A SQUID Microscope Using a Hollow-Structured Cryostat for Scanning Room-Temperature Rock Samples

We developed a high-resolution superconducting quantum interference device (SQUID) microscope, which employs a hollow cryostat, for magnetic field imaging of rock samples at room temperature. A directly coupled low-temperature SQUID with a 0.2 × 0.2 mm² pick-up loop, mounted on a sapphire conical rod, is separated from a sample at room temperature through a vacuum gap and a 40-micrometer-thick sapphire window. Precise and repeatable adjustment of the vacuum gap is performed by rotating a micrometer spindle connected to the sapphire rod through the hollow of the cryostat. We have achieved the separation of 0.23 mm between the SQUID and a sample. We also demonstrated imaging of the magnetic field of a zircon crystal having magnetite grains.