  We report experimental results on characteristics of SFIFS junctions and
multi-terminal SFIFSIS devices (where S, I, and F denote a superconductor (Nb),
an insulator (AlOx), and a ferromagnetic material (Ni), respectively). The
SFIFS junctions serve as injectors in the SFIFSIS devices which have
transistor-like properties; for this reason we call them
Superconducting-Ferromagnetic Transistors (SFTs). We have found the F (Ni)
thickness at which the SFIFS current-voltage characteristic (CVC) becomes
linear. Furthermore, we investigated the DC and AC characteristics of SFTs of
two types: ordinary devices with a single acceptor (SIS) junction, and devices
with a double acceptor. In the first case, we focused on studying the influence
of the injection current through the SFIFS junction on the maximum Josephson
current of the SIS acceptor. For devices of the second type, we studied voltage
amplification properties when the operating point was chosen in the sub-gap
region of the acceptor CVC. By applying an AC signal (in the kHz range) while
biasing the injector (SFIFS) junction with a constant DC current, we observed a
voltage gain above 25 on the double acceptor. In the reverse transmission
experiment, we applied DC current and an AC modulation to the acceptor junction
and, within the accuracy of the experiment, observed no response on the
injector junction, which implies an excellent input-output isolation in our
SFIFSIS devices. The experiments indicate that, after optimization of the
device parameters, they can be used as input/output isolators and amplifiers
for memory, digital, and RF applications.

Superconductivity;Mesoscale and Nanoscale Physics