Depairing currents in the superconductor/ferromagnet proximity system Nb/Fe

Abstract

We have investigated the behavior of the depairing current ${J}_{\mathrm{dp}}$ in ferromagnet/superconductor/ferromagnet $(F/S/F)$ trilayers as function of the thickness ${d}_{s}$ of the superconducting layers. Theoretically, ${J}_{\mathrm{dp}}$ depends on the superconducting order parameter or the pair-density function, which is not homogeneous across the film due to the proximity effect. We use a proximity-effect model with two parameters (proximity strength and interface transparency), which can also describe the dependence of the superconducting transition temperature ${T}_{c}$ on ${d}_{s}.$ We compare the computations with the experimentally determined zero-field critical current ${J}_{c0}$ of small strips (typically $5\ensuremath{-}\ensuremath{\mu}\mathrm{m}$ wide) of Fe/Nb/Fe trilayers with varying thickness ${d}_{\mathrm{Nb}}$ of the Nb layer. Near ${T}_{c}$ the temperature dependence ${J}_{c0}(T)$ is in good agreement with the expected behavior, which allows extrapolation to $T=0.$ Both the absolute values of ${J}_{c0}(0)$ and the dependence on ${d}_{\mathrm{Nb}}$ agree with the expectations for the depairing current. We conclude that ${J}_{\mathrm{dp}}$ is correctly determined, notwithstanding the fact that the strip width is larger than both the superconducting penetration depth and the superconducting coherence length, and that ${J}_{\mathrm{dp}}{(d}_{s})$ is correctly described by the model.