Аннотация:In recent years the use of Josephson junctions with ferromagnetic F layers in superconducting electronics is one of the main directions of their development. The oscillating nature of superconductive proximity effect in the magnetics leads to appearance of π state and provides opportunities to develop a wide range of applications. Among them there are memory elements, improved RSFQ circuits and metamaterial elements.
Unfortunately, characteristic voltage, ICRN, of conventional SFS junctions is much smaller than that of tunnel SIS devices, which are widely used in superconductive electronics. This leads to deficient value of characteristic frequency and to difficulties in integration with other circuits.
Recently, successful realization of switchable Nb-Al/AlOx-Nb-Pd0.99Fe0.01-Nb junctions was reported in [1-3]. These junctions are of SIsFS type, i.e., a serial connection of the SIs tunnel junction and sFS sandwich. These SIsFS structures have high characteristic voltage ICRN up to 300 mV, due to the presence of tunnel barrier I. At the same time they behave as a single junction with respect to an external magnetic field, Hext, since intermediate layer s is too thin to screen it. As a result, the magnetic field entering the Pd0.99Fe0.01 layer modifies its effective magnetization, facilitating the critical current control. According to Ref. [4], effective magnetization in the dilute Pd0.99Fe0.01 is controlled by Fe-rich nano-clusters which can be easily reordered by a weak magnetic field.
We have performed a theoretical study of magnetic SIsFS Josephson junctions in the frame of Usadel equations. Josephson current is calculated numerically in self-consistent manner as a function of s and F layers thickness, temperature T and exchange energy of F film. We outline several modes of operation of these junctions and demonstrate their unique ability to have high ICRN product in the π state, comparable to that in SIS tunnel junctions commonly used in SFQ circuits. Moreover we develop a model describing switching of the Josephson critical current in these devices by external magnetic field and explain asymmetric nature of Fraunhofer pattern IC(Hext) in it. The results are in good agreement with the experimental data and promise to make these structures suitable for integration into high-speed energy-efficient SFQ digital circuits.
References:
[1] T. I. Larkin, V. V. Bol'ginov, V. S. Stolyarov, V. V. Ryazanov, I. V. Vernik, S. K. Tolpygo, and O. A. Mukhanov, Appl. Phys. Lett. 100, 222601 (2012).
[2] I. V. Vernik, V. V. Bol'ginov, S. V. Bakurskiy, A. A.Golubov, M. Y. Kupriyanov, V. V. Ryazanov, O. A. Mukhanov, IEEE Trans. Appl. Supercond. 23, 1701208 (2013).
[3] V. V. Ryazanov, V. V. Bol'ginov, D. S. Sobanin, I. V. Vernik, S. K. Tolpygo, A. M. Kadin, O. A. Mukhanov, Physics Procedia 36, 35 (2012).
[4] L. S. Uspenskay, A. L. Rahmanov, L. A. Dorosinskiy, A. A. Chugunov, V. S. Stolyarov, O. V. Skryabina, and S. V. Egorov, Pis'ma v ZhETF 97, 176 (2013).