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

We have investigated the behavior of the depairing current J(dp) in ferromagnet/superconductor/ferromagnet (F/S/F) trilayers as function of the thickness d(s) of the superconducting layers. Theoretically, J(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-mum wide) of Fe/Nb/Fe trilayers with varying thickness d(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(Nb) agree with the expectations for the depairing current. We conclude that J(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(dp)(d(s)) is correctly described by the model.