Imaging defects in Cu-clad NbTi wire using a high-T/sub c/ scanning SQUID microscope

We have used a sensitive magnetic microscope based on the dc Superconducting Quantum Interference Device (SQUID) to examine room-temperature samples of Cu-clad NbTi wire. Our SQUID microscope allows the wire (in air at room temperature) to be brought to within about 0.05 mm of the SQUID, although typically the separation is 0.3-0.4 mm. The wires have round or rectangular cross-sections and are about 2 mm across. With the wire aligned parallel to the x-direction in the x-y scanning plane we apply currents of a few mA through the wire at a frequency of a few hundred Hz, and image the x-component of the magnetic field. Defects cause a redistribution of current and corresponding features in the magnetic field image. We show images of wires, and discuss the advantages of using the parallel component of the field.