as close to the sample surface as possible within a few nm by an STM feedback control. The SQUID detects the magnetic field from the end of probe. It is also important to bring the probe end close to the SQUID. In this paper, we have investigated the magnetic coupling between the SQUID and the probe in the STM-SQUID microscope setup by decreasing the gap distance between the SQUID and the end of probe from 4.5 mm to 0.1 mm. The magnetic signal detected by the SQUID was increased as the distance between the SQUID and the probe was decreased. As the result, the magnetic images of a nickel thin film sample became clearer with decreasing the distance.