The physics of superconductor-ferromagnet hybrid structures

Abstract

In this review, we summarize the foundations underlying a variety of phenomena in superconductor-ferromagnet hybrid structures, with a focus on recent advances in several key areas. These include: (i) the fundamental understanding of proximity effects in superconductor-ferromagnet based systems; (ii) spin-valve effects in superconductor-ferromagnet and superconductor-ferromagnet-superconductor Josephson junctions; and (iii) the design and realization of superconducting memory elements, particularly in hybrid Josephson junctions. We also discuss the experimental progress in fabricating and characterizing spin-valve structures.

Figures (4)

• Figure 1: (a, b) Experimental (dots) and simulated (solid lines) specular neutron reflectivity curves measured at $T = 13$ K in applied magnetic fields of $H = 300$ Oe (a) and $H = 30$ Oe (b). Insets show: (a) schematic of the sample and PNR setup; (b) magnetic field dependence of the $j = 1/2$ Bragg peak (highlighted in blue). The sample consists of a periodic [Co(2 nm)/Nb(8 nm)]$\times$12 spin-valve structure Klenov2019. Numbers above the peaks denote the Bragg reflection order. (c) Scanning electron microscope (SEM) image of an SFS junction fabricated using focused ion beam (FIB) etching. (d) $I$–$V$ characteristics of a Nb/Ni (7 nm)/Nb junction at various temperatures $T$, illustrating superconducting the Josephson effect Kapran2021. (e) Calculated dependence of critical temperature $T_c$ on ferromagnetic layer thickness $d_F$; $T_{cs}$ is the intrinsic $T_c$ of the superconducting layer in isolation Karabassov2019. (f) Experimental $T_c(d_F)$ data from Ref. Khaydukov2018 (red dots) and Ref. Jiang1995 (black dots). Solid lines are theoretical fits based on Usadel equations for both $0$ and $\pi$ phase states Karabassov2019 in S/F/S structures using material parameters from the measurements Khaydukov2018Jiang1995. Inset shows the temperature dependence of the magnetic moment for the set of samples marked in the main panel (from Ref. Khaydukov2018). Figures 1 (a,b) were reproduced from Klenov2019 (© 2019 N. Klenov et al., published by the Beilstein- Institut, distributed under the terms of the Creative Commons Attribution 4.0 International License, https://creativecommons.org/licenses/by/4.0). Figures 1 (c,d) were reproduced from Kapran2021 (© 2021 O. M. Kapran et al., published by the American Physical Society, distributed under the terms of the Creative Commons Attribution 4.0 International License, https://creativecommons.org/licenses/by/4.0). Figure 1 (e) was reprinted with permission from Ref. Karabassov2019, Copyright 2019 by the American Physical Society. This content is not subject to CC BY 4.0. Figure 1 (f) was reprinted with permission from Ref. Khaydukov2018, Copyright 2018 by the American Physical Society. This content is not subject to CC BY 4.0.
• Figure 2: The transition temperature $T_c$ for a series of Nb/Cu$_{0.59}$Ni$_{0.41}$ bilayers. The thickness of the flat Nb layer was fixed in different sample series: $d_{\mathrm{Nb}} \approx 14.1$ nm (S23), $7.8$ nm, $7.3$ nm, and $6.2$ nm. For $d_{\mathrm{Nb}} \approx 6.2$ nm, the transition temperature exhibits fully reentrant behavior. Reproduced from Ref. Zdravkov2010.
• Figure 3: (a) Dependence of the critical current density $J_C$ of the SIsFS junction on the F-layer thickness $d_F$ for two values of the intermediate s-layer thickness: $d_s = 5\xi_S(T) > d_{sc}$ (solid line) and $d_s = 0.5\xi_S(T) < d_{sc}$ (dashed line). The calculations are performed at $T = 0.9T_C$ for $H = 10\pi T_C$ and $\Gamma = 5$. (b–d) Current-phase relations $J_S(\varphi)$ in the vicinity of $0$–$\pi$ transitions. Each panel includes an inset that enlarges a corresponding section of the $J_C(d_F)$ curve from part (a), marked by the letters b–d. The numbered points in the insets indicate the specific $d_F$ values at which the $J_S(\varphi)$ curves were computed. Dashed lines in panels (b)–(d) trace the critical points where $J_S(\varphi)$ attains its maximum value $J_C(d_F)$. Reprinted with permission from Ref. Bakurskiy2, Copyright 2013 by the American Physical Society. This content is not subject to CC BY 4.0.
• Figure 4: Multi-valued $R_{N}J_{S}(\varphi )$ dependencies in an SIsFS junction for different values of the first (A) and second (B) harmonics of the CPR of the sFS junction: a) A=0.1, B=0.8; b) A=0.1, B=1.0; c) A=0.1, B=1.2; d) A=1.4, B=1.0; e) A=1.0, B = -0.3; f) A=0.5, B = -0.5; g) A=0.1, B = -0.8; g) A=0.1, B = -1.0. Solid and dashed lines show stable and unstable states, respectively.