INITIAL-VORTEX-ENTRY-RELATED MAGNETIC HYSTERESIS IN THIN-FILM SQUID MAGNETOMETERS

We report measurements and modeling of magnetic hysteresis in SQUID magnetometers cycled in fields of 0.1 to 10 G peak to peak. We show that the observed hysteresis is caused by the penetration of magnetic vortices around the edges of the superconducting thin-film structures forming the SQUID. A threshold field for the onset of hysteresis is predicted and subsequently observed experimentally. The threshold corresponds to the situation when the Lorentz force driving a vortex into the SQUID washer first exceeds the sum of the surface barrier and pinning forces. The threshold field depends on the pinning strength of the superconducting material forming the edges of the device, and is thus distinguished from the lower critical field, Hc1, for flux entry in type-II superconductors. It is, to our knowledge, the first time a threshold field for flux entry is observed to relate to the local pinning strength of a structure. Comparison of our earlier experimental data with model calculations revealed the existence of a degradation in critical current density at edges of our devices. Subsequent optimization of our high-Tc device fabrication process lead to significant improvement in hysteresis characteristics. The threshold field in the improved devices is clearly observable, near which at 77 K a thousandfold reduction in hysteresis is obtained. We believe the mechanism of magnetic flux entry studied in this work is also relevant to the understanding of the hysteresis loss in high-Tc superconductors at microwave frequencies. Such hysteresis, resulting in a strongly power-dependent loss, is a major factor limiting the high-power applications of thin-film high-Tc microwave devices. © 1994 The American Physical Society.