Spin-polarized Electron Tunneling Research Articles

Time-dependent tunneling of spin-polarized electrons in coupled quantum wells

We have solved the in-plane momentum-dependent effective-mass nonlinear Schrödinger equation for a spin-polarized electron wave packet in a InAs double quantum well system with an interlayer voltage. Considering a time-dependent Hartree potential, we have calculated the spin-polarized nonlinear electron dynamics between both quantum wells at different in-plane momentum values and applied bias. The spin-splitting caused by the Rashba effect is combined with the level matching between the spin dependent resonant tunneling levels making possible the observed local spin density oscillations which depend on the applied bias value. The filtering efficiency has been studied using time-dependent calculations.

Anti-phase boundaries pinned abnormal positive magnetoresistance in Mg doped nanocrystalline zinc spinel ferrite

Ni and Mg substituted Zn 1− x M x Fe 2O 4 (M = Ni, Mg) nanocrystals with variable concentration were synthesized by thermal decomposition of EDTA precursor complexes at 600 °C. The crossover from negative to positive magnetoresistance (MR) is realized by replacing the doping ions from Ni to Mg in nanocrystalline zinc spinel ferrites. In Zn 1− x Ni x Fe 2O 4 nanocrystalline ferrite, the negative MR is originated from spin-polarized electron tunneling across the grain boundaries. In Zn 1− x Mg x Fe 2O 4 nanocrystalline ferrite, the electron hopping between the sublattices of A- and B-site is deeply decreased due to Mg doping, which hence suppresses the spin-polarized electron tunneling across the neighbouring grains. Moreover, the enhanced magnetic scattering across the antiferromagnetic-coupled anti-phase grain boundaries favors the abnormal positive MR effect. The combination of both the effects causes the abnormal positive MR effect.

Influence of polyparaphenylene on the magnetotransport of manganite/polymer composites

The ferromagnetic, polycrystalline manganese oxide La 0.7Sr 0.3MnO 3 (LSMO) and the conjugated polymer polyparaphenylene (PPP) were prepared by conventional solid-state reaction processing in air and synthesis by cationic polarization of benzene, respectively. The so-obtained compounds were individually characterized after their electrical, magnetic, structural or thermal properties by using conventional analysis methods. By mixing the pre-prepared LSMO and PPP powders, [LSMO] 1− x [PPP] x ( x stands for the weigh fraction of PPP) two-phase manganite/polymer granular composites were obtained and their microstructural, transport and magnetic features carefully investigated. The resistance of composites calcined at 400 °C for 1 h in air atmosphere increased with x on the whole temperature range. In turn, these samples displayed a pronounced low-field magnetoresistance (LFMR) effect within a wide temperature range. Spin-polarized tunneling of conduction electrons across interfaces or grain boundaries was invoked as the dominant mechanism controlling this effect. The addition of the polymer may modify the LSMO grain surfaces adjusting the conduction carrier's spin-dependent tunneling distance, which sensitively depends on the nature of the interface or on interface-related disorder. Concretely, the PPP should provoke spin disorder at the LSMO interface.

Large low field magnetoresistance in ultrathin nanocrystalline magnetite Fe3O4 films at room temperature

Ultrathin (15nm) Fe3O4 nanocrystalline films with (111) spinel texture have been prepared by rapid annealing of amorphous ion oxide films. Large low field magnetoresistance (LFMR), with the values of about −6.3% at 300K and −10% at 200K under a field of 0.5T, has been observed in the films. The LFMR is mainly attributed to the boundary tunneling of high spin-polarized electrons in Fe3O4 grains of the films and nearly follows a simple relationship between MR and polarization for intergranular tunneling. The fabricating method here seems to be a good approach to prepare high quality Fe3O4 nanocrystalline films with a large LFMR at room temperature.

Anomalous exchange coupling in transition-metal-oxide based superlattices with antiferromagnetic spacer layers

A direct correlation is seen between the coercive field (HC) and the magnetic-field-dependent resistivity (MR) in SrMnO3/SrRuO3 superlattices of perpendicular magnetic anisotropy. The magnetoresistance shows a sharp jump at HC for in-plane current and the out-of-plane magnetic field. Both HC and high-field MR also oscillate with the thickness of the SrMnO3 spacer layers separating the metallic ruthenate. Since the spacer in these superlattices has no mobile carriers to facilitate an oscillatory coupling, we attribute the observed behavior to the spin-polarized quantum tunneling of electrons between the ferromagnetic layers and antiferromagnetically ordered t2g spins of SrMnO3.

