Investigation of Current Gain in Superconducting-Ferromagnetic Transistors With High-j ${}_{\rm c}$ Acceptor

We report the results of a study of the current gain in high Josephson critical current density (j
_c
) superconducting-ferromagnetic transistors with the SISFIFS structure [where S, I, and F denote a superconductor (Nb), an insulator (AlO
_∞
), and a ferromagnetic material (permalloy, Py), respectively]. The Nb/AlOx/Nb trilayer, which serves as the acceptor (SIS) junction, is estimated to have Josephson critical current density j
_c
of 19 kA/cm
²
. The Al/Py/Al/AlO
_∞
/Py/Al/Nb multilayer is deposited in a separate vacuum run after in situ ion milling of about 8 nm of the top Nb layer. The devices are patterned using optical lithography and tested at 4.2 K. We have observed a small-signal current gain in the range of 5-9. We demonstrate that proper device engineering allows one to efficiently control the maximum Josephson current in the SISF acceptor junction using the quasiparticle injection.