We are studying on information processing using half-flux quanta as an information carrier with a new parameter called "softness" by using an artificial spin circuit with a magnetic Josephson junction. An information carrier capable of dynamically adjusting the energy consumption associated with switching in a wide range is called an "information carrier with variable softness". This can be freely adjusted from a state in which information is stably held to a state in which it is susceptible to external influences. In applications such as machine learning or combinatorial optimization, exact solutions are not always required, which have been in increasing demand in recent years. Information carriers with variable-softness will fill this gap, enabling uncertain but overwhelmingly low-energy information retention and computation.

We fabricated Nb/CuNi/Nb (S/F/S) Josephson junction using CuNi as the magnetic layer material for the purpose of establishing the magnetic Josephson junction fabrication process to realize the artificial spin circuit and evaluating the uniformity. For magnetic Josephson junctions, While Yamashita et al. demonstrated devices that use PdNi as a magnetic layer [1], we are investigating uniformity, controllability, and reproduction for the SFS device and are working to improve the nature and establish the fabrication process.

DC magnetron sputtering was used to prepare the electrode Nb, and electron beam deposition was used to prepare the magnetic layer CuNi in situ. It was found that the resistivity of CuNi can be controlled from 3.79 μΩcm to 8.06 μΩcm by changing the composition of Ni in the range of 20 wt% to 40 wt%[2]. In addition, for plasma etching of the Nb electrode of the conventional Nb/AlOx/Nb Josephson junction, SF₆ gas is used, but when etching the Nb electrode on the CuNi layer, the Nb electrode was not uniformly etched due to chemical reaction of the copper and sulfur of SF₆ gas. Therefore, we tried the plasma etching with CF₄ gas instead of SF₆ gas for the Nb electrode on the CuNi layer, the electrodes were uniformly patterned without alteration of CuNi. Currently, we are working on the fabrication of SFS Josephson junction arrays to evaluate the uniformity. The SFS junction has an over-dumped characteristic without hysteresis like an SNS junction, and the uniformity of the critical current of the junction array cannot be evaluated even with an I-V characteristics, but the uniformity could be evaluated indirectly from the height of the Shapiro step by applying microwaves[3]. We believe that these approaches will contribute to realize reliable fabrication process and evaluate the junction characteristics for an artificial spin circuit.

The authors would like to thank Nobuhito Kokubo and Go Fujii for their helpful discussions. This research was supported by Japan Science and Technology Agency (JST) CREST (No. JPMJCR20C5). This work was carried out in part in the clean room for analog-digital superconductivity (CRAVITY), National Institute of Advanced Industrial Science and Technology, Tsukuba, Japan.

[1] T.Yamashita, A.Kawakami, and H.Terai, “NbN-Based Ferromagnetic 0 and π Josephson Junctions,” Phys. Rev. Applied., Vol. 8, p. 054028, 2017.
[2] V.Zdravkov et al. “Reentrant superconductivity in Nb/Cu1-xNix Bilayers,” Phys. Rev. Lett. Vol. 97 057004, 2006.
[3] H. Selleier, C Baraduc, F. Lefloch, and R Calemczuk, “Half-Integer Shapiro Steps at the 0-pi Crossover of a Ferromagnetic Josephson Junction” Phys. Rev Lett. Vol. 92 257005, 2004.

Keywords: half-flux quanta, variable-softness, Nb/CuNi/Nb (S/F/S) Josephson junction, magnetic Josephson junction