We investigate SFF structures (S-superconductor, F feromagnetic metal) with noncollinear magnetizations of F films with arbitrary transparency of FF interface. We show the existence of phase slips both at SF and FF interfaces which manifest themselves in the anomalous dependence of the spin-triplet correlations on misorientation angle between magnetization vectors in the F-layers. We discuss how these effects can be observed in experiments with Josephson $π$-junctions.
Using a self-consistent Bogoliubov-de Gennes approach, we theoretically study the proximity-induced density of states (DOS) in clean SFF spin-valves with noncollinear exchange fields. Our results clearly demonstrate a direct correlation between the presence of a zero energy peak (ZEP) in the DOS spectrum and the persistence of spin-1 triplet pair correlations. By systematically varying the geometrical and material parameters governing the spin-valve, we point out to experimentally optimal system configurations where the ZEPs are most pronounced, and which can be effectively probed via scanning…
The Andreev current and the subgap conductance in a superconductor/ insulator/ ferromagnet (SIF) structure in the presence of a small spin-splitting field show novel interesting features (A. Ozaeta et al., Phys. Rev. B 86, 060509(R), 2012). For example, the Andreev current at zero temperature can be enhanced by a spin-splitting field h, smaller than the superconducting gap, as has been recently reported by the authors. Also at finite temperatures the Andreev current has a peak for values of the spin-splitting field close to the superconducting gap. Finally, the differential subgap conductance …
Using a Keldysh formalism, we study supercurrent (SC) transport in several types of diffusive ferromagnet(F)/superconductor(S) configurations. We separate out the even- and odd-frequency components and identify the relative contributions from the singlet and triplet channels. We first consider 1D Josephson structures consisting of a uniform SFFS structure and a trilayer SFFFS configuration. Our results demonstrate that for SCs flowing perpendicular to the FF interfaces, incorporating a conical texture yields the most effective way to observe the signatures of the long-ranged SC. We also consid…
We have studied the proximity-induced superconducting triplet pairing in CoO$_x$/Py1/Cu/Py2/Cu/Pb spin-valve structure (where Py = Ni$_{0.81}$Fe$_{0.19}$). By optimizing the parameters of this structure we found a triplet channel assisted full switching between the normal and superconducting states. To observe an "isolated" triplet spin-valve effect we exploited the oscillatory feature of the magnitude of the ordinary spin-valve effect $ΔT_c$ in the dependence of the Py2-layer thickness $d_{Py2}$. We determined the value of $d_{Py2}$ at which $ΔT_c$ caused by the ordinary spin-valve effect (th…
We study the critical temperature T_c of SFF trilayers (S is a singlet superconductor, F is a ferromagnetic metal), where the long-range triplet superconducting component is generated at noncollinear magnetizations of the F layers. We demonstrate that T_c can be a nonmonotonic function of the angle αbetween the magnetizations of the two F layers. The minimum is achieved at an intermediate α, lying between the parallel (P, α=0) and antiparallel (AP, α=π) cases. This implies a possibility of a "triplet" spin-valve effect: at temperatures above the minimum T_c^{Tr} but below T_c^{P} and T_c^{AP},…
The difference in the density of states for up- and down-spin electrons in a ferromagnet (F) results in spin-dependent scattering of electrons at a ferromagnet / nonmagnetic (F/N) interface. In a F/N/F spin-valve, this causes a current-independent difference in resistance ($ΔR$) between antiparallel (AP) and parallel (P) magnetization states. Giant magnetoresistance (GMR), $ΔR = R(AP) - R(P)$, is positive due to increased scattering of majority and minority spin-electrons in the AP-state. If N is substituted for a superconductor (S), there exists a competition between GMR and the superconducti…
Superconducting spin valves based on the superconductor/ferromagnet (S/F) proximity effect are considered to be a key element in the emerging field of superconducting spintronics. Here, we demonstrate the crucial role of the morphology of the superconducting layer in the operation of a multilayer S/F1/F2 spin valve. We study two types of superconducting spin valve heterostructures, with rough and with smooth superconducting layers, using transmission electron microscopy in combination with transport and magnetic characterization. We find that the quality of the S/F interface is not critical fo…
