Enhancement of the Josephson current by an exchange field in superconductor-ferromagnet structures

We calculate the dc Josephson current for two superconductor/ferromagnet (S/F) bilayers separated by a thin insulating film. It is demonstrated that the critical Josephson current I(c) in the junction strongly depends on the relative orientation of the effective exchange field h of the bilayers. We found that in the case of an antiparallel orientation I(c) increases at low temperatures with increasing h and at zero temperature has a singularity when h equals the superconducting gap Delta. This striking behavior contrasts with the suppression of the critical current by the magnetic moments aligned in parallel and is an interesting new effect of the interplay between superconductors and ferromagnets.     

Zeeman effects on Josephson current in d-wave superconductor/d-wave superconductor junctions

This paper solves a self-consistent equation for the d-wave superconducting gap and the effective exchange field in the mean-field approximation, and studies the Zeeman effects on the d-wave superconducting gap and thermodynamic potential. The Josephson currents in the d-wave superconductor(S)/insulating layer(I)/d-wave S junctions are calculated as a function of the temperature, exchange field, and insulating barrier strength under a Zeeman magnetic field on the two d-wave Ss. It is found that the Josephson critical currents in d-wave S/d-wave S junction to a great extent depend on the relative orientation of the effective exchange field of the two S electrodes, and the crystal orientation of the d-wave S. The exchange field under certain conditions can enhance the Josephson critical current in a d-wave S/I/d-wave S junction.     

Josephson current in ferromagnetic d-wave superconductor/ferromagnetic d-wave superconductor junction

We solve a self-consistent equation for the d-wave superconducting gap and the effective exchange field in the mean-field approximation, study the Zeeman effects on the d-wave superconducting gap and thermodynamic potential. The Josephson currents in the d-wave superconductor (S)/insulating layer (I)/d-wave S junction are calculated as a function of the temperature, exchange field, and insulating barrier strength under a Zeeman magnetic field on the two d-wave Ss. It is found that the Josephson critical currents in d-wave S/d-wave S junction depend to a great extent on the relative orientation of the effective exchange field of the two S electrodes, and the crystal orientation of the d-wave S. The exchange field can under certain conditions enhance the Josephson critical current in a d-wave S/I/d-wave S junction.     

Josephson effect through a ferromagnetic layer

We report transport measurements on Superconductor/Ferromagnet/Superconductor (S/F/S) junctions: Nb/Al/Gd/Al/Nb where gadolinium (Gd) is a weakly polarized ferromagnet. A sizeable critical current I _c is observed in the I(V) characteristics. This current can be modulated by a weak magnetic field, as expected for a Josephson current. With these experiments, we establish that superconducting coherent transport survives across a small ferromagnetic layer. The penetration depth of Cooper pairs in Gd has been measured. An extensive study of the Josephson critical current in temperature for different thicknesses of magnetic compounds is presented. A comparison of transport measurements with S/N/S junction is given through measurements made on Nb/Al/Y/Al/Nb, where yttrium (Y) is used as non magnetic rare earth metal. Received 20 September 2000 and Received in final form 22 February 2001     

Josephson effects in long superconducting bridges with variable thickness

We have studied the properties of long (L≈20μm) superconducting threads (Pb or Sn) formed by pulse electrical breakdown in monocrystalline silicon. These long bridges with variable thickness have well defined d.c. and a.c. Josephson effects in large temperature range (T/T_c?0.2 and 0.4 for Pb and Sn, respectively) and high magnetic fields (up to 0.14T for Pb at 4.2 K). In the whole temperature range, the critical current has linear temperature dependence, as for “ideal” Josephson junctions. The results are interpreted by coherent motion of vortices. Some interesting results about satellites of quasiparticle gap structures and the asymmetry of these structures with respect to the current direction are presented.     

Peculiarities of the proximity effect in superconductor-multilayered ferromagnet structures with collinear magnetizations in ferromagnetic layers

The proximity effect is investigated theoretically in different superconducting structures including multilayered ferromagnet consisting of arbitrary number of metallic ferromagnet (F) layers. The in-plane exchange fields in the F-layers are supposed to be collinear. Different cases of the exchange fields ordering including the case of antiferromagnetic one. It is shown for the last case that the proximity effect (for the fixed thickness of the M-interlayer) increases with the growing number of the layers N and significantly depends on whether this number is odd or even. It is shown that under the condition of diffusive electron transport, the anomalous proximity effect is exhibited which is related with singlet component of the condensate Green’s function. Peculiarities of the proximity effect are analyzed for S-M, S-I-M-S, and S-M-S superconducting structures (S is a superconductor, I is an insulating layer). It is shown that when the M-structure consists of N seriously connected F₁-N-F₂-N links in which F layers are separated by normal metallic layers (N), for antiferro-magnetic ordering of the magnetization the exchange field induced enhancement of the critical current may occur.     

