Critical Current Gain in High-j_c Superconducting-Ferromagnetic Transistors

We report experimental and theoretical results on the current gain in superconductor-ferromagnetic transistors (SFTs) with the SISFIFS structure [where S, I, and F denote a superconductor (Nb), an insulator (AlO_x), and a ferromagnetic material (Ni), respectively]. The Josephson critical current density j_ca of the acceptor (SISF) junction in the devices is above 9 kA/cm², which is higher than that in our previous devices. The critical current gain is defined as /δI_ca/ /δI_i/, where a change /δI_i/ in the injector (SFIFS junction) current produces a change /δI_ca/ in the acceptor maximum Josephson current. We observed a small-signal gain as high as 7.8 and a large-signal current gain of about 2.2. In addition, we found that the Josephson current of the acceptor junction is sensitive to the state of the ferromagnetic layers. A theoretical model is proposed to describe the nonequilibrium processes in the SFT devices, which agrees with the experimental observations. The calculation shows that, in the present device configuration, the dominant contribution to the gap suppression in the middle Nb electrode is due to the quasiparticle injection; spin injection plays a secondary role. We demonstrate that proper device engineering allows one to efficiently control the maximum Josephson current in the SISF acceptor junction using the quasiparticle injection. We conclude that SFT devices can be used as input/output isolators and amplifiers for memory, digital, and RF applications.