Superconducting probe of electronic correlations and exchange field based on the proximity effect in F/S nanostructures

The proximity effect and competition between the BCS and LOFF states are studied in the Cooper limit for thin F/S and F/S/F nanostructures, where F is a ferromagnet and S is a superconductor. The dependences of the critical temperature Tc on the exchange field I, electron correlations λf, and the thickness df of the F layer are derived for F/S bilayers and F/S/F trilayers. Two new π-phase superconducting states with electron-electron repulsion in the F layers of F/S/F trilayers are predicted. A 2D LOFF state in F/S/F trilayers is possible only in the presence of a weak magnetic field and the appropriate parameters of the F and S layers. The absence of the suppression of 3D superconductivity in short-period Gd/La superlattices is explained and the electron-electron coupling constant in gadolinium is predicted. A method of superconducting probe spectroscopy based on the proximity effect is proposed for determining the symmetry of the order parameter, the magnitude and sign of electron correlations, and the exchange field in various nanomagnets F. © (2012) Trans Tech Publications

Nonmonotonic behavior of the superconducting transition temperature in bimetallic ferromagnet-superconductor structures

For layered ferromagnet/superconductor (F/S) structures we develop a theory of the proximity effect. In contrast to previous approaches, this theory allows for a finite transmission coefficient of the interface between the two metals and competition between the diffusion and spin-wave types of quasiparticle motion in the ferromagnet's strong exchange field. The superconductivity in F/S systems proves to be a superposition of BCS pairing with a constant-sign pair amplitude in the S-layers and Larkin-Ovchinnikov-Fulde-Ferrell (LOFF) pairing with an oscillating wave function in the F-layers. We show that the oscillatory behavior of the superconducting transition temperature Tc is due to oscillations of the Cooper pair flux from the S-layer to the F-layer, which are the result of oscillations of the discontinuity (jump) of the pair amplitude at the F/S boundary as the thickness df of the F-layer increases. The presence of nonmagnetic impurities leads to heavy damping of the oscillations of the LOFF pair amplitude and rapid deterioration of the coherent coupling of the boundaries of the F-layer in which the Ts vs. df dependence reaches a plateau as df increases. In F/S superlattices, in contrast to F/S double-layer junctions, there are two forms of the LOFF state, the 0-phase and the π-phase, which differ in their symmetry with respect to the center of the F layer. This gives rise to additional oscillations in the Tc (df) dependence due to the 0-π transitions. As the most vivid manifestation of LOFF states in F/S-systems, we predict the existence of recurrent and periodically recurrent superconductivities. We give a qualitative explanation of the different behavior of the superconducting transition temperature observed by different groups of experimenters dealing with the same ferromagnet-superconductor structures. © 1998 American Institute of Physics

The superconducting phase transitions for the asymmetrical FS superlattices with interelectronic interaction in ferromagnet layers

The superconducting and magnetic states coexistence in the FS superlattice, where F is ferromagnetic metal and S is superconductor, is investigated on base of microscopically derived boundary value problem for the Eilenberger function. The asymmetrical four-layered structure FSF' S' is considered as elementary cell. The second order phase transitions are explored for case of ideal boundary transparencies and clean Cooper limit. Each layer is characterized by its own thickness and electronic structure. Materials are also differed by electron-electron interaction constants (note these constants for ferromagnets are nonzero!). It is shown, that 0- and π-phase superconducting states of clean thin superlattices FS are defined by value and sign of electronic correlations in all four layers of elementary cell. The competition between uniform BCS pairing and non-uniform Fulde-Ferrell-Larkin-Ovchinnikov pairing are also taken into account. We predict that the complex system under consideration may have up to 8 different states which are characterized by phase shifts between superconducting order parameters in S(F) and S'(F') and mutual orientation of magnetizations in the F and F' layers. The states with π-phase magnetism can fully explain surprising experimental behavior of short-range Gd/La superlattice, i.e the coincidence of superlattice critical temperature Tc with Tc(La) = 5 K for different thickness of the Gd layers exceeding the La layer thickness

Manifestations of the Larkin-Ovchinnikov-Fulde-Ferrell state in bimetal ferromagnet-superconductor structures

Reentrant and periodically reentrant superconductivity in contacts and ferromagnet/superconductor (F/S) superlattices are predicted on the basis of the theory developed in this letter. These effects are consequences of the realization of the Larkin-Ovchinnikov-Fulde-Ferrell state in F layers. An explanation is given for the qualitatively different behavior of the critical temperature observed by different experimental groups on identical F/S structures. © 1997 American Institute of Physics

Competition between superconductivity and magnetism in ferromagnet/superconductor heterostructures

