Slow relaxation and avalanches in the spin-paramagnetic transition of a thin film granular superconductor
Abstract
We report measurements of the superconducting to normal transition of ultra-thin granular Al films in parallel magnetic fields, H{norm of matrix}. The parallel critical field,Hc, in our samples is Pauli spin-paramagnetically limited. We find that the critical field transition goes from second-order to strongly first-order at a tricritical point, Ttr, near 250 mK. The first-order transition is characterized by a giant hysteresis in the critical field, ΔHc{norm of matrix} ∼ 2.5 kG. In the hysteretic region we find that the films are far out of thermodynamic equilibrium and exhibit very slow, τ ∼ 104s, glass-like, stretched-exponential relaxation. In addition, we observe significant avalanches. These observations are discussed in terms of a random array of Josephson junctions. © 1995.