Superconducting spin valve effect in Fe/Si$_3$N$_4$/Pb/Si$_3$N$_4$/Fe heterostructures

TL;DR

This work investigates superconducting spin valve behavior in Fe/Si$_3$N$_4$/Pb/Si$_3$N$_4$/Fe heterostructures with ultrathin insulating spacers to tailor S/F interface transparency. By varying the superconducting Pb thickness $d_ ext{Pb}$ and insulating Si$_3$N$_4$ thickness $d_ ext{Si_{3}N_{4}}$, the authors observe a sizable full SSV effect, with $\Delta T_c$ reaching up to $0.36$ K under $H_0 \approx 1$ kOe. The experimental findings are interpreted via a symmetric F1/S/F2 proximity-model treating $d_ ext{Si_{3}N_{4}}$ as an interface resistance parameter $\gamma_b$, yielding qualitative agreement and quantitative parameter fits (e.g., $\xi_S=40$ nm, $\xi_F=5$ nm, $\gamma=0.038$, $h=0.075$ eV). The results demonstrate tunable, robust SSV behavior in conventional materials and controlled insulating barriers, informing future spintronic device designs that leverage the proximity effect.

Abstract

The structures of the superconducting spin valve (SSV) Fe/Si$_3$N$_4$/Pb/Si$_3$N$_4$/Fe (where Si$_3$N$_4$ is a dielectric insulating layer of controlled thickness) were investigated. The dependence of the magnitude of the SSV effect on the thicknesses of the superconducting (S) and insulating (I) layers was studied. Optimization of the S and I layer thicknesses enabled a complete switching between the normal and superconducting states when the mutual orientation of the magnetizations of the ferromagnetic (F) layers changed from antiparallel to parallel. A maximal SSV effect value of 0.36\,K was achieved in an external magnetic field of 1\,kOe. These results demonstrate that SSV structures with tunable S/F interface transparency controlled by insulating interlayers are promising for achieving a significant magnitude of the effect. This opens new avenues for the development of such systems and their potential applications in spintronic devices.

Figures (7)

• Figure 1: Design of the prepared SSV heterostructures: CoO$_x$(3.5nm)/ Fe1(3nm)/ Si$_3$N$_4$1($d_\mathrm{Si_{3}N_{4}}$)/ Pb($d_\mathrm{Pb}$)/ Si$_3$N$_4$2($d_\mathrm{Si_{3}N_{4}}$)/ Fe2(3nm)/ Si$_3$N$_4$(85nm) with variable Si$_3$N$_4$ layers thickness $d_\mathrm{Si_{3}N_{4}}$ in the range from 0 to 1.2 nm and Pb layer $d_\mathrm{Pb}$ thickness in the range of 40 to 60 nm.
• Figure 2: Magnetic hysteresis loop $M(H)$ for the sample Series 1-3 measured after the field cooling procedure from room temperature to $T = 10$ K in a magnetic field of $+8$ kOe.
• Figure 3: Dependence of $T_c$ on the thickness $d_\mathrm{Si_{3}N_{4}}$ for three series of samples measured without external magnetic field ($H_0 = 0$ Oe). Shaded area is the optimal range of insulating layer thicknesses for observing the SSV effects (see the text for details).
• Figure 4: Superconducting transitions curves for the P and AP orientations of the Fe1 and Fe2 layers magnetization for three samples (a) Series 3-2 ($d_\mathrm{Pb} = 60$ nm), (b) Series 2-3 ($d_\mathrm{Pb} = 50$ nm), and (c) Series 1-2 ($d_\mathrm{Pb} = 40$ nm).
• Figure 5: Dependence of the magnitude of the SSV effect $\Delta T_c$ on the thickness of the Si$_3$N$_4$ layers $d_\mathrm{Si_{3}N_{4}}$ for three series of samples. The calculated theoretical points correspond only to $d_{\text{Si}_{3}\text{N}_{4}} =0$, $0.3$, $0.6$, and $1.2$ nm; they are connected by straight solid lines to aid visual comparison with the experimental data. The fitting parameters are discussed in the main text.