Spin-polarized inelastic electron tunneling spectroscopy of a molecular magnetic tunnel junction

Molecular electronic devices with spin-dependent tunneling transport behavior offer an innovative and extremely enticing direction towards spin electronics, both from fundamental and technological points of view. In this work, inelastic electron tunneling spectroscopy provides unambiguous experimental evidence of the existence of molecular species in the fabricated molecular magnetic tunnel devices. Tunneling spectroscopy is also utilized to investigate the spin-polarized inelastic electron tunneling processes in the molecular device. The results show that inelastic scattering due to molecular vibrations, instead of magnon excitations, may be the main cause of the observed junction magnetoresistance bias dependence.

Twin-microstructure enhanced magnetoresistance in La 0.67Ba 0.33MnO 3 oxides

The grain growth dependence of microstructure and its effects on magnetic and transport properties are studied in the polycrystalline La 0.67Ba 0.33MnO 3 oxides. It is found that a lateral growth manner along a certain direction and a concentric terrace pattern along three orthogonal axes occur in the samples sintered at 1573 and 1673 K, respectively. Lamella-like twin microstructure forms in the concentric terrace growth pattern and the magnetoresistance properties can be enhanced by the twin microstructure. It suggests that the twin-boundaries in twin-grains may possibly induce spin-dependent scattering of electrons that is field reduced, or spin-polarized tunneling of electrons that is field enhanced, thus strengthening the effect of grain boundaries.

Theoretical study of molecule mediated spin-polarized electron tunneling between magnetic materials

Spin-polarized electron tunneling between a Ni tip and a self-assembled monolayer, consisting of σ-bonded bicyclo[2.2.2]octane-1,4-dithiol molecules, deposited on Ni(1 1 1) surface is calculated by density functional approach within Bardeen, Tersoff, and Hamann formalism. The magnitude of the tunneling current is predicted to depend on the spin alignments of the probe tip and the substrate. Comparison with a similar study involving benzene-dithiol, a π-conjugated molecule, suggests that the magnitude of the tunnel current as well as the spin-dependent current is strongly influenced by the nature of the chemical bonds in the molecular structure.

Observation of Fowler–Nordheim hole tunneling across an electron tunnel junction due to total symmetry filtering

Using a ${\mathrm{SrTiO}}_{3}(001)$ barrier and ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_{3}(001)$ electrodes, we study the case of coherent spin-polarized tunneling of electrons that exhibit a symmetry mismatch with respect to the lower electron barrier height. Due to this mismatch, electrons with energy above this barrier height continue to tunnel thanks to total symmetry filtering. This allows us to observe symmetry-matched hole tunneling across this electron tunnel junction, and in particular spin-polarized oscillations in the Fowler-Nordheim hole regime above the hole barrier height, thereby enriching the present theoretical picture of coherent spin-polarized tunneling at finite bias.

Engineering of spin filtering in double epitaxial tunnel junctions

Double-barrier fully epitaxial crystalline magnetic tunnel junctions employing a class of artificial antiferromagnetic (AAF) subsystem are elaborated by molecular-beam epitaxy. Our specific AAF subsystem is constituted by an Fe(10nm)∕MgO(0.7nm)∕Fe(20nm) trilayer stack where the antiferromagnetic coupling between the two Fe layers occurs by spin-polarized tunneling of electrons across the three, atomic-layer thin MgO insulating barrier. In our junctions, the efficiency of spin filtering is validated by the tunnel magnetoresistance effects of about 140% at room temperature and a high output voltage up to 500 mV at 1.3 V.

Spin fluctuations and indirect tunneling of itinerant electrons in a- Fe/Si multilayers

We propose the new mechanism of interlayer exchange coupling (IEC) in the amorphous silicon/iron (a-Si/Fe) multilayers, taking into consideration the iron diffusion through Fe/Si interface and formation of the iron silicide (c-FeSi) inside the spacer. This mechanism is based on the competition between the antiferromagnetic (AFM) IEC, provided by the indirect tunneling of spin-polarized itinerant electrons across the spacer and the ferromagnetic (FM) IEC, which is due to the strong spin fluctuations inside the spacer. We calculated the dependence of the IEC integral J( T,L) on the temperature and the spacer thickness and concluded that qualitative coincidence exists between theory and experiment.