The mutual influence of superconductivity and magnetism in FS systems, i.e. systems of alternating ferromagnetic (F) and super-conducting (S) layers, is comprehensively reviewed. For systems with ferromagnetic metal (FM) layers, a theory of the proximity effect in the dirty limit is constructed based on the Usadel equations. For a FM/S bilayer and a FM/S superlattice, a boundary-value problem involving finite FM/S boundary transparency and the diffusion and wave modes of quasi-particle motion is formulated; and the critical temperature Tc is calculated as a function of FM- and S-layer thicknesses. A detailed analysis of a large amount of experimental data amply confirms the proposed theory. It is shown that the superconducting state of an FM/S system is a superposition of two pairing mechanisms, Bardin - Cooper - Schrieffer's in S layers and Larkin - Ovchinnikov - Fulde - Ferrell's in FM ones. The competition between ferromagnetic and antiferromagnetic spontaneous moment orientations in FM layers is explored for the 0- and π-phase superconductivity in FM/S systems. For FI/S structures, where FI is a ferromagnetic insulator, a model for exchange interactions is proposed, which, along with direct exchange inside FI layers, includes indirect Ruderman - Kittel - Kasuya - Yosida exchange between localized spins via S-layer conduction electrons. Within this framework, possible mutual accommodation scenarios for superconducting and magnetic order parameters are found, the corresponding phase diagrams are plotted, and experimental results explained. The results of the theory of the Josephson effect for S/F/S-contacts are presented and the application of the theory of spin-depending transport to F/S/F contacts is discussed. Application aspects of the subject are examined

πPhase superconductivity and magnetism in ferromagnet/superconductor/ ferromagnet trilayers

On the base of new boundary-value problem for the Eilenberger function we investigate the superconducting and magnetic states in ferromagnet/ superconductor (FM/S) nanostructures, where superconductivity is a superposition of the BCS pairing with zero total momentum in the S layers and the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) pairing with nonzero 3D coherent momentum k in the FM layers. We originally study the interplay between the BCS and FFLO states in the pure thin FM/S/FM trilayers and two novel π-phase superconducting states with electronelectron repulsion in the FM layers are predicted. The modulated FFLO states are possible in such trilayers only in presence of external magnetic field at suitable parameters of the FM and S layers. In the FM/S superlattices there are also two π-phase magnetic states (0π and ππ) with compensation of the exchange field paramagnetic effect. This fact allows us to explain a surprisingly high Tc ∼ 5K in the short period Gd/La superlattice and to predict the sign and value of the electron-electron interaction in the ferromagnetic Gd metal

Decoupled superconductivity and hierarchy of transition temperatures in the tetralayer ferromagnet/superconductor nanostructure and control devices

We predict the decoupled superconductivity for the four-layered F′/S′/F″/S″ nanostructure consisting of dirty superconducting (S) and ferromagnetic (F) metals. The predicted hierarchy of critical temperatures is found to manifest itself in its most striking way through arising of different critical temperatures in different superconducting layers S′ and S″. In common case the phase diagram including four different regions is found. Conceptual sketch of the new control nanodevice based on this tetralayer system are proposed. It is shown that they can have up to seven various states. © 2006 American Institute of Physics

Ferromagnet/superconductor layered superlattices

New 0π and ππ states with an antiparallel orientation of magnetization in the adjacent FM layers are predicted for ferromagnetic metal/superconductor (FM/S) superlattices. If the thickness ds.of the S layers is less than the threshold value dπs, these states have a higher critical temperature Tc than the earlier known 00 and π0 LOFF states. A new type of logic device combining the advantages of superconducting and magnetic recording channels in one sample is proposed on the basis of FM/S superlattices. Good quantitative agreement with known experimental phase diagrams is obtained

Adjustment of superconductivity and ferromagnetism in few-layered ferromagnet-superconductor nanostructures

The phase diagrams of few-layered nanosystems consisting of dirty superconducting (S) and ferromagnetic (F) metals are investigated within the framework of the modern theory of the proximity effect taking into account the boundary conditions. The F/S tetralayer and pentalayer are shown to have considerably richer physics than the F/S bi- and trilayer (due to the interplay between the 0 and π phase superconductivity and the 0 and π phase magnetism and nonequivalence of layers) and even the F/S superlattices. It is proven that these systems can have different critical temperatures and fields for different S layers. This predicted decoupled superconductivity is found to manifest itself in its most striking way for F/S tetralayer. It is shown that F/S/F′/S′ tetralayer is the most promising candidate for use in superconducting spin nanoelectronics. © 2006 American Institute of Physics

π magnetic states of ferromagnet/superconductor superlattices

For the ferromagnetic metal/superconductor (FM/S) superlattices, 0 π and ππ states with antiferromagnetic ordering of the FM layers magnetizations are predicted. If the S layer thickness ds is less than the threshold value ds π, these states have a higher critical temperature Tc than the earlier known ferromagnetic states 00 and π0. Therefore, the Tc oscillation origin at dsds π it is related to the sequence of transitions 00-π0-00. A type of logical device combining the advantages of the superconducting and magnetic recording channels in one sample is offered on the FM/S superlattice base