Change in the sign of the magnetoresistance effect in bilayer superconductor/ferromagnet structures under change in the type of the domain structure in the ferromagnet

The magnetoresistance effects in the bi- and trilayer hybrid planar superconductor/ferromagnet (S/F) structures based on Py (permalloy) and Nb near the superconducting transition temperature T _C are considered. It has been experimentally shown that the sign of the observed magnetoresistance peaks in the bilayer S/F systems changes from negative to positive at the permalloy layer thickness corresponding to the change in the type of domain walls from Néel to Bloch. For the Néel walls at the ferromagnet coercive fields, the negative magnetoresistance effect, which is due to a decrease in the depairing action of the exchange field E _ex, is observed in the S/F bilayers. For the Bloch domain walls, the magnetoresistance of the bilayer S/F structures is determined by the dissipative motion of Abrikosov vortices in the superconducting layer. In the trilayer F/S/F structures, the magnetoresistance is mainly due to the suppression of the superconducting order parameter in the superconducting layer under the action of the accumulation of the spin-polarized carriers near the S/F interfaces.     

Transition from the 0 state to the π state in S-FNF-S Josephson structures

The critical current I _C of S-FNF-S Josephson structures has been calculated as a function of the distance between the superconducting (S) electrodes, L, via the Usadel semiclassical equations for the case of specifying the supercurrent in the direction parallel to the interface between the ferromagnetic (F) and normal (N) films of the composite weak-link region. It has been shown that, owing to the interaction between the F and N films, both the typical decrease scale of the critical current and the period of its oscillations to lengths of the scale ξ_N can be much larger than the respective quantities for the SFS junctions. Moreover, this interaction changes both the magnitude and sign of the critical current. It has been shown that the critical current in a structure with the collinear magnetization vectors of the films can be significantly different from the critical current in a structure with the antiparallel magnetization of the F films.     

The oscillation dependence of the critical current on the exchange field of ferromagnetic metals (F) in Josephson junction S-F-S

The Josephson current of the junction S-F-S is calculated in the case of ODSEE model and for broad junction. The critical current depends on the exchange field and on the thickness of a pure ferromagnetic metal F. The oscillations are absent in the dirty ferromagnet. In a superconducting ring with S-F-S junction a spontaneous current is present under some conditions.     

Superconducting quantum interference device without Josephson junctions

A new type of a superconducting quantum interference device (SQUID) based on a single superconducting loop without Josephson junctions and with asymmetric contacts has been proposed. This SQUID offers advantages in simplicity of fabrication and a steeper dependence of measured quantities on the magnetic flux. To confirm the possibility of making this type of SQUID, the magnetic field dependence of the critical current in an aluminum ring with asymmetric contacts has been experimentally investigated.     

Magnetically dependent superconducting transport in oxide heterostructures with an antiferromagnetic layer

The superconducting current in hybrid superconducting structures Nb/Au/Ca_1?x Sr _x CuO₂/YBa₂Cu₃O_7?δ with an antiferromagnetic layer is experimentally shown to have a Josephson nature, and the deviation from the sinusoidal dependence of the superconducting current on the phase difference between superconducting electrodes is about 20% of the second harmonic. These heterostructures are found to have sensitivity to an applied magnetic field that is much higher than that of conventional Josephson junctions. The experimental shape of the magnetic-field dependence of the critical current in the heterostructures differs from the usual Fraunhofer shape by oscillation with a significantly smaller period along a magnetic field.     

Angular dependence of the superconducting transition temperature in ferromagnet-superconductor-ferromagnet trilayers

The superconducting transition temperature T(c) of a ferromagnet (F)-superconductor (S)-ferromagnet trilayer depends on the mutual orientation of the magnetic moments of the F layers. This effect has been previously observed in F/S/F systems as a T(c) difference between parallel and antiparallel configurations of the F layers. Here we report measurements of T(c) in CuNi/Nb/CuNi trilayers as a function of the angle between the magnetic moments of the CuNi ferromagnets. The observed angular dependence of T(c) is in qualitative agreement with a F/S proximity theory that accounts for the odd triplet component of the condensate predicted to arise for noncollinear orientation of the magnetic moments of the F layers.     

Superconducting Josephson structures with high linearity of transformation of magnetic signal into voltage

Possible approaches to the creation of Josephson superconducting circuits with a high linearity of transformation of a magnetic signal into voltage and a wide dynamic range have been described. Realization of such circuit by the creation of a special spatial inhomogeneity in chains of Josephson junctions and connection of these chains into a differential circuit has been considered. A new method for linearizing the transfer function of a single two-junction superconducting quantum interference device, which is an elementary cell of the chain, by connecting to it an additional loop playing the role of a nonlinear transformer of the magnetic flux has been described. For both approaches, the basic analytical expressions describing the synthesized circuits are presented. The data on numerical calculation and experimental measurements of samples produced on the basis of niobium technology with the critical current density of Josephson junctions of 4.5 kA/cm² are reported. The approaches developed in the work can serve as a basis for the creation of highly efficient gigahertz superconducting amplifiers for cellular and satellite communications and can be applied for improving the characteristics of SQUID-based devices.     

