Magnetization of Synthetic Filamentary Superconductors. A. The Dependence of Flux Jumping on Temperature and Magnetic Field Sweep Rate

Abstract

Synthetic high‐field superconductors have been prepared by impregnating porous glasses of various pore size with indium, using pressures up to 70 000 psi. The magnetic field H_fi at which the first flux jump occurred in the initial magnetization of the sample was measured as a function of temperature T and magnetic field sweep rate dH/dt. Values of dH/dt between 24 and 375 Oe/sec were used. For a given value of dH/dt, H_fi increased initially as T was lowered, reached a maximum, and finally fell with T. For a given T, H_fi was reduced as dH/dt increased. For large values of dH/dt, H_fi was almost independent of T and dH/dt. H_fi was reproducible to 50 Oe from cycle to cycle except for T H_fi was reduced. The samples showed hysteresis in the magnetization. Flux jumping occurred around the hysteresis loop; the values of field at which flux jumps occurred were reproducible from cycle to cycle. The field interval Δ between flux jumps had a temperature dependence similar to H_fi; Δ is slightly reduced for large values of the applied magnetic field. The size of the flux jumps were also reduced for large values of magnetic field. For small values of T, the flux jumping behavior became erratic. Measurements of the critical field and transition temperature have been made using a low‐frequency mutual‐inductance technique. The critical field, that is the magnetic field for the normal‐superconducting transition, for a sample with a 65‐Å pore diameter was 25 770 Oe at 2.67°K. At a reduced temperature of 0.6°K, samples with pore diameters of 65, 117, and 250 Å have critical fields of 156, 102, and 39 times the critical field of bulk indium (176 Oe). The transition temperatures for the samples were 4.24°, 4.04°, and 3.61°K, respectively, compared with 3.408°K for bulk indium. The over‐all behavior of these samples was similar to inhomogeneous type II superconductors.