Nonlinear response of low temperature superconducting thin film gradiometer sense loops

Experimental measurements of the nonlinear response of superconducting thin-film gradiometer sense loops to changes in magnetic field at 4.2 K are described. The sense loops were fabricated from niobium and niobium nitride. The basic experiment involved the ramp of a uniform magnetic field from zero up to a predetermined value. The performance of the loops was measured by means of a superconducting-quantum-interference-device (SQUID)-based readout system inductively coupled to the samples. Threshold values of external field were observed to characterize the onset of nonlinear behavior for all samples measured. The performance of the loops as measured by this onset of nonlinearity is correlated with important parameters including material, aspect ratio, edge quality, and the effect of crossovers. The empirical results are compared with predictions from an ideal diagmagnet model to describe demagnetization effects.<>