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1.
The temperature dependence of the upper critical fields, both perpendicular Hc2 and parallel Hc2 to layer planes of ferromagnet/superconductor bi- and multilayers, is theoretically investigated. The secular equation of the superconducting order parameter for determining the phase diagram (H, T) is obtained by solving exactly the linearized Usadel equations in the multimode method taking into account the material parameter values. For the bilayers system, the influence of the boundary resistivity on the critical fields, and the dimensional crossover behavior of Hc2(T) are studied in details. For the multilayered structure, the effects of the π-phase state on both the superconducting transition temperature Tc and the upper critical fields (Hc2, and Hc2) are also considered. The nonmonotonic Tc behaviors are predicted. The interplay between 0- and π-phases leading to the strong oscillations of Tc as well as the temperature dependence of the zero temperature critical fields on the ferromagnetic layer thickness are investigated theoretically. 相似文献
2.
《Physica C: Superconductivity and its Applications》2007,451(1):55-58
The superconducting proximity effect in normal metal/insulator/ferromagnet/d-wave superconductor (N/I/F/D) structures is studied based on an extended Blonder–Tinkham–Klapwijk (BTK) theory. The transition from the “0” to “π” state is found in the conductance spectra with increasing thickness of F or the ferromagnetic exchange energy. The superconducting proximity effect is drastically changed by the orientation angle α, as α increases the proximity effect is enhanced, being strongest for α/π = 0.25. 相似文献
3.
The superconducting proximity effect in normal metal/insulator/ferromagnet/triplet p -wave superconductor (N/I/FP) structures is studied based on an extended Blonder–Tinkham–Klapwijk (BTK) theory. Three kinds of pairings for the P side are chosen: px, py, px+ipy waves. The transition from the “0” to “π” state is found in the conductance spectra with increasing the thickness of F or the ferromagnetic exchange energy. The large amplitude of the normalized conductance suggests the possible coexistence of the ferromagnetism and p-wave superconductivity in a small region near the F/P interface induced by the proximity effect. 相似文献
4.
Sun Guoya Xing D.Y. Shen R. Lin H.Q. 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,30(1):33-38
The Nambu spinor Green's function approach is applied to calculating the density of states (DOS) and superconducting order
parameter in normal-metal/insulator/ferromagnet/superconductor (NM/I/FM/SC) junctions. It is found that the s-wave superconductivity and ferromagnetism can coexist near the FM/SC interface, which is induced by proximity effect. On
the SC side, the spin-dependent DOS appears both within and without the energy gap. On the FM side, the superconducting order
parameter displays a damped oscillation and the DOS exhibits some superconducting behavior. The calculated result for the
DOS in FM for “0 state” and “π state” can reproduce recent tunneling spectra in Al/Al2O3/PdNi/Nb tunnel junctions.
Received 1st July 2002 Published online 19 November 2002 相似文献
5.
The Bogoliubov-de Gennes equation and Nambu spinor Green's
function approach are applied to studying the Josephson current in
superconductor/ferromagnet/superconductor (S/F/S) Josephson
junctions in the clean limit. It is found that the critical
current exhibits a damped oscillation with the F thickness d,
the oscillation period equal to 2πξF with ξF the
coherence length of the F. The change of the critical current from
positive to negative is determined by factor cosφ′
with φ′=d/ξF as the F-induced phase difference.
The exponent decay of the critical current is close related to
that of the superconductor order parameter in the F, both of them
having the same decay length. 相似文献
6.
《Physics letters. A》2004,325(2):166-174
By using the Bogoliubov–de Gennes equation and the Nambu spinor Greens function approach, we have theoretically studied the dc Josephson current and the coupling phase state of superconductor/ferromagnet/superconductor (SC/FM/SC) junctions, where the FM is of weak ferromagnetism. From the behavior of the temperature-dependent dc Josephson current (Ic), we confirm that such SC/FM/SC junction may change from 0-phase to π-phase state with increasing the temperature (T), for particular parameters of the thickness and the strength of ferromagnetism of the FM interlayer. We attribute such changement to an extra phase difference between the two SCs. The results are qualitatively consistent with an experiment [Phys. Rev. Lett. 86 (2001) 2427], which shows a sharp cusp structure on the Ic−T curves of Nb/Cu0.48Ni0.52/Nb junction for specific thickness of the Cu0.48Ni0.52, indicating the junction changes from 0-phase state at high temperatures to π-phase state at low temperatures. 相似文献
7.
8.
Gruyters M 《Physical review letters》2005,95(7):077204
The magnetic behavior of CoO nanoparticles and layered CoO/ferromagnetic (FM) structures has been investigated by magnetization and hysteresis loop measurements. In the amorphous CoO, a large uncompensation of spins is found that is closely related to spin-glass-like behavior below a freezing temperature T(F) approximately 215-220 K. The spin-glass-like phase may be described by the de Almeida-Thouless line for Ising spin systems. The exchange bias in the layered CoO/FM structures is explained by the spin-glass-like state in the nanoparticles constituting the CoO film. 相似文献
9.
Coexistence of ferromagnetism and superconductivity in normal mental/superconductor/ferromagnet structures 下载免费PDF全文
<正>We extend the Blonder,Tinkham and Klapwijk theory to the study of the inverse proximity effects in the normal mental/superconductor/ferromagnet structures.In the superconducting film,there are the gapless superconductivity and the spin-dependent density of states both within and without the energy gap.It indicates an appearance of the inverse-proximity-effect-induced ferromagnetism and a coexistence of ferromagnetism and superconductivity near the interface.The influence of exchange energy in the ferromagnet and barrier strength at the superconductor/ferromagnet interface on the inverse proximity effects is discussed. 相似文献
10.
