The influence of the Dresselhaus spin--orbit coupling on the tunnelling magnetoresistance in ferromagnet/ insulator /semiconductor/ insulator /ferromagnet tunnel junctions

This paper investigates the effect of Dresselhaus spin--orbit coupling on the spin-transport properties of ferromagnet/insulator/semiconductor/insulator/ferromagnet double-barrier structures. The influence of the thickness of the insulator between the ferromagnet and the semiconductor on the polarization is also considered. The obtained results indicate that (i) the polarization can be enhanced by reducing the insulator layers at zero temperature, and (ii) the tunnelling magnetoresistance inversion can be illustrated by the influence of the Dresselhaus spin--orbit coupling effect in the double-barrier structure. Due to the Dresselhaus spin--orbit coupling effect, the tunnelling magnetoresistance inversion occurs when the energy of a localized state in the barrier matches the Fermi energy E_F of the ferromagnetic electrodes.     

Magnetoresistance in the ferromagnet/insulator/ferromagnet tunnel junction

Recently experiments and theories show that the tunnel magnetoresistance （TMR） does not only depend on the ferromagnetic metal electrodes but also on the insulator. Considering the rough-scattering effect and spin-flip effect in the insulator, this paper investigates the TMR ratio in a ferromagnet/insulator/ferromagnet （FM/I/FM） tunnelling junction by using Slonczewsik＇s model. A more general expression of TMR ratio as a function of barrier height, interface roughness and spin-flip effect is obtained. In lower barrier case, it shows that the TMR ratio depends on the roughscattering effect and spin-flip effect.     

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.     

Tunneling conductance in topological insulator ferromagnet/p-wave superconductor junctions

The tunneling conductance in topological insulator (TI) ferromagnet/p-wave superconductor (FM/pS) junction is studied based on the Blonder–Tinkham–Klapwijk (BTK) theory. The Fermi energy mismatch between FM and pS as well as the finite quasiparticle lifetime are considered. Three kinds of pairings p_x, p_y, and p_x+ip_y-waves for pS are chosen. It is found that the spectrum strongly depend on the magnetic gap, the gate potential, the quasiparticle lifetime as well as the type of the pair potential symmetry. The pair potential symmetry drastically affects the formation of the zero-energy bound states dependent on the magneto effect or the Fermi energy mismatch effect. The finite quasiparticle lifetime effect can suppress the Andreev resonant scattering process at eV=Δ₀ and smear the dips in the conductance.     

Coexistence of superconductivity and ferromagnetism in ferromagnet/superconductor proximity structures

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     

Nonequilibrium spin accumulation and tunneling magnetoresistance in a ferromagnet/semiconductor/ferromagnet double tunnel junction

Taking into account the nonequilibrium spin accumulation, we apply a quantum-statistical approach to study the spin-polarized transport in a two-dimensional ferromagnet/semiconductor/ferromagnet (FM/SM/FM) double tunnel junction. It is found that the effective spin polarization is raised by increasing the barrier strength, resulting in an enhancement of the tunneling magnetoresistance (TMR). The nonequilibrium spin accumulation in SM may appear in both antiparallel and parallel alignments of magnetizations in two FMs, in particular for high bias voltages. The effects of spin accumulation and TMR on the bias voltage are discussed.     

Magneto-transport properties of La0.7Ca0.3MnO3/SrTiO3/La0.7Ce0.3MnO3 tunnel junction

Hole-doped rare-earth manganite La_0.7Ca_0.3MnO₃ and the electron-doped manganite La_0.7Ce_0.3MnO₃ both show a metal-insulator transition around 250 K associated with a ferromagnetic transition and colossal magnetoresistance. In an earlier publication we have reported the rectifying characteristic of La_0.7Ca_0.3MnO₃/SrTiO₃/La_0.7Ce_0.3MnO₃ tunnel junction at room temperature, showing that it is possible to fabricate a diode out of the polaronic insulator regime of doped manganites. Here we report the magneto-transport properties of such a tunnel junction above and below the metal-insulator transition. We show, from the large positive magnetoresistance of the tunnel junction at low temperature, that La_0.7Ce_0.3MnO₃ could be a minority spin carrier ferromagnet. The implication of this observation is discussed.     

