Vortex avalanches in a Pb-porous glass nanocomposite

Magnetic properties of a superconducting lead-porous glass composite were studied. The glass pore size was 7 nm. The onset of superconductivity was observed at 7.22 K with complete diamagnetic screening at lower temperatures. Strong magnetic instabilities were found on the magnetization-versus-field loops in the temperature range from 1.8 to 5.5 K at a sweep rate of 20 Oe/s. The shape of the hysteresis loops above 2.5 K was typical for other types of hard type-II superconductors in the adiabatic limit, the field of the first jump on the virgin magnetization being maximal at 3.5 K. Below 2.5 K the hysteresis loops become complex, showing different behavior at lower and higher fields. The nature of such a loop was discussed. The smooth hysteresis loops just below the superconducting transition had fishtails that completely disappeared down to 6 K. The evolution of magnetization instabilities until complete smoothing away the hysteresis loops with increasing or decreasing the sweep rate was observed at 1.8 or 5 K, respectively.