Comparison of NiFeCr and NiFe in ferromagnetic Josephson junctions

Abstract

Josephson junctions containing ferromagnetic materials are under consideration for applications in digital superconducting logic and memory. Some memory applications rely on the ability to reverse the magnetization direction of a “soft” magnetic layer within the junction using a small local magnetic field generated on the chip. It is crucial, therefore, to find a suitable soft magnetic material with a low switching field and low switching energy. A popular magnetic material for such applications is Ni80Fe20, also known as Permalloy; however, Permalloy has a rather large magnetization, leading to large magnetic switching energies. In this work, we explore Cr-doped Permalloy, specifically Ni73Fe18Cr9, which has a saturation magnetization just under two-thirds that of Permalloy. Josephson junctions containing this NiFeCr alloy undergo a 0-π transition at a NiFeCr thickness of 2.3 nm, and the critical supercurrent decays in the alloy over a short characteristic length of 0.36 nm. Switching fields of a few millitesla are promising, but the short decay length and overall small values of the critical current in the Josephson junctions may preclude the use of NiFeCr in current cryogenic memory technologies.