Tunneling Conductance and Magnetoresistance in Ferromagnet/Ferromagnet/d-Wave Superconductor Double Tunnel Junctions

The tunneling conductance and tunneling magnetoresistance (TMR) are investigated in ferromagnet/insulator/ferromagnet/insulator/d-wave superconductor (FM/I/FM/I/d-wave SC) structures by applying an extended Blonder-Tinkham-Klapwijk (BTK) approach. We study the effects of the exchange splitting in the FM, the magnetic impurity scattering in the thin insulator interface of FM/I/FM, and noncollinear magnetizations in adjacent magnetic layers on the TMR. It is shown (1) that the tunneling conductance and TMR exhibit amplitude-varying oscillating behavior with exchange splitting, (2) that with the presence of spin-flip scattering in insulator interface of FM/I/FM, the TMR can be dramatically enhanced, and (3) that the TMR depends strongly on the angle between the magnetization of two FMs.     

Proximity effect in normal metal/ferromagnet/d-wave superconductor structures

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.     

Bias Dependence in Spin-Polarized Tunneling of Ferromagnet/Ferromagnetic Insulator (Semiconductor)/Ferromagnet Junctions

Based on the nearly-free-electron approximation, the bias dependencies of electron transport properties of ferromagnet/ferromagnetic insulator (semiconductor)/ferromagnet junctions have been studied. Resonances appear in electron transmission probability. These resonances cause oscillations in the zero-temperature tunnel current and the resonances occur in tunnel conductance. Tunnel magnetoresistance (TMR) is an oscillatory function of bias. The TMR can reach a value as high as 100%. The bins dependencies of electron transport properties relate to the magnetic configurations of the junctions.     

Tunnel spin polarization versus energy for clean and doped Al2O3 barriers

The variation of the tunnel spin-polarization (TSP) with energy is determined using a magnetic tunnel transistor, allowing quantification of the energy dependent TSP separately for both ferromagnet/insulator interfaces and direct correlation with the tunnel magnetoresistance (TMR) measured in the same device. The intrinsic TSP is reduced below the Fermi level, and more strongly so for tunneling into empty states above the Fermi level. For artificially doped barriers, the low bias TMR decreases due to defect-assisted tunneling. Yet, this mechanism becomes ineffective at large bias, where instead inelastic spin scattering causes a strong TMR decay.     

Large oscillating tunnel magnetoresistance in ferromagnetic graphene single tunnel junction

Spin-polarized transports of relativistic electrons through graphene-based ferromagnet/insulator/ferromagnet (FG/IG/FG) single junctions have been investigated theoretically. Large oscillating tunnel magnetoresistance (TMR) has been found in monolayer and bilayer FG/IG/FG junctions. The oscillating amplitudes of TMR do not decrease with the increase of the thickness and the height of barrier, in contrast to the exponential decay in conventional ferromagnet/insulator/ferromagnet single junction. The physical origin for such a phenomenon has also been analyzed. It is anticipated to apply such a phenomenon to design the spin-polarized electron device based on the graphene materials.     

Spin-flip scattering effect on the current-induced spin torque in ferromagnet–insulator–ferromagnet tunnel junctions

We have investigated the current-induced spin transfer torque of a ferromagnet–insulator–ferromagnet tunnel junction by taking the spin-flip scatterings into account. It is found that the spin-flip scattering can induce an additional spin torque, enhancing the maximum of the spin torque and giving rise to an angular shift compared to the case when the spin-flip scatterings are neglected. The effects of the molecular fields of the left and right ferromagnets on the spin torque are also studied. It is found that τ^Rx/I_e (τ^Rx is the spin-transfer torque acting on the right ferromagnet and I_e is the tunneling electrical current) does vary with the molecular fields. At two certain angles, τ^Rx/I_e is independent of the molecular field of the right ferromagnet, resulting in two crossing points in the curve of τ^Rx/I_e versus the relevant orientation for different molecular fields.     

