We have developed a microscope to image weak magnetic fields using submillimeter pickup coils made from conventional low-temperature superconducting niobium wire coupled to the input circuit of a superconducting quantum interference device (SQUID). The pickup coil and the SQUID sensor are mounted in the vacuum space of the cryostat and are thermally anchored to the liquid helium reservoir. A 25 μm thick sapphire window separates the room temperature (RT) sample and the vacuum space. The spacing between the pickup coil and RT sample was typically less than 130 μm. The spatial resolution is limited by the diameter of the pickup coil. The pickup coils are easily interchangeable, allowing us to adapt the SQUID microscope to a variety of different measurements. We have achieved a spatial resolution of 250 μm with a magnetic field sensitivity of 850 fT/Hz1/2 or a spatial resolution of 500 μm with a magnetic field sensitivity of 330 fT/Hz1/2. We have used this instrument to measure various biomagnetic and paleomagnetic fields.

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