Intrinsic pinning of vortices as a direct probe of the nonuniform Larkin-Ovchinnikov-Fulde-Ferrell state in layered superconductors

Previously the search for the modulated superconducting Larkin-Ovchinnikov-Fulde-Ferrell (LOFF) state was performed by means of measurements which do not give direct information on spatial modulation of the superconducting state. We propose to measure interlayer conductivity in Josephson-coupled layered superconductors as a function of the strength and the orientation of the parallel magnetic field. We show that interlayer critical current and the conductivity have peaks when the magnetic field is perpendicular to the in-plane wave vector of the LOFF state and when the period of the Josephson vortex lattice induced by the magnetic field is commensurate with the LOFF period.     

超导体/铁磁体绝缘层-超导体隧道结的直流Josephson效应

在超导体铁磁体绝缘层超导体结(SFIS)中,运用BogoliubovdeGennes(BdG)方程和FurusakiTsukada(FT)电流公式,计算铁磁超导共存态的自洽方程和SFIS结中的直流Josephson电流.研究表明,铁磁超导态的磁交换能h对准粒子的Andreev反射有抑制作用,使得SFIS结中的直流Josephson电流随铁磁超导共存态的磁交换能h增大而减弱 关键词： S/F-I-S结 铁磁超导态 直流Josephson电流     

Ferromagnet–superconductor hybrids

The new class of phenomena described in this review is based on the interaction between spatially separated, but closely located ferromagnets and superconductors, the so-called ferromagnet–superconductor hybrids (FSH). Typical FSH are: coupled uniform and textured ferromagnetic and superconducting films, magnetic dots over a superconducting film, magnetic nanowires in a superconducting matrix, etc. The interaction is provided by the magnetic field generated by magnetic textures and supercurrents. The magnetic flux from magnetic structures or topological defects can pin vortices or create them, changing the transport properties and transition temperature of the superconductor. On the other hand, the magnetic field from supercurrents (vortices) strongly interacts with the magnetic subsystem, leading to formation of coupled magnetic–superconducting topological defects.

The proximity of ferromagnetic layer dramatically changes the properties of the superconducting film. The exchange field in ferromagnets not only suppresses the Cooper-pair wavefunction, but also leads to its oscillations, which in turn leads to oscillations of observable values: the transition temperature and Josephson current. In particular, in the ground state of the Josephson junction the relative phase of two superconductors separated by a layer of ferromagnetic metal is equal to?π?instead of the usual zero (the so-called π-junction). Such a junction carries a spontaneous supercurrent and possesses other unusual properties. Theory predicts that rotation of magnetization transforms s-pairing into p-pairing. The latter is not suppressed by the exchange field and serves as a carrier of long-range interaction between superconductors.     

Josephson Effect in FS/I/N/I/FS Tunnel Junctions

The Bogoliubov de Gennes equation is applied to the study ofcoherence effects in the ferromagnetic superconductor/insulator/normalmetal/insulator/ferromagnetic/superconductor (FS/I/N/I/FS) junction. We calculated the Josephson current in FS/I/N/I/FS as a function of exchange field in ferromagnetic superconductor, temperature, and normal metal thickness. It is found that the Josephson critical current in FS/I/N/I/FS exhibits oscillations as a function of the length of normal metal. The exchange field always suppresses the Josephson critical current I_p for a parallel configuration of the magnetic moments of two ferromagnetic superconductor (FS) electrodes. In the antiparallel configuration, the Josephson critical current I_Ap at the minimum values of oscillation increases with the exchange field for strong barrier strength and at low temperatures.     

磁性d波超导/铁磁/磁性d波超导结中的约瑟夫森效应

通过求解磁性d波超导中的能隙和磁交换能的自洽方程, 研究磁性d波超导/铁磁/磁性d波超导结中的约瑟夫森电流. 计算结果表明: 1)临界电流随中间的铁磁层厚度呈现出两种不同周期的振荡混合, 通过增强铁磁层中的磁交换能q₀和铁磁/磁性d波超导界面处的势垒强度z₀, 短周期分量可从长周期中分离出来, 反之, 通过降低q₀和z₀, 长周期分量可从短周期中分离出来; 2)在两边磁性d波超导的磁化方向取平行时, 在取一些特定的铁磁层厚度下, 磁性d波超导中的磁交换能可增强系统的临界电流. 关键词： 磁性d波超导体 铁磁体 约瑟夫森电流     

Magnetotransmission in a Magnetostrictive CoFe<Subscript>2</Subscript>O<Subscript>4</Subscript> Crystal in the Faraday Geometry

An analytical approach that makes it possible to reconstruct the current–phase relation (CPR) in Josephson structures included in one of the arms of a two-junction superconducting quantum interference device (SQUID), where the second junction has a significantly higher critical current and a known (sinusoidal) CPR, has been developed. The developed methods of analytical and numerical studies of current–flow transformations in two-junction SQUIDs make it possible to reconstruct the CPR of a junction with a low critical current taking into account both the existence of the self-inductance of the interferometer contour and a possible asymmetry in the supply current system. The efficiency of this approach has been confirmed by the experimental study of niobium–aluminum/aluminum oxide–niobium test structures with the known CPR.