Abstract
When using SQUID magnetic sensors to inspect metallic contaminants in magnetic matrices, such as battery components, it is advantageous to use a gradiometer rather than a magnetometer because a higher signal-to-noise ratio can be obtained. In this study, we reviewed the design of directly coupled SQUID gradiometers and optimized their pickup loops. Several gradiometers with pickup loops of different sizes were fabricated. To evaluate these gradiometers, we applied a small magnetic field produced by a magnetic field coil with a small diameter (effective diameter: 0.65 mm) to them. The sensitivity distribution of these gradiometers was evaluated using a special mechanism that allows for complete scanning over the device substrate. As a result, the magnitude of the peak value could be explained by an inductance imbalance between the pickup loop and the SQUID. Finally, we could find the optimal pickup coil design conditions. Furthermore, we could capture the sensitivity distribution of a small SQUID loop placed in the center of the device.
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