V. N. Kushnir

Magnetic field depedennce of pinning mechanisms in Bi2Sr2Ca1Cu2O8+x thin films

Abstract We performed investigation of the pinning mechanisms in Bi 2 Sr 2 CaCu 2 O 8+ x (BSCCO) thin films as a function of the temperature, magnetic field and bias current. The specimens were prepared by molecular beam epitaxy deposition technique with a subsequent “ex-situ” annealing in air atmosphere. Transition temperatures T c exceeded 86 K and critical current densities J c were of the order of 10 5 A/cm 2 at T = 4.2 K. Analyzing the temperature dependence of the resistivity ϱ ( T ) data we show that they could be well described by an activation process. The absoute value of the activation energy U depends crucially on the elaboration procedure, while the obtained magnetic field dependence U ( H ) does not depend on the selected model. Introducing a pinning strength distribution we were able to describe the U ( H ) dependence taking into account a crossover from an individually pinned vortex regime to a collective behavior. This crossover was also confirmed by the current dependence of U . In fact, below the crossover field H 0 the curves U ( J ) were well described by a spatial washboard-type variation of the pinning energy, while for H > H 0 the U ( J ) curves showed a logarithmic behavior.

Critical states of superconductivity and their crossover in multilayer superconductor/ferromagnet structures

In order to calculate the critical temperature of multilayer S/F structures (where S is a superconductor and F is a ferromagnet), a matrix method for solving linearized Usadel equations has been proposed. The spectrum of critical temperatures T(k) for the F/Nbl(S/F) structure has been obtained in the single-mode approximation. Eigenfunctions describing the spatial distribution of superconducting correlations in the direction perpendicular to the S-F interfaces have been calculated for each T(k) value. It has been found that dependences of T(k) on the thickness of F layers have a jump near the transition from 0 to π-state; any of the calculated T(k) values can be implemented in the region of jumps. It has been shown that the crossover of eigenstates is characterized by the suppression of superconductivity in outer S layers and by induced countercurrents in F layers. The possibility of the experimental implementation of a state corresponding to a given value from the spectrum of T(k) has been discussed.

The influence of a submicrometre antidot array on the vortex topology and the pinning mechanism in layered superconductors

Measurements of the critical current density dependence on the direction of the external magnetic field vector H have been performed on Nb/CuMn multilayers with and without a regular square lattice of submicrometre antidots. (i) At small angles between H and the surface of the sample, the presence of the antidot array strongly influences the topology of the flux lines. In the multilayers without the antidot array the vortex topology is mainly due to the anisotropy of the system with the formation of kinked vortices in the samples with higher values of the anisotropy parameter. In antidotted samples, the presence of kinked vortices is not related to the anisotropy of the system but to the geometry of the antidot array. (ii) At large angles between H and the surface of the sample, the dimensions of the antidots determine the pinning mechanism, with the prevalence of edge pinning when the antidot diameter is larger than the magnetic penetration depth λ and the presence of electromagnetic pinning when the diameter is comparable to λ. A phenomenological expression for the angular dependence of the pinning force at intermediate and large angles is proposed, satisfactorily describing the experimental data.

Flux creep-flux flow crossover in disordered superconductors

Abstract We propose a method for the correct definition of the effective pinning potential U eff and the critical current density J ∗ in the case of large spread in the values of pinning energies (disordered superconductor). This physical situation corresponds to the high temperature superconductors. We show that, in the limit of small variances σ u of the pinning potential, the current dependence of the effective pinning energy, U eff ( J ), is determined by the interaction of vortices with microinhomogeneities under the deterministic spatial shape U(x) . At large σ u (i.e. σ u ⪢ U eff > k B T ) U eff is shown to have a logarithmic J -dependence in a wide range of J .

Parametric spin-valve effect in superconductor/ferromagnet structures

The equations of the microscopic theory of superconductivity in the diffusion limit (Usadel equations) are used to interpret experimental data from a study of the spin-valve effect in epitaxial Nb/Ho structures. The cause of the quasimetallic behavior of their critical dependences is determined. The influence of the triplet component of the superconducting condensate on the critical temperature is studied.

Resistive transitions in Nb/Cu0.41Ni0.59/Nb trilayers

The superconducting phase transition in Nb/Cu0.41Ni0.59/Nb trilayers, with superconducting (S) Nb and ferromagnetic (F) Cu0.41Ni0.59, has been experimentally studied as a function of the F-layer thickness by measuring the temperature dependence of the electrical resistance R(T). It is shown that the shape and the width of the R(T) curves depends on the Cu0.41Ni0.59 thickness, in particular in the regime where π is the coupling between the S layers, which can be expected. To explain the data, we developed a qualitative model which makes the interconnection between the superconducting phase transition and the 0 to π transition in SFS structures are more evident.

Basic Superconducting Spin Valves

The short review is devoted to the state of the art of a booming field of research in spintronics—superconducting spintronics. The spin valve properties of hybrid structures consisting of alternating layers of superconductor (S) and ferromagnetic (F) of nanoscale thicknesses and superconducting due to the proximity effect are considered in detail. The experimental data for the weak and strong ferromagnetic materials are analyzed; the role of the domain structure of the ferromagnet and the scattering of electrons with spin flip at the SF interfaces in the magnitude of the valve effect is considered. Theoretical works describing the effects of the spin valve for diffusive and clean limits are analyzed. The necessity of consideration of the multiplicity of configurations of the superconducting order parameter in multilayer SF heterostructures is underlined.

Resistive Transitions In S/F/S Trilayers

The phase transition of Nb/Cu0.41Ni0.59/Nb triple layers from the normal to the superconducting state has been studied experimentally by measuring the temperature dependence of the electrical resistance, R(T). It is shown that the shape of the R(T) curves is different depending on the Cu0.41Ni0.59 thickness. To explain the experimental data we developed a qualitative model which makes more evident the interconnection between the superconducting phase transition and the 0 to  crossover in SFS structures.

Superconducting triplet proximity and Josephson spin valves

A heterostructure comprising several ferromagnetic and superconducting layers acquires functionality of managing the superconducting properties of a system applying external magnetic field. At non-collinear magnetic configurations of the ferromagnetic layers, spin-triplet pairings can be induced in these heterostructures. The triplet pairing channel brings additional degrees of freedom to manage superconducting transition temperature in proximity effect superconducting spin valves. Applied to Josephson junctions’ physics, a robust long-range pairing in ferromagnetic weak links produces spin-polarized Josephson currents available for manipulations with magnetic fields and currents. The unique features of the spin-triplet pairings in superconductor–ferromagnet heterostructures make them promising for superconducting spintronics (supertronics).

Superconducting Critical Temperature and Magnetic Inhomogeneities in Superconductor/Ferromagnet/Superconductor Trilayers

The effect of ferromagnetic layer inhomogeneity on the superconducting resistive transition in Superconductor/Ferromagnet/Superconductor (S/F/S) trilayers is studied. The critical temperature, Tc, and the resistive transition shape versus the F layer thickness, dF, in Nb/Cu0.41Ni0.59/Nb and Nb/Pd0.81Ni0.19/Nb trilayers were analyzed. It is shown that the Tc(dF) dependence is sensitive to the Ni concentration variation along the F layer for thickness of dF corresponding to the π-superconducting state and to the 0-π crossover thickness.