Comparison of NiFeCr and NiFe in ferromagnetic Josephson junctions

TL;DR

This study compares NiFeCr and NiFe in ferromagnetic Josephson junctions, focusing on magnetic switching properties and their implications for superconducting memory applications.

Contribution

It introduces NiFeCr as a low-magnetization alternative to Permalloy and characterizes its behavior in Josephson junctions, highlighting its potential and limitations.

Findings

NiFeCr undergoes a 0-$$ transition at 2.3 nm thickness.

Critical supercurrent decays rapidly with a length of 0.36 nm.

Switching fields are in the millitesla range, promising for memory applications.

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 Ni$_{80}$Fe$_{20}$, 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 Ni$_{73}$Fe$_{18}$Cr$_{9}$, which has a saturation magnetization just under two-thirds that of Permalloy. Josephson junctions containing this NiFeCr alloy undergo a 0-$\pi$ 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.