Superconducting spin-valve and triplet superconductivity

Recent experimental results on the superconducting spin-valve effect and generation of the long-range triplet superconductivity in a F1/F2/S structure are reviewed (here, F1 and F2 are uncoupled ferromagnetic layers, and S is the superconducting layer). The main results are the following: (i) the maximum of the magnitude of the superconducting spin-valve effect increases with decreasing the exchange field h in the ferromagnetic layer; (ii) a full switching between the normal and superconducting states may be realized with the aid of the triplet contribution to the spin-valve effect.     

Peculiarities of performance of the spin valve for the superconducting current

The spin valve effect for the superconducting current based on the superconductor/ferromagnet proximity effect has been studied for a CoO _x /Fe1/Cu/Fe2/Cu/Pb multilayer. The magnitude of the effect ΔT _c = T _c ^AP ? T _c ^P , where T _c ^P and T _c ^AP are the superconducting transition temperatures for the parallel (P) and antiparallel (AP) orientation of magnetizations, respectively, has been measured for different thicknesses of the Fe1 layer d _Fe1. The obtained dependence of the effect on d _Fe1 reveals that ΔT _c can be increased in comparison with the case of a half-infinite Fe1 layer considered by the previous theory. A maximum of the spin valve effect occurs at d _Fe1 ～ d _Fe2. At the optimal value of d _Fe1 almost full switching from the normal to the superconducting state when changing the mutual orientation of magnetizations of the iron layers Fe1 and Fe2 from P to AP is demonstrated.     

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.     

Suppression of interlayer segregation in spin-valve NiFe/Cu/NiFe/FeMn multilayers

Experimental results show that Cu atoms can float out to or segregate to the NiFe/FeMn interface for Ta/NiFe/Cu/NiFe/FeMn/Ta spin-valve multilayers, which results in a drop of the exchange-coupling field (H_ex) of NiFe/FeMn in the spin-valve multilayers. However, when a small amount of Bi atoms is deposited between the Cu and the pinned NiFe layers, Cu segregation to the NiFe/FeMn interface can be suppressed. At the same time, H_ex of NiFe/FeMn in the spin-valve multilayers with a Bi interfacial layer can be effectively increased. PACS 75.70.Cn; 82.80.Pv     

Electric field-induced magnetoresistance in spin-valve/piezoelectric multiferroic laminates for low-power spintronics

Electric field-induced magnetic anisotropy has been realized in the spin-valve-based {Ni₈₀Fe₂₀/Cu/Fe₅₀Co₅₀/IrMn}/piezoelectric multiferroic laminates. In this system, electric-field control of magnetization is accomplished by strain mediated magnetoelectric coupling. Practically, the magnetization in the magnetostrictive FeCo layer of the spin-valve structure rotates under an effective compressive stress caused by the inverse piezoelectric effect in external electrical fields. This phenomenon is evidenced by the magnetization and magnetoresistance changes under the electrical field applied across the piezoelectric layer. The result shows great potential for advanced low-power spintronic devices.     

High sensitivity GMR with small hysteresis in Ni–Fe/Cu multilayers

Giant magnetoresistance (GMR) effect and magnetisation reversal processes have been investigated in Py/Cu(Py=Ni₈₃Fe₁₇,permalloy) multilayers (Mls) obtained by face-to-face sputtering method. The investigated films had constant sublayer thicknesses both for Py and Cu (d_Cu=2nm,d_Py=2nm) and various numbers of ferromagnetic sublayers. It has been shown that for such Mls a high field sensitivity of GMR effect (S≈0.4%/Oe) and negligible hysteresis can be obtained for a low number of Py layers.     

Spin-valve magnetoresistive structures based on Co/Tb multilayer films

The effect of the layer thickness on the magnetic properties of {Co/Tb}_n, {Co/Tb}_n/Co, and {Co/Tb}_n/Co/Cu/Co multilayer films is studied. The dependence of the hysteresis and magnetoresistive properties of {Co(1 nm)/Tb(1 nm)}_n/Co(5 nm)/Cu(L _Cu)/Co(5 nm) structures on the thickness of the {Co/Tb}_n layer and copper spacing are obtained. The feasibility of spin-valve structures based on {Co/Tb}_n multilayer films with in-plane anisotropy is demonstrated.     

Triplet superconducting correlations in oxide heterostructures with a composite ferromagnetic interlayer

The superconducting current induced by the penetration of the long-range triplet component of superconducting correlations into a composite ferromagnetic interlayer has been detected in mesa-heterostructures based on oxide cuprate superconductors YBa₂Cu₃O_{7 ? δ} and Au/Nb bilayer films with the composite oxide interlayer that is made of ferromagnetic films of manganite La_0.7Sr_0.3MnO₃ and ruthenate SrRuO₃ and has a thickness much larger than the length of correlations determined by the exchange field. The deviation of the superconducting current in the mesa-heterostructure with the fraction of the second harmonic of 13% from a sinusoidal current-phase relation has been detected; this deviation can also be due to the generation of the triplet component of superconducting correlations in the ferromagnet.     

