Depairing current behavior in superconducting Nb/Pd81Ni19 bilayers

We have investigated superconductor/ferromagnetic bilayers consisting of Nb/Pd81Ni19 with varying thickness of the ferromagnet (F) layer d(F) in order to compare the behavior of the superconducting transition temperature T-c with that of the depairing current density J(dp). For T-c(d(F)), we find the usual behavior, with a minimum around d(F)=3 nm, which signifies the transition to an oscillatory order parameter in the F layer. For J(dp), which was measured down to T/T-c approximate to 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 J(dp)proportional to(1-T/T-c)(3/2) in the Ginzburg-Landau (GL) regime close to T-c. Extrapolating the GL regime to T=0 yields J(dp)(GL)(0), which, as a function of d(F), behaves similarly to T-c(d(F)) with a shallow minimum around d(F)=3-4 nm. At some temperature below T-c, most samples break away from the KL curve to higher values of J(dp), 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.