The spin-triplet correlations in superconducting spin valve structures arising in the presence of noncollinear textures of magnetic moment are shown to enhance strongly the electromagnetic proximity effect, i. e. the long-range leakage of the magnetic field from the ferromagnet (F) to the superconducting (S) layer. Both the dirty and clean limits are studied on the basis of the Usadel and Eilenberger theory, correspondingly. Our results suggest a natural explanation for the puzzling enhancement of the spontaneous magnetic fields induced by the noncollinear magnetic structures observed by the m…
Heterostructures consisting of a cuprate superconductor YBa2Cu3O7x and a ruthenate/manganite (SrRuO3/La0.7Sr0.3MnO3) spin valve have been studied by SQUID magnetometry, ferromagnetic resonances and neutron reflectometry. It was shown that due to the influence of magnetic proximity effect a magnetic moment is induced in the superconducting part of heterostructure and at the same time the magnetic moment is suppressed in the ferromagnetic spin valve. The experimental value of magnetization induced in the superconductor has the same order of magnitude with the calculations based on the induced ma…
We theoretically study the Josephson effect in a superconductor/normal metal/superconductor ({\it S}/{\it N}/{\it S}) Josephson junction composed of $s$-wave {\it S}s with {\it N} which is sandwiched by two ferromagnetic insulators ({\it F}s), forming a spin valve, in the vertical direction of the junction. We show that the 0-$π$ transition of the Josephson critical current occurs with increasing the thickness of {\it N} along the junction. This transition is due to the magnetic proximity effect (MPE) which induces ferromagnetic magnetization in the {\it N}. Moreover, we find that, even for fi…
A hybrid ferromagnet-superconductor spin valve is proposed. Its operation relies on the interplay between nonequilibrium transport and proximity-induced exchange coupling in superconductors. Huge tunnel magnetoresistance values as large as some 10^6% can be achieved in suitable ferromagnet-superconductor combinations under proper voltage biasing. The controllable spin-filter nature of the structure combined with its intrinsic simplicity make this setup attractive for low-temperature spintronic applications where reduced power dissipation is an additional requirement.
We present the Usadel theory describing the superconducting proximity effect in heterostructures with a half-metallic layer. It is shown that the full spin polarization inside the half-metals gives rise to the giant triplet spin-valve effect in superconductor (S) - ferromagnet (F) - half-metal (HM) trilayers as well as to the $φ_0$-junction formation in the S/F/HM/F/S systems. In addition, we consider the exactly solvable model of the S/F/HM trilayers of atomic thickness and demonstrate that it reproduces the main features of the spin-valve effect found within the Usadel approach. Our results …
Spin-singlet Cooper pairs consisting of two electrons with opposite spins cannot directly penetrate from a superconductor to a half-metal (fully spin polarized ferromagnets) which blocks the superconducting proximity effect between these materials. In this paper we demonstrate that, nevertheless, two half-metallic layers electrically coupled to the superconducting film substantially affect its critical temperature and produce the spin valve effect. Within the tight-binding model for the atomically thin multilayered spin valves we show that depending on the details of the electron energy spectr…
Coupling a conventional s-wave superconductor to a ferromagnet allows, via the proximity effect, to generate superconducting triplet correlations. This feature can be employed to achieve a superconducting triplet spin-valve effect in superconductor-ferromagnet (SF) hybrid structures, for example by switching the magnetizations of the ferromagnets between parallel and antiparallel configurations in F1SF2 and SF1F2 trilayers, or in SF bilayers with both Rashba and Dresselhaus SOC. It was recently reported that geometric curvature can control the generation of long ranged triplets. We use this pr…
We have measured the transport properties of Ferromagnet - Superconductor nanostructures, where two superconducting aluminum (Al) electrodes are connected through two ferromagnetic iron (Fe) ellipsoids in parallel. We find that, below the superconducting critical temperature of Al, the resistance depends on the relative alignment of the ferromagnets' magnetization. This spin-valve effect is analyzed in terms of spin accumulation in the superconducting electrode submitted to inverse proximity effect.
