Spin-valve Josephson junctions for cryogenic memory

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 π 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.

Cryogenic Memory Element Based on an Anomalous Josephson Junction

“…One of the key challenges towards this objective is the fabrication of a reliable and scalable cryogenic memory architecture. Superconductor-ferromagnet-superconductor (SFS) junctions are promising structures suggested for such memories [6][7][8][9][10][11][12][13][14][15]. Indeed, the interplay between the intrinsic exchange field and the induced superconductivity in the ferromagnet leads to the so-called π-junction, i.e., a Josephson junction exhibiting an intrinsic π-phase shift in its ground state.…”

Spin-valve Josephson junctions with perpendicular magnetic anisotropy for cryogenic memory

“…The first experimental report by Bell et al used a Co/Cu/Permalloy PSV to control the critical current of the junction 28 . Promising electrical engineering architectures which integrate the ferromagnetic Josephson junction bits into scalable memory cells compatible with current technology exists, and recently PSV devices have been demonstrated by several groups [29][30][31][32][33][34][35][36][37][38] . To date, all experimental works on PSV Josephson junctions have considered in-plane F layers as used by Bell et al.…”

“…In ferro-magnetic Josephson junctions however, the magnetization of the F layer can contribute to the magnetic flux density in the junction, introducing H shift proportional to the magnetization of the F layer collinear with H app . This can be significant in magnetization in-plane devices, where µ 0 H shift can be 10's mT causing the soft F layer to switch prior to reaching I c0 in the I c (B) sweep 35 . In our junctions, where the magnetization of the F layers is perpendicular to plane, we do not expect this to occur as the layers do not contribute significant in-plane magnetization components and cannot be switched by an inplane applied field.…”

Electrodynamics of Highly Spin-Polarized Tunnel Josephson Junctions

“…Ferromagnetic Josephson junctions (SFS JJs) have attracted considerable attention in the emerging fields of superconducting spintronics [1][2][3][4][5] and as quantum and classical devices, since they have been proposed as energy-efficient memories [6][7][8][9] and as passive π shifters (phase inverters) in quantum circuits [10][11][12]. However, in standard metallic SFS JJs, the I c R N product is of the order of a few microvolts or less [2,3,11], I c and R N being, respectively, the critical current and the normal state resistance.…”