NONLINEAR RESPONSE IN THIN FILM HIGH TEMPERATURE SUPERCONDUCTING MAGNETOMETER SENSE LOOPS

Measurements of nonlinear response in Tl2CaBa2Cu2O8+y and YBa2Cu3O7-x. thin film magnetometer sense loops were performed at 4.2 K. The thallium-based films were deposited by laser ablation onto lanthanum aluminate substrates and the yttrium-based films were prepared by sputtering onto strontium titanate substrates. Both types of superconducting films were patterned using photolithography into sense loops, each consisting of a magnetometer input loop connected to a magnetometer output loop by a pair of closely spaced parallel strip lines. The thin film output loop of a given sample was coupled inductively to a sensitive measurement system employing a superconducting quantum interference device to measure magnetic flux. A uniform dc magnetic field applied to the sample input loop was slowly ramped to plus and minus a predetermined value and then ramped back to zero. The sample output was monitored to determine the value of applied field corresponding to the threshold of nonlinear response of the film loop. Film loops having various thicknesses and line widths were studied to correlate aspect ratio to nonlinear effects. Possible mechanisms for the observed nonlinear behavior are discussed.