Design could improve cryogenic memory performance

Cryogenic memory could enable practical implementation of quantum computers, but current technologies are not dense enough for large-scale systems. New approaches using ferromagnetic materials inside Josephson junctions look promising, but the magnetic layers must be made very thinly and precisely, making scalability difficult. Mishra et al. reported a materials system that enables the implementation of thicker magnetic layers in cryogenic-memory technologies.

The authors fabricated Josephson junctions containing a ruthenium layer and, crucially, two sandwiching nickel layers with antiparallel magnetizations. Whereas ferromagnetic Josephson junctions typically exhibit rapidly decaying supercurrents with increasing thickness, the supercurrent in the team’s design decayed very slowly with thickness.

“The [effect from antiparallel magnetization] is only effective at suppressing the supercurrent decay if electrons travel through the junction without scattering much,” co-author Norman Birge said. “We expected our system [to have a lot of scattering]. Hence, we were surprised the supercurrent decayed very slowly as we increased the thicknesses of the two nickel layers.”

To fabricate their junction, the team used lithography and sputtering, varying the thicknesses of the two nickel layers from 1 to 3 nanometers. The authors measured current-voltage curves in a cryogenic transport probe.

The authors plan to modify their multimaterial junction to work with current cryogenic memory devices.

“We hope to use an unbalanced nickel/ruthenium/nickel synthetic antiferromagnet as the fixed layer in place of the single nickel layers we have used until now,” Birge said. “If the magnetic behavior of the synthetic antiferromagnet is more consistent than that of a plain nickel layer, that could improve the performance of our memory devices.”

Source: “Supercurrent transmission through Ni/Ru/Ni synthetic antiferromagnets,” by Swapna Sindhu Mishra, Reza Loloee, and Norman O. Birge, Applied Physics Letters (2021). The article can be accessed at https://doi.org/10.1063/5.0068524 .