Ferromagnets can sustain supercurrents through the formation of equal spin triplet Cooper pairs and the mechanism of odd-frequency pairing. Since such pairs are not broken by the exchange energy of the ferromagnet, superconducting triplet correlations are long-ranged and spin-polarized, with promises for superconducting spintronics devices. The main challenge is to understand how triplets are generated at the superconductor (S)/ ferromagnet (F) interface. Here we use the concept of a so-called triplet spin valve (TSV) to investigate the conversion of singlets in a conventional superconductor t…
The spin valve effect for the superconducting current based on the superconductor/ferromagnet proximity effect has been studied for a CoO_x/Fe1/Cu/Fe2/Cu/Pb multilayer. The magnitude of the effect $ΔT_c$ = T_c^{AP} - T_c^{P}, where T_c^{P} and T_c^{AP} are the superconducting transition temperatures for the parallel (P) and antiparallel (AP) orientation of magnetizations, respectively, has been measured for different thicknesses of the Fe1 layer d_{Fe1}. The obtained dependence of the effect on d_{Fe1} reveals that $ΔT_c$ can be increased in comparison with the case of a half-infinite Fe1 laye…
Superconductor/ferromagnet (S/F) spin valve effect theories based on the S/F proximity phenomenon assume that the superconducting transition temperature Tc of F1/F2/S or F1/S/F2 trilayers for parallel magnetizations of the F1- and F2-layers (TcP) are smaller than for the antiparallel orientations (TcAP). Here, we report for CoOx/Fe1/Cu/Fe2/In multilayered systems with varying Fe2-layer thickness the sign-changing oscillating behavior of the spin valve effect ΔTc=TcAP-TcP. Our measurements revealed the full direct spin valve effect with TcAP>TcP for Fe2-layer thickness dFe2<1 nm and the full in…
We study the proximity effect in hybrid structures consisting of superconductor and ferromagnetic insulator separated by a normal diffusive metal (S/N/FI structures). These stuctures were proposed to realize the absolute spin-valve effect. We pay special attention to the gaps in the density of states of the normal part. We show that the effect of the ferromagnet is twofold: It not only shifts the density of states but also provides suppression of the gap. The mechanism of this suppression is remarkably similar to that due to magnetic impurities. Our results are obtained from the solution of on…
Competition between superconducting and ferromagnetic ordering at interfaces between ferromagnets (F) and superconductors (S) gives rise to several proximity effects such as odd-triplet superconductivity and spin-polarized supercurrents. A prominent example of an S/F proximity effect is the spin switch effect (SSE) observed in S/F/N/F superconducting spin-valve multilayers, in which the superconducting transition temperature T$_c$ is controlled by the angle $ϕ$ between the magnetic moments of the F layers separated by a nonmagnetic metallic spacer N. Here we present an experimental study of SS…
Spin-valve structures in which a change of magnetic configuration is responsible for magnetoresistance have enabled impressive advances in spintronics, focusing on magnetically storing and sensing information. However, this mature technology also offers versatile control of the underlying fringing fields and entirely different applications by realizing topologically-nontrivial states. Together with proximity-induced superconductivity in a two-dimensional electron gas, these fringing fields realized in commercially-available spin valves could control Majorana bound states (MBS). Detailed suppor…
The trilayer and pentalayer spin valve structures are revisited to determine the behavior of pair correlations and Josephson current when the magnetic layers are canted at arbitrary angle. The two systems display markedly different behaviors in the center magnetic layer. While the trilayer generates a triplet component that is weakly affected by canting, the pentalayer tunes in singlet pair correlations depending heavily on canting. We also show that a minimum with depleted $m=\pm1$ triplet components, rather than a $0-π$ transition, may be observed in the current profile $I_c(d_F)$ of a trila…
We investigate heat and charge transport through a diffusive SIF1F2N tunnel junction, where N (S) is a normal (superconducting) electrode, I is an insulator layer and F1,2 are two ferromagnets with arbitrary direction of magnetization. The flow of an electric current in such structures at subgap bias is accompanied by a heat transfer from the normal metal into the superconductor, which enables refrigeration of electrons in the normal metal. We demonstrate that the refrigeration efficiency depends on the strength of the ferromagnetic exchange field h and the angle α between the magnetizations o…
A detailed investigation of a superconducting spin-triplet valve is presented. This spin-valve consists of a superconducting film covering a metal with an intrinsic spiral magnetic order, which could result from competing isotropic exchanges or, if the crystal lattice breaks central symmetry, from asymmetric Dzyaloshinskii-Moriya exchange. Depending on the anisotropy, such a metal may change its magnetization either from a spiral to uniform order, as seen in Ho and Er, or in the direction of the spiral itself, as in crystals of the B20-type structure (such as MnSi, (Fe,Co)Si, FeGe, etc.). The …
Josephson diode and spin-valve effects on the surface of altermagnet CrSb
Institute of Solid State Physics of the Russian Academy of Sciences（俄罗斯科学院固体物理研究所）；V.L. Ginzburg Research Centre for High-Temperature Superconductivity and Quantum Materials, P.N. Lebedev Physical Institute of RAS（俄罗斯科学院列别杰夫物理研究所V.L.金兹堡高温超导与量子材料研究中心）
