Ferromagnetic-Insulator-Based Superconducting Junctions as Sensitive Electron Thermometers

We present an exhaustive theoretical analysis of charge and thermoelectric transport in a normal-metalferromagnetic- insulator-superconductor junction and explore the possibility of its use as a sensitive thermometer. We investigate the transfer functions and the intrinsic noise performance for different measurement configurations. A common feature of all configurations is that the best temperature-noise performance is obtained in the nonlinear temperature regime for a structure based on an Europium chalcogenide ferromagnetic insulator in contact with a superconducting Al film structure. For an opencircuit configuration, although the maximal intrinsic temperature sensitivity can achieve 10 nK Hz(-1/2), a realistic amplifying chain will reduce the sensitivity up to 10 mu KHz(-1/2) . To overcome this limitation, we propose a measurement scheme in a closed-circuit configuration based on state-of-the-art superconducting-quantum- interference-device detection technology in an inductive setup. In such a case, we show that temperature-noise can be as low as 35 nK Hz(-1/2). We also discuss a temperature-to-frequency converter where the obtained thermovoltage developed over a Josephson junction operated in the dissipative regime is converted into a high-frequency signal. We predict that the structure can generate frequencies up to approximately 120 GHz and transfer functions up to 200 GHz/K at around 1 K. If operated as an electron thermometer, the device may provide temperature-noise lower than 35 nK Hz(-1/2) thereby being potentially attractive for radiation-sensing applications.