Superexchange Nonresonant Tunneling Current across a Molecular Wire

The spin-valve effect in superconductor hybrid structures based on magnetic materials with a noncollinear magnetic ordering is described theoretically. It is shown that the following effect is possible in such structures: when the sample is magnetized, the critical temperature of the superconducting transition increases. The origin is the suppression of long-range triplet superconducting correlations. Such structures can be used as newtype memory elements for superconducting spintronics.     

Superconductivity of Nb/Cu superlattices

The superconducting transition temperature T_c of Nb/Cu superlattices has been investigated as a function of layer thickness. The dependence of T_c above 300 Å layer thickness agrees well with proximity effect theory with no adjustable parameters. Below 300 Å, the data in conjunction with current proximity theory shows that T_c of Nb decreases with layer thickness. This is interpreted as changes in the electronic density of states due to a decrease in the mean-free path.     

Oscillations of the Critical Temperature in a (Fe/Cr/Fe)/V/Fe Heterostructure

The superconducting and magnetic properties of the (Fe/Cr/Fe)/V/Fe layered system with variable thickness of the chromium layer have been experimentally and theoretically studied. The magnetic properties of the system have been studied by the ferromagnetic resonance method, and the superconducting transition temperature has been measured from the jump in the magnetic susceptibility. A wide variety of magnetic states are observed in the system; in particular, the structure of small domains can arise in the iron layer placed between vanadium and chromium. It has been shown experimentally that the critical temperature T_c of the superconducting transition undergoes nonmonotonic oscillations with a noticeable amplitude in the given system with the change in the thickness of the Cr layer. The proposed model based on the proximity effect theory makes it possible to relate these T_c oscillations to the features of the magnetic structure of the samples.     

Influence of magnetic scattering on superconductivity of ferromagnet/superconductor/ferromagnet spin-valve

Ferromagnet/superconductor/ferromagnet (F/S/F) spin-valve systems in the dirty limit, described by Usadel equations, was theoretically investigated with respect to superconducting transition temperature. Their superconducting characteristics strongly depend not only on the mutual orientation and thickness of the ferromagnetic layers, but also on the interface transparency as well as magnetic scattering. Especially, the introduction of magnetic scattering drastically reduces the spin-valve effect in our work. The obtained results could be used to understand experimental values of and to provide guidelines for optimizing the experimental systems.     

Studies of the microstructure of a superconducting Nb<Subscript>3</Subscript>Sn-Band obtained from (Cu/Nb)/Cu12Sn nanocomposite

A multilayer layer from the superconducting compound Nb₃Sn was obtained by the thermal treatment of composite (Cu/Nb)/Cu12Sn conductor consisting of Cu-12% Sn and Cu/Nb interlayers consisting in turn of nanosized copper and niobium layers. The conductor was coated with copper from the outside, which served as a stabilizer. The microstructure of the cross section of the composite band in dependence on the volume ratio of Cu/Nb and bronze interlayers and the microstructure of the Cu/Nb interlayers themselves in dependence on the annealing temperature were investigated. The optimum ratio of composite components was as follows: t _Nb N _Nb ≈ 0.288t _CuSn N _CuSn, where t and N are the thickness and number of the niobium and bronze layers, respectively. Trying to maintain the optimum design of the conductor forced us to increase the bronze volume content relative to the content of Cu/Nb interlayers, which had a negative impact on the composite microstructure. This resulted in disruptions of Cu/Nb interlayers. A technique has been developed for producing the (Cu/Nb)/Cu12Sn composite as a precursor to the band from the Nb₃Sn compound with allowance for the first experiment’s shortcomings.     

Improvement of the structural and magnetic properties of Permalloy/Gadolinium multilayers with Mo spacers

We have studied the influence of the growth conditions on the structural characteristics of sputtered Py(Fe₂₀Ni₈₀)/ Gd/Py(Fe₂₀Ni₈₀) thin films. Auger electron spectroscopy reveals the existence of Ni in the Gd layer. The Ni concentration profile appears asymmetric with a higher concentration close to the top Py layer. This asymmetrical Ni concentration produces a different coercivity of each Py layer. It is possible to reduce the Ni interdiffusion by using multilayers based on Py/Gd bilayers separated by Mo spacers between the Py/Gd bilayers. In these samples we have obtained an enhancement of the structural and magnetic properties. PACS 81.15.Cd; 66.30.-h     

Role of the external surfaces on the superconducting properties of superconductor/normal metal trilayers

Superconducting proximity effect is studied in superconductor/normal metal trilayers. The dependences of the superconducting transition temperature _Tc$T_c$ versus Nb thickness in Cu/Nb/Cu systems and versus Cu thickness in Nb/Cu/Nb ones are described by different values of the microscopical parameters. We attribute this difference to the influence of the external surfaces of the Nb/Cu/Nb hybrids on the superconducting properties of the system.     

