Ferromagnetic-insulator-based superconducting junctions as sensitive electron thermometers

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY).We present an exhaustive theoretical analysis of charge and thermoelectric transport in a normal-metal-ferromagnetic-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 open-circuit 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 μK Hz-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.F. G. acknowledges the European Research Council (ERC) under the European Union’s Seventh Framework Program No. FP7/2007-2013 ERC Grant No. 615187-COMANCHE for funding. F. G. and P. S. acknowledge the MIUR-FIRB2013 Project Coca (Grant No. RBFR1379UX) for partial financial support. P. S. has received funding from the European Union Program No. FP7/2007-2013 under REA Grant No. 630925 COHEAT. A. B. thanks for the support of the MIUR-FIRB2012 Project HybridNanoDev (Grant No. RBFR1236VV). The work of F. S. B is supported by the Spanish Ministerio de Economía y Competitividad through the Project No. FIS2014-55987-P and Grupos Consolidados UPV/EHU del Gobierno Vasco (Grant No. IT-756-13).Peer Reviewe