Measurement Results of the Superconducting-Ferromagnetic Transistor

We report on the measurement results of the superconducting-ferromagnetic transistors (SFTs) made at Northwestern University and Hypres, Inc. [IEEE Trans. Appl. Supercond. vol. 24, 1800506 (2014); vol. 25, 1800705 (2015)]. SFT is a multi-terminal device with the SIS'FIFS structure (where S, I, and F denote a superconductor, an insulator, and a ferromagnetic material, respectively) exploiting intense quasiparticle injection in order to modify the non-linear I-V curve of a superconducting tunnel junction. Potentially, SFT is capable of providing voltage, current and power amplification while having good input/output isolation. We characterized the devices at frequencies up to 5 MHz at 4 K. Our setup did not allow for accurate measurement of the voltage gain of low-impedance SFT devices because of contribution of resistance of the bias-T connected in series with the SFT. Nevertheless we observed a voltage gain above unity for some measurement configurations. It is very interesting that we confirmed that the isolation between the input and output of the device is quite good. We suggest that further improvement of the SFT device parameters is possible in optimized devices, so that the device potentially may serve as a preamplifier for readout of output signals of cryogenic detectors and be useful as an element of other superconductor-based circuits.