Institute of Electronic Engineering and Nanotechnologies ASM

Moldova

Institute of Electronic Engineering and Nanotechnologies ASM

Moldova

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Pugach N.G.,Moscow State University | Kupriyanov M.Y.,Moscow State University | Goldobin E.,University of Tübingen | Koelle D.,University of Tübingen | And 4 more authors.
NanoScience and Technology | Year: 2011

We study the Josephson effect in junctions with a ferromagnetic (F) barrier having its properties (interface transparency or the F-layer thickness) artificially modulated on a scale less than the Josephson penetration length. Within the framework of the quasiclassical Usadel equations, we describe SIFS and SIFNS (S is a superconductor, I is an insulator, N is a normal metal) structures with a steplike transparency of the FS or NS interface. The step-like change in parameters may lead to oscillations (including sign change) of the critical current density J C (y) along the junction in the vicinity of the step, resulting in the formation of a 0-π nano-junction near the step. Such structures exhibit an unusual behaviour in an external magnetic field H. The properties of arrays of nano-junctions with several transparency steps are also investigated. We propose a method to realize a φ Josephson junction by combining alternating 0 and π parts made of "clean" SFS sub junctions with different F-layer thickness and an intrinsically non-sinusoidal current-phase relation (CPR). The latter can significantly enlarge the parameter range of the φ ground state and make the practical realization of φ Josephson junctions feasible. Such junctions may also have two different stable solutions, such as 0 and π, 0 and φ, or φ and π. © Springer-Verlag Berlin Heidelberg 2011.


Zdravkov V.I.,University of Augsburg | Zdravkov V.I.,Institute of Electronic Engineering and Nanotechnologies ASM | Kehrle J.,University of Augsburg | Obermeier G.,University of Augsburg | And 11 more authors.
Superconductor Science and Technology | Year: 2011

The theoretical description of the Fulde-Ferrell-Larkin-Ovchinnikov like state establishing in nanostructured bilayers of ferromagnetic (F) and superconducting (S) material leads to critical temperature oscillations and reentrant superconductivity as the F layer thickness gradually increases. The experimental realization of these phenomena is an important prerequisite for the fabrication of the ferromagnet/superconductor/ferromagnet core structure of the superconducting spin-valve. A switching of the spin-valve is only expected if such non-monotonic critical temperature behavior is observed in F/S bilayers as well as in the S/F bilayers, a combination of which the spin-valve core structure can be regarded to consist of. In our former investigations we could demonstrate the required non-monotonic behavior of the critical temperature in S/F bilayers. In this study we succeeded in the preparation of F/S bilayers, where the superconducting material is now grown on top of the ferromagnetic metal, which shows deep critical temperature oscillations as a function of the ferromagnetic layer thickness as well as an extinction and recovery, i.e.a reentrant behavior, of superconductivity. In particular, the latter is necessary to obtain a spin-valve with a large critical temperature shift between the parallel and antiparallel configurations of magnetizations in the F layers. © 2011 IOP Publishing Ltd.


Zdravkov V.I.,University of Augsburg | Zdravkov V.I.,Institute of Electronic Engineering and Nanotechnologies ASM | Kehrle J.,University of Augsburg | Obermeier G.,University of Augsburg | And 9 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2013

The theory of superconductor-ferromagnet heterostructures with two ferromagnetic layers predicts the generation of a long-range, odd-in-frequency triplet pairing at noncollinear alignment (NCA) of the magnetizations of the F layers. This triplet pairing has been detected in a Nb/Cu41Ni 59/normal conducting- (nc-) Nb/Co/CoOx spin-valve-type proximity effect heterostructure, in which a very thin Nb film between the F layers serves as a spacer of nc metal. 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 collinear alignment of the Cu41Ni 59 and Co layer magnetic moments but switches to the normal conducting state at a NCA configuration. This indicates that the superconducting transition temperature Tc for NCA is lower than the fixed measuring temperature. The existence of a minimum Tc, at the NCA regime below that one for parallel or antiparallel alignments of the F-layer magnetic moments, is consistent with the theoretical prediction of a singlet superconductivity suppression by the long-range triplet pairing generation. © 2013 American Physical Society.


