Depairing current behavior in superconducting Nb/Pd_81Ni_19 bilayers

We have investigated superconductor/ferromagnetic bilayers consisting of Nb/Pd81Ni19 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 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 around 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) (Sov. J. Low Temp. Phys. 6, 210 (1980))), and therefore also by Jdp prop to (1-T/Tc)^3/2 in the Ginzburg-Landau (GL) regime close to Tc . Extrapolating the GL regime to T=0 yields Jdp^GL (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.