Magnetic properties of a quasi-one-dimensional organic ferromagnet

A quasi-one-dimensional alternating-spin Heisenberg model is used to describe the charge-transfer organic ferromagnet, which is studied by the Green's function method. The magnetic properties of the charge-transfer organic ferromagnet in different temperature regions are obtained. A formula for the critical temperature T_c is obtained and is found to be related to the spin of donors, to the intrachain exchange interaction, and to the spatial anisotropy parameter. This conclusion can explain T_c of the organic ferromagnet. The spatial anisotropy parameter of the organic ferromagnet [FeCp₂¹][TCNE] we obtained is consistent with the estimated value of Narayan et al. The Curie-Weiss temperature Θ for the organic ferromagnet [CrCp₂¹][TCNE] we obtained is Θ = 15.6 K which is near the experimental result of θ = 22±1 K.     

Critical temperature of superconductor/ferromagnet bilayers

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.     

Spin-precession-induced currents through ferromagnet/nanomagnet/superconductor junctions

The spin-precession-induced current through ferromagnet/nanomagnet/superconductorjunctions is investigated by using the nonequilibrium Green’s function method. It is foundthat the charge current I _c for the spinprecession frequency ω less than the energy gap Δ onlyarises from the equal-spin Andreev reflection, which is independent of the spinpolarization p of the ferromagnetic lead, while that forω > Δ mainly originates from the quasiparticle’scontribution. Both equal-spin AR and quasiparticle scattering processes contribute to thespin current I _s and the quasiparticlescattering process plays a dominant role. WhileI _c forω < Δ can be enhanced by the spin polarizationp, I _s decreases withp. These features may be of interest for ongoing experiments in thefield of molecular spintronics.     

Tunnel spin injection into graphene using Al2O3 barrier grown by atomic layer deposition on functionalized graphene surface

We demonstrate electrical tunnel spin injection from a ferromagnet to graphene through a high-quality Al₂O₃ grown by atomic layer deposition (ALD). The graphene surface is functionalized with a self-assembled monolayer of 3,4,9,10-perylene tetracarboxylic acid (PTCA) to promote adhesion and growth of Al₂O₃ with a smooth surface. Using this composite tunnel barrier of ALD-Al₂O₃ and PTCA, a spin injection signal of ∼30 Ω has been observed from non-local magnetoresistance measurements at 45 K, revealing potentially high performance of ALD-Al₂O₃/PTCA tunnel barrier for spin injection into graphene.     

Conductivity, magnetoresistance, and the Hall effect in granular Fe/SiO2 films

We have investigated the conductance, magnetoresistance, and Hall effect in granular Fe/SiO₂ films with size of the iron grains around 40 Å, whose volume fraction x lies in the range 0.3–0.7. The conduction activation regime has been established for x<0.6. On the insulator side of the transition we observed a giant negative magnetoresistance, falling off sharply as the metal volume fraction decreases. For x<0.4 we observed a large positive magnetoresistance of premagnetized samples, showing up in fields; ～100 Oe and characterized by large response times. The field dependence of the Hall effect in the dielectric samples, as in the metallic samples, correlates with their magnetization. We found that the Hall resistance is proportional to the square root of the longitudinal resistance, which cannot be explained by known models of the anomalous Hall effect.     

Tunneling conductance in ferromagnet/Sr2RuO4 junction: Detection of the d-vector direction

When a spin-triplet superconductor is attached to a ferromagnet, the tunneling conductance depends not only on the degree of the spin polarization but also sensitively on the relative angles between the magnetic moment in ferromagnet and the d-vector in spin-triplet superconductor. We study theoretically the tunneling conductance in ferromagnet/triplet superconductors assuming three nodal unitary gap functions, which are promising candidates for the pairing symmetry of Sr₂RuO₄. Our results suggest that the d-vector direction in Sr₂RuO₄ may be detected by performing angular dependent tunneling spectroscopy in this hybrid structure. We also show that these three gap functions can be distinguished by their distinctive conductance spectra.