Magnetoresistance and magnetization studies through a Cu interlayer in spin valves of NiFe/Cu/NiFe/FeMn

The influence of the Cu layer thickness on the magnetic and magnetotransport properties has been investigated in Ta/NiFe/Cu/NiFe/FeMn spin valves. The magnetization and magnetoresistance measurements were carried out for magnetic field applied along the easy-axis direction. A phenomenological model, which assumes formation of a planar domain wall at the anti-ferromagnetic side of the interfaces as well as bilinear coupling between the ferromagnetic layers, was used to derive the anisotropy characteristics and orientation of each NiFe layer magnetization. The anisotropy and spin valve magnetoresistance were simulated numerically and compared with the experiment. It was found that the anisotropy magnetoresistance is negligible and that there is a poor agreement for the spin-valve one, which was attributed to the model (valid for ferromagnetic layers in single-domain state only) used for its calculation. It was found that the increase of the Cu layer thickness provokes a decrease of the interdiffusion between the NiFe and FeMn layers, and, as consequence, changes of the uniaxial anisotropy of the pinned NiFe layer, of the exchange interaction between the pinned NiFe layer and the FeMn ones, as well as of the exchange-bias field of the pinned NiFe layer.     

Unusual behavior of nuclear relaxation in CeCu2Si2 “possible evidence for triplet superconductivity”

Nuclear relaxation of ⁶³Cu in the superconducting state of the Kondo-lattice system CeCu₂Si₂ has been studied with the use of the ⁶³Cu nuclear quadrupole resonance technique under zero field and down to 65mK. The nuclear spin-lattice relaxation rate (1/T₁) decreases drastically just below T_c=0.67 K down to 0.5T_c without the apparent enchanced behavior and then is found to be almost temperature independent below 0.3T_c. These results suggest that the superconductivity in CeCu₂Si₂ is not in the usual BCS regime. The analysis based upon the existing triplet pairing model with an anisotropic energy gap describes well the behavior from T_c down to 0.5T_c, while the temperature independence below 0.3T_c remains unexplained.     

MgB2 coated superconducting tapes with high critical current densities fabricated by hybrid physical-chemical vapor deposition

The MgB₂ coated superconducting tapes have been fabricated on textured Cu (0 0 1) and polycrystalline Hastelloy tapes using coated conductor technique, which has been developed for the second generation high temperature superconducting wires. The MgB₂/Cu tapes were fabricated over a wide temperature range of 460-520 °C by using hybrid physical-chemical vapor deposition (HPCVD) technique. The tapes exhibited the critical temperatures (T_c) ranging between 36 and 38 K with superconducting transition width (ΔT_c) of about 0.3-0.6 K. The highest critical current density (J_c) of 1.34 × 10⁵ A/cm² at 5 K under 3 T is obtained for the MgB₂/Cu tape grown at 460 °C. To further improve the flux pinning property of MgB₂ tapes, SiC is coated as an impurity layer on the Cu tape. In contrast to pure MgB₂/Cu tapes, the MgB₂ on SiC-coated Cu tapes exhibited opposite trend in the dependence of J_c with growth temperature. The improved flux pinning by the additional defects created by SiC-impurity layer along with the MgB₂ grain boundaries lead to strong improvement in J_c for the MgB₂/SiC/Cu tapes. The MgB₂/Hastelloy superconducting tapes fabricated at a temperature of 520 °C showed the critical temperatures ranging between 38.5 and 39.6 K. We obtained much higher J_c values over the wide field range for MgB₂/Hastelloy tapes than the previously reported data on other metallic substrates, such as Cu, SS, and Nb. The J_c values of J_c(20 K, 0 T) ∼5.8 × 10⁶ A/cm² and J_c(20 K, 1.5 T) ∼2.4 × 10⁵ A/cm² is obtained for the 2-μm-thick MgB₂/Hastelloy tape. This paper will review the merits of coated conductor approach along with the HPCVD technique to fabricate MgB₂ conductors with high T_c and J_c values which are useful for large scale applications.     

Antiferromagnetism and superconductivity in a model with extended pairing interactions

The competition between antiferromagnetism and the d + id superconducting state is studied in a model with near and next near neighbour interactions in the absence of any on-site repulsion. A mean field study shows that it is possible to have simultaneous occurrence of an antiferromagnetic and a singlet d + id superconducting state in this model. In addition, such a coexistence generates a triplet d + id superconducting order parameter with centre of mass momentum Q = (π,π) dynamically having the same orbital symmetry as the singlet superconductor. Inclusion of next nearest neighbour hopping in the band stabilises the d_xy superconducting state away from half filling, the topology of the phase diagram, though, remains similar to the near neighbour model. In view of the very recent observation of a broad region of coexistence of antiferromagnetic and unconventional superconducting states in organic superconductors, the possibility of observation of the triplet state has been outlined. Received 30 November 2000 and Received in final form 27 March 2001