High-resolution magnetic mapping using a SQUID magnetometer array

A four-channel, high-resolution superconducting quantum interference device (SQUID) magnetometer array was used to map magnetic fields from various samples. Each SQUID has a 3-mm-diameter pickup coil located 4.4 mm from the adjacent channel. The spacing between the cryogenic array and the room-temperature sample is adjustable from 1.5 mm to 4.0 mm. The authors mapped the field from a 350-μm-diameter hole in an 11 cm×15 cm×60 μm copper sheet that was carrying a current of 100 mA. Field shape and strength were compared with predictions from analytical and finite-element models, which indicate that this technique should be able to detect an order of magnitude smaller flaws in flat plates. The ability is demonstrated to detect magnetic contamination in a 230-μm-deep by 1.1-mm-long slot that was electric-discharge-machined into a nonmagnetic tube, and to determine the orientation of the slot with respect to the tube axis. Slices of pyroclastic rock of thickness as low as 30-μm-thick have also been mapped