Spin-valve Josephson junctions for cryogenic memory

TL;DR

This study investigates spin-valve Josephson junctions with ferromagnetic layers to understand their magnetic and superconducting properties, aiming to enable their use in cryogenic memory by mapping phase control as a function of layer thickness.

Contribution

The paper provides a comprehensive experimental analysis of single spin-valve Josephson junctions with varying ferromagnetic layer thicknesses, advancing understanding of their phase control capabilities.

Findings

Critical currents and Fraunhofer pattern shifts depend on magnetic states.

Switching fields of magnetic layers are characterized.

Partial phase diagram mapping for different layer thicknesses.

Abstract

Josephson junctions containing two ferromagnetic layers are being considered for use in cryogenic memory. Our group recently demonstrated that the ground-state phase difference across such a junction with carefully chosen layer thicknesses could be controllably toggled between zero and $\pi$ by switching the relative magnetization directions of the two layers between the antiparallel and parallel configurations. However, several technological issues must be addressed before those junctions can be used in a large-scale memory. Many of these issues can be more easily studied in single junctions, rather than in the Superconducting QUantum Interference Device (SQUID) used for the phase-sensitive measurements. In this work, we report a comprehensive study of spin-valve junctions containing a Ni layer with a fixed thickness of 2.0 nm, and a NiFe layer of thickness varying between 1.1 and 1.8 nm in steps of 0.1 nm. We extract the field shift of the Fraunhofer patterns and the critical currents of the junctions in the parallel and antiparallel magnetic states, as well as the switching fields of both magnetic layers. We also report a partial study of similar junctions containing a slightly thinner Ni layer of 1.6 nm and the same range of NiFe thicknesses. These results represent the first step toward mapping out a ``phase diagram" for phase-controllable spin-valve Josephson junctions as a function of the two magnetic layer thicknesses.