Theory of proximity effect in superconductor/ferromagnet heterostructures

We present a microscopic theory of proximity effect in the ferromagnet/superconductor/ferromagnet (F/S/F) nanostructures where S is s-wave low-T_c superconductor and F's are layers of 3d transition ferromagnetic metal. Our approach is based on the solution of Gor'kov equations for the normal and anomalous Green's functions together with a self-consistent evaluation of the superconducting order parameter. We take into account the elastic spin-conserving scattering of the electrons assuming s-wave scattering in the S layer and s-d scattering in the F layers. In accordance with the previous quasiclassical theories, we found that due to exchange field in the ferromagnet the anomalous Green's function F(z) exhibits the damping oscillations in the F-layer as a function of distance z from the S/F interface. In the given model a half of period of oscillations is determined by the length \xi_m^0 = \pi v_F/E_ex, where v_F is the Fermi velocity and E_ex is the exchange field, while damping is governed by the length l_0 = (1/l_{\uparrow} + 1/l_{\downarrow})^{-1} with l_{\uparrow} and l_{\downarrow} being spin-dependent mean free paths in the ferromagnet. The superconducting transition temperature T_c(d_F) of the F/S/F trilayer shows the damping oscillations as a function of the F-layer thickness d_F with period \xi_F = \pi/\sqrt{m E_ex}, where m is the effective electron mass. We show that strong spin-conserving scattering either in the superconductor or in the ferromagnet significantly suppresses these oscillations. The calculated T_c(d_F) dependences are compared with existing experimental data for Fe/Nb/Fe trilayers and Nb/Co multilayers.Comment: 13 pages, REVTeX4, 8 PS-figures; improved version, submitted to PR

Unternehmensinterne Arbeitsvermittlung als Alternative zur Entlassung: erste Fallstudien zur Wirksamkeit interner Arbeitsmärkte

"Bei den Stichworten Restrukturierung und Personalabbau denken viele spontan an Fälle wie die Schließung des Bochumer Nokia-Werks im Jahr 2008 oder die Insolvenz von BenQ Mobile 2006/2007. Neben den spektakulären Fällen gibt es aber auch zahlreiche Unternehmen und öffentliche Verwaltungen, die – von der Öffentlichkeit vergleichsweise unbemerkt – Restrukturierungen ohne Entlassungen bewältigen, indem sie das frei gewordenes Personal mittels eigener organisatorischer Einrichtungen auf frei werdende oder neue Stellen des internen Arbeitsmarktes vermitteln. Einzelne Unternehmen und Verwaltungen zeigen, dass dies auch bei tief greifenden Umstrukturierungsmaßnahmen mit großen Personalüberhängen gelingen kann. Diese Einrichtungen zur Gestaltung interner Arbeitsmärkte sind Gegenstand des von der Hans-Böckler-Stiftung geförderten Forschungsprojekts WEGA, das am Institut Arbeit und Qualifikation durchgeführt wird. Das Projekt hat seit seinem Start im Sommer 2006 bundesweit mehr als fünfzig Interne Arbeitsmärkte in Unternehmen und Verwaltungen identifiziert, acht dieser Internen Arbeitsmärkte sowie ein Interner Arbeitsmarkt in Schweden wurden in Form von Fallstudien intensiv untersucht. Das vorliegende Papier beinhaltet die drei ersten Fallstudien des WEGA-Projekts. Im Mittelpunkt der Fälle stehen unternehmens- oder verwaltungsinterne Mobilitätsbüros, oder, mit anderen Worten, Einrichtungen zur Gestaltung interner Arbeitsmärkte. Die Fälle im Einzelnen sind: IAM, der Interne Arbeitsmarkt der Medizinischen Hochschule Hannover; PIA, das Projekt Interner Arbeitsmarkt der Hamburgischen Verwaltung; TSO, die Abteilung Omställning von TeliaSonera, Stockholm, Schweden. Die Fallstudien dieses Berichts zielen auf eine erste und explorative Darstellung der Organisation interner Arbeitsmärkte. Ziel des Papiers ist es, verschiedenen Varianten von internen Arbeitsmärkten in Bezug auf Vermittlungskonzepte, Regelungen, Rahmenbedingungen und Effektivität der Vermittlung abzubilden. Durch die Vielfalt der gewählten Perspektiven gibt das Papier erste Hinweise auf den Zusammenhang von Organisation und Effektivität der internen Vermittlung; die Fälle legen zudem typische Interessenkonflikte und konzeptionelle Widersprüche der internen Mobilitätsförderung offen. Sie zeigen sowohl Möglichkeiten als auch Grenzen, die Interessen der beteiligten betrieblichen Akteure auf internen Arbeitsmärkten auszubalancieren." (Autorenreferat

Cryptoferromagnetic state in superconductor-ferromagnet multilayers

We study a possibility of a non-homogeneous magnetic order (cryptoferromagnetic state) in heterostructures consisting of a bulk superconductor and a ferromagnetic thin layer that can be due to the influence of the superconductor. The exchange field in the ferromagnet may be strong and exceed the inverse mean free time. A new approach based on solving the Eilenberger equations in the ferromagnet and the Usadel equations in the superconductor is developed. We derive a phase diagram between the cryptoferromagnetic and ferromagnetic states and discuss the possibility of an experimental observation of the CF state in different materials.Comment: 4 pages, 1 figur

Inhomogeneous magnetism induced in a superconductor at superconductor-ferromagnet interface

