Infinite magnetoresistance from the spin dependent proximity effect in symmetry driven bcc-Fe/V/Fe heteroepitaxial superconducting spin valves

Superconducting spin valve effect in Co/Pb/Co heterostructures with insulating interlayers

We report the superconducting properties of Co/Pb/Co heterostructures with thin insulating interlayers. The main specific feature of these structures is the intentional oxidation of both superconductor/ferromagnet (S/F) interfaces. We study the variation of the critical temperature of our systems due to switching between parallel and antiparallel configurations of the magnetizations of the two magnetic layers. Common knowledge suggests that this spin valve effect, which is due to the S/F proximity effect, is most pronounced in the case of perfect metallic contacts at the interfaces. Nevertheless, in our structures with intentionally deteriorated interfaces, we observed a significant full spin valve effect. A shift of the superconducting transition temperature Tc by switching the mutual orientation of the magnetizations of the two ferromagnetic Co layers from antiparallel to parallel amounted to ΔTc = 0.2 K at the optimal thickness of the superconducting Pb layer. Our findings verify the so far unconfirmed earlier results by Deutscher and Meunier on an F1/S/F2 heterostructure with oxidized interlayers [Deutscher, G.; Meunier, F. Phys. Rev. Lett. 1969, 22, 395. https://doi.org/10.1103/PhysRevLett.22.395] and suggest an alternative route to optimize the performance of superconducting spin valves.

Emergent Multifunctional Magnetic Proximity in van der Waals Layered Heterostructures

Proximity effect, which is the coupling between distinct order parameters across interfaces of heterostructures, has attracted immense interest owing to the customizable multifunctionalities of diverse 3D materials. This facilitates various physical phenomena, such as spin order, charge transfer, spin torque, spin density wave, spin current, skyrmions, and Majorana fermions. These exotic physics play important roles for future spintronic applications. Nevertheless, several fundamental challenges remain for effective applications: unavoidable disorder and lattice mismatch limits in the growth process, short characteristic length of proximity, magnetic fluctuation in ultrathin films, and relatively weak spin-orbit coupling (SOC). Meanwhile, the extensive library of atomically thin, 2D van der Waals (vdW) layered materials, with unique characteristics such as strong SOC, magnetic anisotropy, and ultraclean surfaces, offers many opportunities to tailor versatile and more effective functionalities through proximity effects. Here, this paper focuses on magnetic proximity, i.e., proximitized magnetism and reviews the engineering of magnetism-related functionalities in 2D vdW layered heterostructures for next-generation electronic and spintronic devices. The essential factors of magnetism and interfacial engineering induced by magnetic layers are studied. The current limitations and future challenges associated with magnetic proximity-related physics phenomena in 2D heterostructures are further discussed.

Mesoscale Phase Separation of Skyrmion-Vortex Matter in Chiral-Magnet-Superconductor Heterostructures

We investigate theoretically the equilibrium configurations of many magnetic skyrmions interacting with many superconducting vortices in a superconductor-chiral-magnet bilayer. We show that miscible mixtures of vortices and skyrmions in this system break down at a particular wave number for sufficiently strong coupling, giving place to remarkably diverse mesoscale patterns: gel, stripes, clusters, intercalated stripes, and composite gel-cluster structures. We also demonstrate that, by appropriate choice of parameters, one can thermally tune between the homogeneous and density-modulated phases.

Superconducting straintronics via the proximity effect in superconductor-ferromagnet nanostructures

We propose the concept of superconducting straintronic devices based on the proximity effect at superconductor-ferromagnet interfaces and the ability of ferromagnets with a strong magnetoelastic coupling to change magnetization orientations under strain. Since strains can be generated electrically with the aid of a piezoelectric transducer attached to a superconductor-ferromagnet nanostructure, the power consumption of the resulting hybrid straintronic device could be greatly reduced in comparison with a superconducting spin valve operated by external magnetic field. To demonstrate the feasibility of our proposal, we theoretically describe the strain-mediated magnetoelectric control of the critical temperature Tc in asymmetric ferromagnet-superconductor-ferromagnet trilayers integrated onto a ferroelectric substrate. To that end, Tc is first calculated as a function of the misorientation angle Δφ between the in-plane magnetizations of the two ferromagnetic layers. Numerical calculations performed for CoFe/Nb/NiCu trilayers show that, at optimal nanoscale thicknesses of superconducting and ferromagnetic films, the variation of Δφ may change Tc significantly (ΔTc ∼ 1 K). Then electric-field-induced magnetization rotations are determined for CoFe/Nb/NiCu trilayers coupled to the Pb(Zn1/3Nb2/3)O3-6%PbTiO3 single crystal. It is found that, owing to the opposite sign of the magnetoelastic coupling in CoFe and NiCu alloys, the misorientation angle Δφ can be changed by up to 90° without the introduction of an antiferromagnetic pinning layer. As a result, the electrical switching of the Nb interlayer between the superconducting state and a finite-resistance state appears to be possible in a narrow temperature range. Hence the described multiferroic hybrid can be employed as an electrically controlled resistive switch useful for memory and logic applications in cryogenic electronics.