We have measured the superconducting transition temperature Tc of Ni/Nb/Ni trilayers when the magnetizations of the two outer Ni layers are parallel (P) and antiparallel (AP). The largest difference in occurs when the Nb thickness is just above the critical thickness at which superconductivity disappears completely. We have observed a difference in Tc between the P and AP states as large as 41 mK--a significant increase over earlier results in samples with higher Tc and with a CuNi alloy in place of the Ni. Our result also demonstrates that strong elemental ferromagnets are promising candidates for future investigations of ferromagnet/superconductor heterostructures. 相似文献
11.
The Josephson effect in the superconductor/ferromagnet/superconductor (SFS) graphene Josephson junction is studied using the Dirac Bogoliubov-de Gennes (DBdG) formalism. It is shown that the SFS graphene junction drives 0–π transition with the increasing of p=h0L/vF?, which captures the effects of both the exchange field and the length of the junction; the spin-down current is dominant. The 0 state is stable for p 〈 pc (critical value pc ≈ 0.80) and the π state is stable for p 〉 pc, where the free energy minima are at φg=0 and φg=π, respectively. The coexistence of the 0 and π states appears in the vicinity of pc. 相似文献
12.
Separate re-entrant superconductivity in asymmetric ferromagnet/superconductor/ferromagnet trilayers
The superconducting and magnetic states of asymmetric ferromagnet/superconductor/ferromagnet (F/S/F′) nanostructures have
been investigated using the boundary value problem for the Eilenberger function. It has been shown that 0- and π-phase superconducting
states of pure thin F/S/F′ trilayers are controlled by the magnitude and sign of electron correlations in the F and F′ layers,
as well as by the competition between homogeneous Bardeen-Cooper-Schrieffer (BCS) pairing and inhomogeneous Larkin-Ovchinnikov-Fulde-Ferrell
(LOFF) pairing. The LOFF-BCS-LOFF separate re-entrant superconductivity has been predicted for F/S/F′ trilayers. A continuous
control of the pair-breaking factor in the Eilenberger function and transition to the state with re-entrant superconductivity
is achieved by varying the thickness of the F′ layer. Sine-modulated 2D LOFF states in asymmetric F/S/F′ trilayers are possible
not only for parallel, but also for antiparallel orientations of the magnetizations of the F and F′ layers; this fact significantly
facilitates the experimental implementation of the predicted phenomena. 相似文献
13.
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/N
bl(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. 相似文献
14.
We combine parameter-free calculations of the transmission and reflection matrices for clean and dirty interfaces with a scattering-theory formulation of Andreev reflection (AR) generalized to spin-polarized systems in order to critically evaluate the use of an extended Blonder-Tinkham-Klapwijk (BTK) model to extract values of the spin polarization for ferromagnetic metals from measurements of point-contact AR. Excellent agreement with the experimental conductance data is found for Pb/Cu but it is less good for Pb/Ni and poor for Pb/Co, indicating that the BTK formalism does not describe transport through superconducting/ferromagnetic interfaces correctly. 相似文献
15.
Critical temperature of superconductor/ferromagnet bilayers 总被引:1,自引:0,他引:1
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 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. 相似文献
16.
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. 相似文献
17.
《Physics letters. A》2005,339(6):488-496
The local density of states (LDOS) in a ferromagnet (FM)/superconductor (SC) bilayer is obtained by using the Nambu Green's function approach. Besides the transition from the “0” state to “π” state found in the spatial variation of the LDOS in the FM, the dependence of the LDOS spectrum on the thicknesses of the FM and SC is also investigated. It is shown that the LDOS in a thin FM film in contact to the SC shows a strong superconducting feature suggesting the possible coexistence of the ferromagnetism and s-wave superconductivity induced by the proximity effects. 相似文献
18.
F.S. Bergeret A.F. Volkov K.B. Efetov 《Applied Physics A: Materials Science & Processing》2007,89(3):599-601
We review the main features of odd triplet superconductivity in superconductor-ferromagnet (S/F) structures. We discuss the
different types of superconducting condensate that can be experimentally observed and pay special attention to the triplet
component induced in a ferromagnet which is in contact with a superconductor. The triplet component is an even function of
the momentum and an odd function of the frequency and leads to novel phenomena.
PACS 74.78.Fk; 73.23.-b; 74.50.+r; 75.70.-i 相似文献
19.
Peña V Sefrioui Z Arias D Leon C Santamaria J Martinez JL Te Velthuis SG Hoffmann A 《Physical review letters》2005,94(5):057002
We show magnetoresistance in excess of 1000% in trilayers containing highly spin-polarized La0.7Ca0.3MnO3 and high-Tc superconducting YBa2Cu3O7. This large magnetoresistance is reminiscent of the giant magnetoresistance (GMR) in metallic superlattices but with much larger values, and originates at spin imbalance due to the injection of spin-polarized carriers. Furthermore, in contrast to ordinary GMR, the magnetoresistance is intimately related to the superconductivity in the YBa2Cu3O7 layer and vanishes in the normal state. This result, aside from its fundamental importance, may be of interest for the design of novel spintronic devices based on ferromagnet/superconductor structures. 相似文献
20.
Spontaneous Josephson spin current in triplet superconductor/ferromagnet/triplet superconductor junctions 下载免费PDF全文
This paper theoretically studies Josephson spin current
through triplet superconductor/ferromagnet/triplet superconductor
junctions. At the ferromagnet/superconductor interfaces, the
ferromagnetic scattering potential gives rise to coupling between
the Andreev bound states and lifts their spin degeneracy. These
spin-split Andreev states carry the Josephson spin current through
the junctions. The generated spin supercurrent can be controlled by
the magnetization of a ferromagnetic thin layer and bias voltage
across the junctions. 相似文献