We study a magnetic proximity effect at superconductor (S) - ferromagnet (F) interface. It is shown that due to an exchange of electrons between the F and S metals ferromagnetic correlations extend into the superconductor, being dependent on interface parameters. We show that ferromagnetic exchange field pair breaking effect leads to a formation of subgap bands in the S layer local density of states, that accommodate only one spin-polarized quasiparticles. Equilibrium magnetization leakage into the S layer as function of SF interface quality and a value of ferromagnetic interaction have also been calculated. We show that a damped-oscillatory behavior versus distance from SF interface is a distinguished feature of the exchange-induced magnetization of the S layer.Comment: 10 pages, 7 Postscript figure

Critical temperature of superconductor/ferromagnet bilayers

Superconductor/ferromagnet bilayers are known to exhibit nontrivial dependence of the critical temperature T_c on the thickness d_f of the ferromagnetic layer. We develop a general method for investigation of T_c as a function of the bilayer's parameters. It is shown that interference of quasiparticles makes T_c(d_f) a nonmonotonic function. The results are in good agreement with experiment. Our method also applies to multilayered structures.Comment: 4 pages, 2 EPS figures; the style file jetpl.cls is included. Version 2: typos correcte

Andreev bound states in ferromagnet-superconductor nanostructures

We discuss the properties of a ferromagnet - superconductor heterostructure on the basis of a Hubbard model featuring exchange splitting in the ferromagnet and electron - electron attraction in the superconductor. We have solved the spin - polarized Hartree - Fock - Gorkov equations together with the Maxwell's equation (Ampere's law) fully self-consistently. We have found that a Proximity Effect - Fulde - Ferrell - Larkin - Ovchinnikov state is realized in such a heterostructure. It manifests itself in an oscillatory behavior of the pairing amplitude in the ferromagnet and spontaneously generated spin polarized current in the ground state. We argue that it is built up from the Andreev bound states, whose energy can be tuned by the exchange splitting and hence can coincide with the Fermi energy giving rise to a current carrying $\pi$-state. We also suggest experiments to verify these predictions.Comment: 12 pages, 5 figures included; to be published in Physica

Thermodynamic properties of ferromagnetic/superconductor/ferromagnetic nanostructures

The theoretical description of the thermodynamic properties of ferromagnetic/superconductor/ferromagnetic (F/S/F) systems of nanoscopic scale is proposed. Their superconducting characteristics strongly depend on the mutual orientation of the ferromagnetic layers. In addition, depending on the transparency of S/F interfaces, the superconducting critical temperature can exhibit four different types of dependences on the thickness of the F-layer. The obtained results permit to give some practical recommendations for the spin-valve effect experimental observation. In this spin-valve sandwich, we also expect a spontaneous transition from parallel to anti-parallel ferromagnetic moment orientation, due to the gain in the superconducting condensation energy.Comment: 20 pages, 5 figures, submitted to PR

Ferromagnetic/superconducting proximity effect in La0.7Ca0.3MnO3 / YBa2Cu3O7 superlattices

We study the interplay between magnetism and superconductivity in high quality YBa2Cu3O7 (YBCO) / La0.7Ca0.3MnO3(LCMO)superlattices. We find evidence for the YBCO superconductivity depression in presence of the LCMO layers. We show that due to its short coherence length superconductivity survives in the YBCO down to much smaller thickness in presence of the magnetic layer than in low Tc superconductors. We also find that for a fixed thickness of the superconducting layer, superconductivity is depressed over a thickness interval of the magnetic layer in the 100 nm range. This is a much longer length scale than that predicted by the theory of ferromagnetic/superconducting proximity effect.Comment: 10 pages + 5 figures, submitted to Phys. Rev.

Feasibility of study magnetic proximity effects in bilayer "superconductor/ferromagnet" using waveguide-enhanced Polarized Neutron Reflectometry

A resonant enhancement of the neutron standing waves is proposed to use in order to increase the magnetic neutron scattering from a "superconductor/ferromagnet"(S/F) bilayer. The model calculations show that usage of this effect allows to increase the magnetic scattering intensity by factor of hundreds. Aspects related to the growth procedure (order of deposition, roughness of the layers etc) as well as experimental conditions (resolution, polarization of the neutron beam, background etc) are also discussed. Collected experimental data for the S/F heterostructure Cu(32nm)/V(40nm)/Fe(1nm)/MgO confirmed the presence of a resonant 60-fold amplification of the magnetic scattering.Comment: The manuscript of the article submitted to Crysstalography Reports. 23 pages, 5 figure

Magnetic anisotropies of sputtered Fe films on MgO substrates

Ferromagnetic resonance (FMR) and superconducting quantum interference device (SQUID) measurements have been used to study the magnetic properties of rf sputtered Fe films on MgO(001) substrates. The dependences of the FMR spectra parameters on the direction of the dc magnetic field turning in the plane of the films were measured in a wide temperature range (20400 K) for films with thickness L in the range 25500. The analysis of the angular dependence of the resonance field H0 allowed us to determine the fourfold cubic anisotropy constant K1 and the effective magnetization value Meff. It was found that both values decrease with decreasing L and approach a constant value below a certain thickness. A theory of FMR is outlined demonstrating that for the case of the dc magnetic field lying in a film plane, the anisotropy constant can be interpreted as a combination of a volume anisotropy contribution and a 1/L-dependent contribution from the surface anisotropy up to the thickness L103. This means that for the experimentally studied thickness range the films may be considered as dynamically thin films with respect to surface perturbations. Then the peculiar thickness dependence of the K1 value can be explained assuming that the relaxation of the strain due to the mismatch between film and substrate extends to distances as far as 45 from the film-substrate interface. Since our SQUID measurements show that the saturation moment does not depend on the thickness, it is concluded that the thickness dependence of the effective magnetization Meff is caused by a second-order uniaxial anisotropy arising mainly from the broken symmetry of the crystal field at surfaces and near the edges of interfacial dislocations. © 1995 The American Physical Society