Depairing current behavior in superconducting Nb Pd81 Ni19 bilayers

Abstract

We have investigated superconductor/ferromagnetic bilayers consisting of Nb Pd81 Ni19 with varying thickness of the ferromagnet (F) layer dF in order to compare the behavior of the superconducting transition temperature Tc with that of the depairing current density Jdp. For Tc (dF), we find the usual behavior, with a minimum around dF =3 nm, which signifies the transition to an oscillatory order parameter in the F layer. For Jdp, which was measured down to T Tc ≈0.5 using a pulsed-current technique, we find that the behavior can be well described by the Kupriyanov-Lukichev (KL) theory (Fiz. Nizk. Temp. 6, 445 (1980)), and therefore also by Jdp (1-T Tc) 3 2 in the Ginzburg-Landau (GL) regime close to Tc. Extrapolating the GL regime to T=0 yields JdpGL (0), which, as a function of dF, behaves similarly to Tc (dF) with a shallow minimum around dF =3-4 nm. At some temperature below Tc, most samples break away from the KL curve to higher values of Jdp, indicating a current-induced breakdown of the inhomogeneous state. Moreover, we find a significantly increased width of the transition to the normal state in the regime of the oscillatory order parameter. © 2007 The American Physical Society.