Zdravkov V.I.,University of Augsburg | Zdravkov V.I.,Institute of Electronic Engineering and Nanotechnologies ASM | Kehrle J.,University of Augsburg | Obermeier G.,University of Augsburg | And 8 more authors.
Journal of Physics: Conference Series | Year: 2012

Interference effects of the superconducting pairing wave function in thin film bilayers of Nb as a superconductor (S) and Cu41Ni59 as ferromagnetic (F) material lead to critical temperature oscillations and reentrant superconductivity for increasing F-layer thickness. The phenomenon is generated by the Fulde-Ferrell Larkin-Ovchinnikov (FFLO) like state establishing in these geometries. So far detailed investigations were performed on S/F bilayers. Recently, we could also realize the phenomena in F/S bilayers where the S-metal now is grown on top of the F-material. Combining both building blocks yields an F/S/F trilayer, representing the core structure of the superconducting spin valve. Also for this geometry we observed deep critical temperature oscillations and reentrant superconductivity, which is the basis to obtain a large spin switching effect, i.e. a large shift in the critical temperature, if the relative orientation of the magnetizations of the F-layers is changed from parallel to antiparallel. © Published under licence by IOP Publishing Ltd.


Kehrle J.,University of Augsburg | Zdravkov V.I.,University of Augsburg | Zdravkov V.I.,Institute of Electronic Engineering and Nanotechnologies ASM | Obermeier G.,University of Augsburg | And 9 more authors.
Annalen der Physik (Leipzig) | Year: 2012

Ferromagnet/Superconductor/Ferromagnet (F/S/F) trilayers, in which the establishing of a Fulde-Ferrell Larkin-Ovchinnikov (FFLO) like state leads to interference effects of the superconducting pairing wave function, form the core of the superconducting spin valve. The realization of strong critical temperature oscillations in such trilayers, as a function of the ferromagnetic layer thicknesses or, even more efficient, reentrant superconductivity, are the key condition to obtain a large spin valve effect, i.e. a large shift in the critical temperature. Both phenomena have been realized experimentally in the Cu 41Ni 59/Nb/Cu 41Ni 59 trilayers investigated in the present work. Ferromagnet/Superconductor/Ferromagnet (F/S/F) trilayers, in which the establishing of a Fulde-Ferrell Larkin-Ovchinnikov (FFLO) like state leads to interference effects of the superconducting pairing wave function, form the core of the superconducting spin valve. The realization of strong critical temperature oscillations in such trilayers, as a function of the ferromagnetic layer thicknesses, are the key condition to obtain a large shift in the critical temperature. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.


Khaydukov Y.,Max Planck Institute for Solid State Research | Morari R.,Institute of Electronic Engineering and Nanotechnologies ASM | Morari R.,Kazan Federal University | Mustafa L.,Max Planck Institute for Solid State Research | And 8 more authors.
Journal of Superconductivity and Novel Magnetism | Year: 2015

Structural, magnetic, and superconducting properties of S/F bilayers Nb/Cu 40Ni 60 deposited on silicon substrate have been characterized using polarized neutron reflectometry and complementary techniques. The study allowed to determine real thicknesses of the S and F layers as well as the r.m.s. roughness of the S/F interfaces. The latter does not exceed 1 nm, showing the high quality of the S/F interface. Using SQUID and a mutual inductance setup, we determined the superconducting transition temperatures of the samples, which are in agreement with the literature data. Using of polarized neutron reflectometry (PNR) for the single S layer allowed to determine the screening length λ of the superconducting layer, λ = 120 nm. This value is higher than the London penetration depth for pure niobium which may indicate that the superconductor is in the dirty limit. PNR and SQUID studies of magnetic properties of the CuNi layer have shown the presence of ferromagnetism in all investigated samples. © 2014, Springer Science+Business Media New York.


Khaydukov Y.,Max Planck Institute for Solid State Research | Khaydukov Y.,Moscow State University | Morari R.,Institute of Electronic Engineering and Nanotechnologies ASM | Morari R.,Kazan Federal University | And 7 more authors.
Journal of Applied Physics | Year: 2015

We report an investigation of the structural and electronic properties of hybrid superconductor/ferromagnet (S/F) bilayers of composition Nb/Cu60Ni40 prepared by magnetron sputtering. X-ray and neutron reflectometry show that both the overall interfacial roughness and vertical correlations of the roughness of different interfaces are lower for heterostructures deposited on Al2O3(1 1 ¯ 02) substrates than for those deposited on Si(111). Mutual inductance experiments were then used to study the influence of the interfacial roughness on the superconducting transition temperature, TC. These measurements revealed a ∼4% higher TC in heterostructures deposited on Al2O3, compared to those on Si. We attribute this effect to a higher mean-free path of electrons in the S layer, caused by a suppression of diffusive scattering at the interfaces. However, the dependence of the TC on the thickness of the ferromagnetic layer is not significantly different in the two systems, indicating a weak influence of the interfacial roughness on the transparency for Cooper pairs. © 2015 AIP Publishing LLC.

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