Superconducting spintronics with electron symmetry filtering and interfacial spin-orbit coupling

TL;DR

This work reviews fully epitaxial V/MgO/Fe heterostructures where electron symmetry filtering and interfacial SOC enable long-range equal-spin triplet superconductivity. It combines growth, normal-state and low-temperature transport, ab-initio SOC calculations, and Bogoliubov–de Gennes modeling to show how $\Delta_2$/$\Delta_1$ orbital filtering and Rashba SOC promote singlet-triplet conversion, yielding large MAAR, magnetization-driven MCA changes, and MacMillan-type resonances. Key findings include bias-enhanced TMR at room temperature, SOC-driven conductance bottlenecks, and giant subgap shot noise indicative of LRT formation, plus the first demonstrations of magnetization-controlled Josephson behavior in lateral S/SOC/F structures. These results suggest a versatile platform for superconducting spintronic devices with potential applications in ultra-low-energy cryogenic memories and quantum information processing.

Abstract

Over the recent years, crossroads of magnetism and superconductivity led to the emerging field of superconducting spintronics. A cornerstone of this venture is the generation of equal-spin triplet Cooper pairs in superconductor-ferromagnet hybrids, enabling long-range spin-polarized supercurrents and magnetic control over superconducting quantum states for the development of energy-efficient cryogenic devices. Until now, nearly all superconducting spintronic devices have relied on direct interfaces between superconductors and ferromagnets, since it was believed that an insulating barrier would decouple spin and charge transport. This assumption, however, appears to be invalid when a thin spin- and orbit-filtering barrier couples epitaxial ferromagnet and the superconductor. Symmetry filtering plays a crucial role in enhancing giant tunneling magnetoresistance (TMR) by selectively allowing specific electronic states to tunnel through the barrier. Such a mechanism is key for high-performance spintronic devices like magnetic random access memories, magnetic sensors or spin-light emitting diodes. This manuscript provides a comprehensive review of superconducting spintronics driven by electron symmetry filtering and interfacial SOC. It emphasizes the critical role of a crystalline MgO barrier in selectively transmitting specific electronic states between V(100) and Fe(100). The manuscript also highlights how interfacial SOC enables symmetry mixing, allowing for the interaction between ferromagnetic and superconducting orderings though MgO(100). This mutual interaction, mediated by interfacial SOC, facilitates the conversion of spin-singlet to spin-triplet Cooper pairs. The work provides key insights into designing SOC based superconductor-ferromagnet hybrid structures for advanced superconducting spintronic functionalities.

Figures (16)

• Figure 1: Sketch of the structure of the types of junctions studied. (a) F/F, (b) N(S)/F, (c) N(S)/F/F, (d) F/N(S)/F. The different colors represent each material (see legend). The Au top layer is not shown as it is the same in all the samples. (e) Table with detailed information of the vertical structure of the samples.
• Figure 2: Monitoring and characterization of the growth procces of the sample S1. (a) Typical reflection high-energy electron diffraction (RHEED) image for the annealed V (40nm) layer. (b) RHEED image. The yellow square marks the area averaged for the (c) intensity oscillatons during the growth of the MgO barrier on top of V are shown. (d) RHEED image for the Fe layer on top of the MgO barrier. All images are taken from [110] azimuth of the MgO substrate.
• Figure 3: (a,b) In-plane TMR in the easy axis [100] for a a N/F/F junction. Both parallel and anti-parallel magnetic alignments are observed, with a very low coercive field ($\sim25-50$ Oe) for the soft layer and a higher ($\sim400-600$ Oe) for the hard one. (c,d) In-plane rotation of the magnetic field, using a field higher than the soft layer coercive field (80 Oe), making it reorient following the field and magnetocrystalline anistropy (MCA), but lower than the hard layer coercive field, so it is fixed during the experiment. (e,f) Out-of-plane TMR at $T=5$ K with an out-of-plane alignment of the soft layer. Panels (a,c,e) sketch the orientation of the magnetic field with respect to the MCA in each of the experimetns. Figure adapted from refs. GonzalezRuano2020GonzalezRuano2021 with the necessary permissions.
• Figure 4: (a) Supercell model for the X/MgO interfaces with X = V, Fe. Calculated band splitting due to Rashba spin-orbit coupling at the (b) V/MgO and (c) Fe/MgO interface in the absence of electric field. Inset of (b): variation of $\alpha_R$ with the electric field in case of V/MgO interface. Charge density at the X/MgO interface (blue rectangle in (a)) for (d) V/MgO and (e) Fe/MgO. (f) Sketch model for the Rashba splitting of the parabolic bands, the $k$ lines in (b) and (c) correspond to $k_y=0$, the direction in the k space (100) corresponds to $k_x>0$ and (-100) to $k_x<0$.
• Figure 5: (a) Transport mechanism in the V/MgO/Fe barrier, sketching the vanadium mixed $\Delta_2$ and $\Delta_1$ surface states, interfacial spin-orbit coupling and symmetry filtering in the MgO. (b) average conductance for different types of samples of the same lateral size ($20\times20~\mu\text{m}^2$), as a function of the number of V/MgO interfaces for each type.