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

Abstract

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 transparency͒, 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-m wide͒ 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 Tϭ0. 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.

Depairing current behavior in superconductingNb∕Pd81Ni19bilayers

“…This is not unexpected. In earlier work on the system Nb/ Fe, 16 we found that, in the Ginzburg-Landau ͑GL͒ regime close to T c , 27 J dp can be described as function of reduced temperature t = T / T c by the well-known expression J dp GL ͑t͒ = J dp GL ͑0͒͑1 − t͒ 3/2 , ͑1͒ also in the case that the order parameter is not constant over the thickness of the superconducting film. As a first step in the analysis, following Eq.…”

“…21 The first issue requires samples with a bridge width w comparable with the penetration depth and the coherence length of the superconductor, respectively. Recent results 16,22 on Nb and MoGe single films and Fe/ Nb/ Fe trilayers, however, give us reason to expect uniform current distributions for w Ϸ 1.5 m. A classical pseudo-4-point geometry ͑meaning two contacts, each used for a current and a voltage lead͒ was defined using e-beam lithography. The structuring process, consisting of two main stages ͑e-beam writing and argon-ion etching͒, provided sharp-edged bridges.…”

“…A simple advantage of J dp over T c is that, where T c probes the maximum value of the superconducting order parameter in the sample, J dp comes from an average over the layer thickness, which also involves lower values of ⌿. 16 The aims of this work are to probe the inhomogeneous character of the superconducting order parameter in bilayers of Nb and Pd 81 Ni 19 and to directly compare the information from T c and J dp upon varying the PdNi layer thickness d PdNi . The paper is organized as follows.…”

Direct observation of the depairing current density in single-crystalline Ba0.5K0.5Fe2As2 microbridge with nanoscale thickness

“…Indeed, the J dp is comparable with the J GL dp only at temperatures nearby the T c , but not at low-T, e.g. J dp (0)/J GL dp (0) ≈ 0.385 [21]. Thus, we get to the value J dp (0) ≈363.6 MA/cm 2 that is still a bit higher than the estimated one (≈ 200 MA/cm 2 [11]) but of the same order or magnitude.…”

Josephson current in a ferromagnetic semiconductor/semiconductor/ferromagnetic semiconductor junction with superconducting contacts

“…For a tunnel barrier, it is given by I = I C sin(φ) as first found by Josephson [9], where I C is the critical Josephson current. When the two SCs coupled by a thin ferromagnetic layer [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29], I C can change sign. Referring to the Josephson current-phase relationship, the change is corresponding to a π-phase shift of φ.…”