Realisation of de Gennes' absolute superconducting switch with a heavy metal interface

Abstract

In 1966, Pierre-Gilles de Gennes proposed a non-volatile mechanism for switching superconductivity on and off in a magnetic device. This involved a superconductor (S) sandwiched between ferromagnetic (F) insulators in which the net magnetic exchange field could be controlled through the magnetisation-orientation of the F layers. Because superconducting switches are attractive for a range of applications, extensive studies have been carried out on F/S/F structures. Although these have demonstrated a sensitivity of the superconducting critical temperature (T_c) to parallel (P) and antiparallel (AP) magnetisation-orientations of the F layers, corresponding shifts in T_c (i.e. ΔT_c = T_c,AP - T_c,P) are lower than predicted with ΔT_c only a small fraction of T_c,AP, precluding the development of applications. Here, we report EuS/Au/Nb/EuS structures where EuS is an insulating ferromagnet, Nb is a superconductor and Au is a heavy metal. For P magnetisations, the superconducting state in this structure is quenched down to the lowest measured temperature of 20 mK meaning that ΔT_c/T_c,AP is practically 1. The key to this so-called 'absolute switching' effect is a sizable spin-mixing conductance at the EuS/Au interface which ensures a robust magnetic proximity effect, unlocking the potential of F/S/F switches for low power electronics.

Fig. 1. A de Gennes’ superconducting switch and structural, superconducting and magnetic properties of NbO_x(3 nm)/EuS(30 nm)/Nb(d_Nb)/SiO₂//Si structures.

Schematic diagrams of a F/S/F superconducting switch in which a superconductor (S) is sandwiched between ferromagnetic insulators (F): a The proximity-induced magnetic exchange field (h_ex) in the S layer from the AP-aligned magnetisations is minimised or is, ideally, zero, preserving the superconducting state with a transition temperature T_c,AP; b For P-aligned magnetisations, h_ex is maximised so the superconducting transition temperature T_c,P is much lower than T_c,AP. c, d Representations of the superconducting density of states diagrams for the S layer for AP and P magnetisations of the F layers: c In the AP-state the density of states shows no evidence of proximity-induced magnetism (i.e. h_ex = 0), whereas in the P-state in (b) there is an energy splitting of 2h_ex in the spin-bands due to the proximity-induced exchange field. e STEM image from a control sample of a NbO_x(3 nm)/EuS(30 nm)/Nb(20 nm)/SiO₂//Si structure, showing the chemistry diagram with Nb (green), Eu (blue) and O (red). The scale bar has a length corresponding to 20 nm. f The left axis shows the zero-field-cooled superconducting transition temperature T_c versus Nb thickness d_Nb (blue) and the right axis shows the superconducting transition width σ_Tc versus d_Nb (black). g Normalised resistance R versus in-plane magnetic field H (R(H)) of an unpatterned NbO_x(3 nm)/EuS(30 nm)/Nb(2 nm)/SiO₂//Si structure at 50 mK, where R_N is the normal state resistance. h Normalised R(H) of an unpatterned NbO_x(3 nm)/EuS(30 nm)/Nb(3 nm)/SiO₂//Si structure at 2 K along with the magnetisation versus in-plane magnetic field M(H) hysteresis loop for a 30-nm-thick EuS film at 1.8 K. Red (black) curves indicate a decreasing (increasing) in-plane magnetic field.

Fig. 2. Superconducting switch performance with or without a heavy metal interface interlayer.

a M(H) (right axis) and R(H) (left axis) from an unpatterned NbO_x(3 nm)/EuS(20 nm)/Nb(4 nm)/EuS(10 nm)/SiO₂//Si structure (Device 1) at 4.2 K. Single arrows indicate the magnetic field sweep directions and double arrows represent possible magnetisation directions of the top and bottom EuS layers. Top left inset: schematic cross-section of the structure. b R_AP(T)/R_N(T) (in green) and ΔR(T)/R_N(T) (in pink) of each R(H) scan. c M(H) at 1.8 K (right axis) and R(H) at 20 mK (left axis) of an unpatterned NbO_x(3 nm)/EuS(20 nm)/Au(20 nm)/Nb(4 nm)/EuS(10 nm)/SiO₂//Si structure (Device 2). Top left inset: schematic cross-section of the structure. d R_AP(T)/R_N(T) (in green) and ΔR(T)/R_N(T) (in pink) of each R(H) scan, showing absolute switching with ∆T_c/T_c,AP equal to 1 (approximately). Data below 1 K are for the same structure measured in a different cooling in a dilution fridge.

Fig. 3. Literature survey of superconducting switch efficiencies for F/S/F structures with different materials combinations, including transition metal ferromagnets and f-orbital ferromagnets.

PCMO is Pr_0.8Ca_0.2MnO₃, PCCO is Pr_1.85Ce_0.15CuO₄, LCMO is La_0.7Ca_0.3MnO₃ and YBCO is YBa₂Cu₃O₇.

Fig. 4. Calculated superconducting switch efficiency of EuS/Au(d_NM)/Nb(4)/EuS structures.

a  T_c,P (in blue) and T_c,AP (in green) as a function of d_Au. b ΔT_c/T_c,AP as a function of d_NM. For optimised proximity-induced magnetic exchange fields of κ_EuS/Au = 1.5 meV·nm at the EuS/Au interface and κ_EuS/Nb = 1.2 meV·nm at the EuS/Nb interface, absolute switching is expected for d_Au ≥ 15 nm (Solid line). The dashed line in (b) corresponds to d_Au = 0. Dark grey data with d_Al = 8 nm indicates the control structure involved inserting an 8-nm-thick Al spacer in a EuS(20 nm)/Al(8 nm)/Nb(4 nm)/EuS(10 nm) structure. The superconducting switch efficiency decreases to 10%.