The physics of superconductor-ferromagnet hybrid structures
Moscow Institute of Physics and Technology（莫斯科物理技术学院）；HSE University（高等经济学院）；Skobeltsyn Institute of Nuclear Physics, Lomonosov Moscow State University（莫斯科国立大学索别利钦核物理研究所）；Technical University of Moldova（摩尔多瓦技术大学）
The theory of superconductor-ferromagnet (S-F) heterostructures with two ferromagnetic layers predicts the generation of a long-range, odd-in-frequency triplet pairing at non-collinear alignment (NCA) of the magnetizations of the F-layers. This triplet pairing has been detected in a Nb/Cu41Ni59/nc-Nb/Co/CoOx spin-valve type proximity effect heterostructure, in which a very thin Nb film between the F-layers serves as a normal conducting (nc) spacer. The resistance of the sample as a function of an external magnetic field shows that for not too high fields the system is superconducting at a coll…
Superconducting spintronics has emerged in the last decade as a promising new field that seeks to open a new dimension for nanoelectronics by utilizing the internal spin structure of the superconducting Cooper pair as a new degree of freedom. Its basic building blocks are spin-triplet Cooper pairs with equally aligned spins, which are promoted by proximity of a conventional superconductor to a ferromagnetic material with inhomogeneous macroscopic magnetization. Using low-energy muon spin rotation experiments, we find an entirely unexpected novel effect: the appearance of a magnetization in a t…
We present theoretical model for the proximity effect in F-SFF-F structures (F is ferromagnet, S is superconductor) with non-collinear magnetizations vectors in the F-layers and with arbitrary magnitudes of exchange fields. Electrical conductance of these structures is analyzed within the Keldysh-Usadel formalism in the diffusive regime as a function of a misorientation angle between magnetizations of the F-layers and transparencies of SF and FF interfaces. We show that long-range triplet superconducting correlations manifest themselves either as a zero-bias peak in the case of perfect transpa…
We theoretically study the proximity effect in the thin-film layered ferromagnet (F) - superconductor (S) heterostructures in F$_1$F$_2$S design. We consider the boundary value problem for the Usadel-like equations in the case of so-called "dirty" limit. The "latent" superconducting pairing interaction in F layers taken into account. The focus is on the recipe of experimental preparation the state with so-called solitary superconductivity. We also propose and discuss the model of the superconducting spin valve based on F$_1$F$_2$S trilayers in solitary superconductivity regime.
We study graphene ferromagnet/superconductor/ferromagnet (F/S/F) nanostructures via a microscopic self-consistent Dirac Bogoliubov-de Gennes formalism. We show that as a result of proximity effects, experimentally accessible spin switching phenomena can occur as one tunes the Fermi level $μ_F$ of the F regions or varies the angle $θ$ between exchange field orientations. Superconductivity can then be switched on and off by varying either $θ$ or $μ_F$ (a spin-controlled superconducting graphene switch). The induced equal-spin triplet correlations in S can be controlled by tuning $μ_F$, effective…
Ferromagnet/Superconductor/Ferromagnet (F/S/F) trilayers constitute the core of a superconducting spin valve. The switching effect of the spin valve is based on interference phenomena occurring due to the proximity effect at the S/F interfaces. A remarkable effect is only expected if the core structure exhibits strong critical temperature oscillations, or most favorable, reentrant superconductivity, when the thickness of the ferromagnetic layer is increased. The core structure has to be grown on an antiferromagnetic oxide layer (or such layer to be placed on top) to pin by exchange bias the ma…
Spin-polarized transport is investigated in normal metal-superconductor (NS) junctions as a function of interface transmissivity as well as temperature when the density of states of a superconductor is Zeeman-split in response to an exchange field (h_exc). Similarly to the "absolute spin-valve effect" predicted by D. Huertas-Hernando et al. [Phys. Rev. Lett. 88, 047003 (2002)] in superconducting proximity structures, we show that NS junctions can be used to generate highly spin-polarized currents, in alternative to half-metallic ferromagnets. In particular, the spin-polarized current obtained …
Interfacing superconductors with strongly spin-polarized magnetic materials opens the possibility to discover new spintronic devices in which spin-triplet Cooper pairs play a key role. Motivated by the recent derivation of spin-polarized quasiclassical boundary conditions capable of describing such a scenario in the diffusive limit, we consider the emergent physics in hybrid structures comprised of a conventional s-wave superconductor (e.g. Nb, Al) and either strongly spin-polarized ferromagnetic insulators (e.g. EuO, GdN) or halfmetallic ferromagnets (e.g. CrO2, LCMO). In contrast to most pre…
We investigate theoretically how the proximity effect in superconductor/ferromagnet hybrid structures with intrinsic spin-orbit coupling manifests in the density of states and critical temperature. To describe a general scenario, we allow for both Rashba and Dresselhaus type spin-orbit coupling. Our results are obtained via the quasiclassical theory of superconductivity, extended to include spin-orbit coupling in the Usadel equation and Kupriyanov--Lukichev boundary conditions. Unlike previous works, we have derived a Riccati parametrization of the Usadel equation with spin-orbit coupling whic…
