Size dependences of the magnetic properties of superconducting lead–porous glass nanostructures

Superconducting structures Pb–PG formed by filling a porous glass matrix with the lead from melt under pressure have been investigated. Samples with characteristic pore structure diameters of $d\approx$ 7, 3, and 2 nm have been studied. It has been found that the critical temperature of the superconducting transition in the samples under study is similar to the corresponding value $T_c\approx$ 7.2 K for bulk lead. At the same time, it has been observed that the critical magnetic field of the nanocomposites, which attains $H_c$ ($T=0$ K) $\approx$ 165 kOe for Pb–PG (3 nm), exceeds several times the value $H_c(0)$ = 803 Oe for bulk lead. The low-temperature magnetic- field dependences of magnetic moment $M(H)$ contain quasi-periodic flux jumps, which vanish with a decrease in the lead nanostructure diameter. A qualitative model of the observed effects is considered.