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 ${}_{\rm c}$ ) superconducting-ferromagnetic transistors with the SISFIFS structure [where S, I, and F denote a superconductor (Nb), an insulator (AlO ${}_{x}$ ), and a ferromagnetic material (permalloy, Py), respectively]. The Nb/AlO ${}_{x}$ /Nb trilayer, which serves as the acceptor (SIS) junction, is estimated to have Josephson critical current density j ${}_{\rm c}$ of 19 kA/cm 2 . The Al/Py/Al/AlO ${}_{x}$ /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.