A scanning superconducting quantum interference device microscope with high spatial resolution for room temperature samples

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Abstract
We have developed a scanning superconducting quantum interference device(SQUID) microscope based on niobiumtunnel junctiondirect current (dc) SQUIDs. It employs either a SQUID magnetometer or a planar first order gradiometer with an effective area of 40×40 μ m 2 and a magnetic field resolution on the order of 1 pT /√ Hz at frequencies above a few hertz. The gradiometer has a base line of 1 mm. The SQUIDs are mounted inside the insulation vacuum of a fiberglass helium Dewar, and are thermally coupled to the helium bath via a brass block. A sapphire window with a thickness of about 50 μm separates the SQUIDs from the room temperature sample. We have also investigated different window materials, such as tungsten or iridium, and found them equally useful. The stand-off distance between SQUID and room temperature sample can be as low as 75 μm, and the spatial resolution of the microscope is about 50–100 μm. The SQUID sensor is read out using conventional dc SQUID electronics with a bandwidth of 1 kHz and a slew rate of 200 mT/s. A computer controlled x–y stage scans the sample below the microscope. With this microscope, we have investigated magnetic signatures of magnetic storage media, ferromagnetic and paramagnetic inclusions in geological and biological samples as well as platinum coins, and have also performed nondestructive testing of stainless steel plates and aircraft parts with high spatial resolution.