Hybrid superconducting-magnetic memory device using competing order parameters

Abstract

In a hybrid superconducting-magnetic device, two order parameters compete, with one type of order suppressing the other. Recent interest in ultra-low-power, high-density cryogenic memories has spurred new efforts to simultaneously exploit superconducting and magnetic properties so as to create novel switching elements having these two competing orders. Here we describe a reconfigurable two-layer magnetic spin valve integrated within a Josephson junction. Our measurements separate the suppression in the superconducting coupling due to the exchange field in the magnetic layers, which causes depairing of the supercurrent, from the suppression due to the stray magnetic field. The exchange field suppression of the superconducting order parameter is a tunable and switchable behaviour that is also scalable to nanometer device dimensions. These devices demonstrate non-volatile, size-independent switching of Josephson coupling, in magnitude as well as phase, and they may enable practical nanoscale superconducting memory devices.

Critical current oscillations of elliptical Josephson junctions with single-domain ferromagnetic layers

“…One method of accomplishing these goals is to make a junction containing a "spin valve", i.e. two F layers whose relative magnetization directions can be switched between parallel and antiparallel 19,23,24 . We demonstrated such a controllable 0-π junction recently, using Ni and NiFe as the two ferromagnetic materials 18 .…”

“…Numerous ideas have been presented in the literature regarding how SFS junctions might be used as practical memory devices [19][20][21][22][23][24][25] . The ferromagnetic (F) layer influences the properties of the junction both through the magnetic field and the exchange field it generates, and ideas have been presented using either of those mechanisms.…”

Controllable 0–π Josephson junctions containing a ferromagnetic spin valve

“…The free layer was chosen as Ni 0.84 Fe 0.16 ('Permalloy') of thickness 1.5 nm to put the junction close to the 0-π transition (J.A.G., M. A. Khasawneh, B.M.N., E.C.G., R.L., W.P.P.Jr, and N.O.B., in preparation). The fixed layer in the junctions is Ni of thickness 1.2 nm, which should add or subtract a small phase increment 15,[18][19][20] . Further information about the materials can be found in Methods.…”

“…Imagine now a Josephson junction with the structure S/F 1 /N/F 2 /S, where F 1 and F 2 may be different ferromagnetic materials [14][15][16][17] . The pair correlation function describing Cooper pairs from the left-hand S accumulates a phase φ 1 = Q If, however, the magnetization of F 2 is antiparallel to that of F 1 , then the role of majority and minority bands is reversed, and the pair correlation function will acquire the opposite phase, −φ 2 .…”

“…Phase-controllable Josephson junctions have applications in superconducting electronics based on single-flux-quantum logic. The most obvious application is in superconducting memory [14][15][16][17] . A single S/F/S Josephson junction with controllable critical current amplitude could function as a superconducting memory cell, but one must find a way to address such a memory cell when it is embedded in a large memory array, and the speed at which the junction switches into the voltage state after the 'read' current is applied is limited by the small I c R N product of the junction.…”