High-resolution room-temperature sample scanning superconducting quantum interference device microscope configurable for geological and biomagnetic applications

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Abstract
We have developed a scanning superconducting quantum interference device(SQUID) microscope system with interchangeable sensor configurations for imagingmagnetic fields of room-temperature (RT) samples with submillimeter resolution. The low-critical-temperature ( T c ) niobium-based monolithic SQUIDsensors are mounted on the tip of a sapphire and thermally anchored to the helium reservoir. A 25 μ m sapphire window separates the vacuum space from the RT sample. A positioning mechanism allows us to adjust the sample-to-sensor spacing from the top of the Dewar. We achieved a sensor-to-sample spacing of 100 μ m , which could be maintained for periods of up to four weeks. Different SQUIDsensor designs are necessary to achieve the best combination of spatial resolution and field sensitivity for a given source configuration. For imaging thin sections of geological samples, we used a custom-designed monolithic low- T c niobium bare SQUIDsensor, with an effective diameter of 80 μ m , and achieved a field sensitivity of 1.5 pT ∕ Hz 1 ∕ 2 and a magnetic moment sensitivity of 5.4 × 10 − 18 A m 2 ∕ Hz 1 ∕ 2 at a sensor-to-sample spacing of 100 μ m in the white noise region for frequencies above 100 Hz . Imaging action currents in cardiac tissue requires a higher field sensitivity, which can only be achieved by compromising spatial resolution. We developed a monolithic low- T c niobium multiloop SQUIDsensor, with sensor sizes ranging from 250 μ m to 1 mm , and achieved sensitivities of 480 – 180 fT ∕ Hz 1 ∕ 2 in the white noise region for frequencies above 100 Hz , respectively. For all sensor configurations, the spatial resolution was comparable to the effective diameter and limited by the sensor-to-sample spacing. Spatial registration allowed us to compare high-resolution images of magnetic fields associated with action currents and optical recordings of transmembrane potentials to study the bidomain nature of cardiac tissue or to match petrography to magnetic field maps in thin sections of geological samples.