Nodal superconducting exchange coupling

A superconducting spin valve consists of a thin-film superconductor between two ferromagnetic layers. A change of magnetization alignment shifts the superconducting transition temperature (ΔΤ_c) due to an interplay between the magnetic exchange energy and the superconducting condensate. The magnitude of ΔΤ_c scales inversely with the superconductor thickness (d_S) and is zero when d_S exceeds the superconducting coherence length (ξ). Here, we report a superconducting spin-valve effect involving a different underlying mechanism in which magnetization alignment and ΔΤ_c are determined by nodal quasiparticle excitation states on the Fermi surface of the d-wave superconductor YBa₂Cu₃O_7-δ sandwiched between insulating layers of ferromagnetic Pr_0.8Ca_0.2MnO₃. We observe ΔΤ_c values that approach 2 K with the sign of ΔΤ_c oscillating with d_S over a length scale exceeding 100ξ and, for particular values of d_S, the superconducting state reinforces an antiparallel magnetization alignment. These results pave the way to all-oxide superconducting memory in which superconductivity modulates the magnetic state.

Increasing the performance of a superconducting spin valve using a Heusler alloy

We have studied superconducting properties of spin-valve thin-layer heterostructures CoO _x /F1/Cu/F2/Cu/Pb in which the ferromagnetic F1 layer was made of Permalloy while for the F2 layer we have taken a specially prepared film of the Heusler alloy Co₂Cr_1-x Fe _x Al with a small degree of spin polarization of the conduction band. The heterostructures demonstrate a significant superconducting spin-valve effect, i.e., a complete switching on and off of the superconducting current flowing through the system by manipulating the mutual orientations of the magnetization of the F1 and F2 layers. The magnitude of the effect is doubled in comparison with the previously studied analogous multilayers with the F2 layer made of the strong ferromagnet Fe. Theoretical analysis shows that a drastic enhancement of the switching effect is due to a smaller exchange field in the heterostructure coming from the Heusler film as compared to Fe. This enables to approach an almost ideal theoretical magnitude of the switching in the Heusler-based multilayer with a F2 layer thickness of ca. 1 nm.

Single Abrikosov vortices as quantized information bits

Superconducting digital devices can be advantageously used in future supercomputers because they can greatly reduce the dissipation power and increase the speed of operation. Non-volatile quantized states are ideal for the realization of classical Boolean logics. A quantized Abrikosov vortex represents the most compact magnetic object in superconductors, which can be utilized for creation of high-density digital cryoelectronics. In this work we provide a proof of concept for Abrikosov-vortex-based random access memory cell, in which a single vortex is used as an information bit. We demonstrate high-endurance write operation and two different ways of read-out using a spin valve or a Josephson junction. These memory cells are characterized by an infinite magnetoresistance between 0 and 1 states, a short access time, a scalability to nm sizes and an extremely low write energy. Non-volatility and perfect reproducibility are inherent for such a device due to the quantized nature of the vortex.

Superconductors can be used in future supercomputers because they can greatly reduce power consumption and facilitate nonvolatile quantized states that are ideal for Boolean logics. Here, the authors provide a proof-of-concept RAM memory based on a single Abrikosov vortex as bit.

Large Superconducting Spin Valve Effect and Ultrasmall Exchange Splitting in Epitaxial Rare-Earth-Niobium Trilayers

Epitaxial Ho/Nb/Ho and Dy/Nb/Dy superconducting spin valves show a reversible change in the zero-field critical temperature (ΔT(c0)) of ∼400  mK and an infinite magnetoresistance on changing the relative magnetization of the Ho or Dy layers. Unlike transition-metal superconducting spin valves, which show much smaller ΔT(c0) values, our results can be quantitatively modeled. However, the fits require an extraordinarily low induced exchange splitting which is dramatically lower than known values for rare-earth Fermi-level electrons, implying that new models for the magnetic proximity effect may be required.

The interface between superconductivity and magnetism: understanding and device prospects

Ferromagnetism and conventional singlet superconductivity can be regarded as competing ordering phenomena. A considerable body of theoretical work over the past twenty years has predicted that at interfaces between the two systems competition or coupling between superconducting and magnetic phenomena are possible. Despite the very short lengthscales over which some of the phenomena exist, many of these predictions have been experimentally realized. The aim of this topical review is to provide an overview of the experimental position and to discuss the potential developments and applications of existing results.

Superconducting spin switch with infinite magnetoresistance induced by an internal exchange field

A theoretical prediction by de Gennes suggests that the resistance in a FI/S/FI (where FI is a ferromagnetic insulator, and S is a superconductor) structure will depend on the magnetization direction of the two FI layers. We report a magnetotransport measurement in a EuS/Al/EuS structure, showing that an infinite magnetoresistance can be produced by tuning the internal exchange field at the FI/S interface. This proximity effect at the interface can be suppressed by an Al(2)O(3) barrier as thin as 0.3 nm, showing the extreme confinement of the interaction to the interface giving rise to the demonstrated phenomena.

Origin of the inverse spin switch effect in superconducting spin valves

Known as the spin switch effect (SSE), the resistance of a ferromagnet/superconductor/ferromagnet (F/S/F) spin valve near its superconducting transition temperature T c is different for parallel (R P) and antiparallel (R AP) configurations of the F layers. Here, we report the observation of the coexistence of the standard (RP>RAP) and inverse (RP<RAP) SSE in Ni81Fe19/Nb/Ni81Fe19/Ir25Mn75 spin valves. Our measurements reveal that the inverse SSE arises from a dissipative flow of vortices induced by stray magnetic fields from Néel domain-wall pairs in the F layers.