Pump–probe spectroscopy of spin-injection dynamics in double quantum wells of diluted magnetic semiconductor

Dynamics of spin injection has been investigated in a double quantum well (DQW) composed of a diluted magnetic semiconductor by the pump–probe transient absorption spectroscopy in magnetic field. The DQW consists of a non-magnetic well (NMW) of CdTe and a magnetic well (MW) of Cd 0.92Mn 0.08Te. The MW shows a transient absorption saturation in the exciton band for more than 200 ps after the optical pumping, while the exciton photoluminescence does not arise from the MW. In the NMW, the circular polarization degree of the transient absorption saturation shows an increase with increasing time. The results are interpreted by the individual tunneling of spin-polarized electrons and holes from the MW to the NMW with different tunneling times. Depolarization processes of the carrier spins in the MW and the NMW are also discussed.

Enhanced grain surface effect on the temperature-dependent behavior of spin-polarized tunneling magnetoresistance of nanometric manganites

We have investigated the effects of nanometric grain size on magnetoresistance (MR), especially on its temperature-dependent behaviors of single-phase nanocrystalline granular La0.7Sr0.3MnO3 and La0.7Ca0.3MnO3 samples with an average grain size in the nanometric regime (12 and 17 nm). Most interestingly, we observed that the magnitude of low-field MR, arising from spin-polarized tunneling of conduction electrons, as well as of high-field MR remains constant up to a sufficiently high temperature (∼200K), and then drops sharply with temperature. With the application of a magnetic field, strong freezing of surface spins occur at the defect sites (having strong pinning strength of spins) of disordered grains surface as a consequence of competitive interactions between grain-boundary pinning strength (k) and magnetic field. Thermal energy (kBT), up to a considerably high temperature, remains unable to flip them from their strained condition, resulting in such a temperature insensitive behavior of MR as well as of surface spin susceptibility (χb).

Spin-polarized electron tunneling across magnetic dielectric

This letter deals with a magnetic tunnel junction having spin filtering by a magnetic barrier. We performed experiments in which a relatively strong external field rotates magnetizations of both ferromagnetic electrodes in the tunnel junction with the magnetic barrier simultaneously so that the two are always parallel to each other. The tunnel magnetoresistance induced in this way was over 16% at 300K. The angular dependency of the tunnel current on the layer magnetizations indicates that the barrier contains antiferromagnetic oxide. To achieve the described effect the magnetic electrode of the junction was oxidized prior to forming the Al2O3 layer.

The mechanism of interlayer exchange coupling in silicon/iron layered structures

We propose a new mechanism of interlayer exchange coupling in a-Si/Fe structures, based on the indirect tunneling of spin-polarized electrons across the semiconductors spacer. This tunneling is effectuated by virtual hopping process over a narrow band, laying close to the Fermi level and leading to the sharp peak in the density of electron states. We take into account the iron-silicide formation inside the spacer and estimate the parameters of an effective exchange.

Spin-polarized electron tunneling between charge-density-wave metals

For junctions between metals partially gapped by charge density waves (CDWs), the quasiparticle tunnel currents J(V) and conductances G(V) in external magnetic fields H are calculated as functions of H, the bias voltage V, temperature T, the dielectric gaps Σ, and the gapped portions μ of the Fermi surface (FS). The paramagnetic effect of H is taken into account, whereas orbital effects are neglected. General expressions are obtained for different CDW metal electrodes. Analytical formulas are obtained for T=0. Explicit numerical calculations are carried out for symmetrical junctions. The results are substantially unlike those for junctions between superconductors. It is shown that due to the interplay between quasiparticles from nested and non-nested FS sections the junction properties involve features appropriate to both symmetrical and asymmetrical setups. In particular, for H=0 discontinuities at eV=±2Σ and square-root singularities at eV=±Σ should coexist. Here e is the elementary charge. For H≠0 the former remain intact, while the latter split. It is suggested to use the splitting as a verification of the CDW nature of the pseudogap in high-Tc superconducting oxides.

Magnetite (Fe3O4) Core−Shell Nanowires: Synthesis and Magnetoresistance

High quality MgO/Fe3O4 core−shell nanowires have been successfully synthesized by depositing an epitaxial shell of Fe3O4 onto single crystal MgO nanowires. The material composition and stoichoimetric ratio have been carefully examined and confirmed with a variety of characterization techniques. These novel structures have rendered unique opportunities to investigate the transport behavior and spintronic property of Fe3O4 in its one-dimensional form. Room-temperature magnetoresistance of ∼1.2% was observed in the as-synthesized nanowires under a magnetic field of B = 1.8 T, which has been attributed to the tunneling of spin-polarized electrons across the anti-phase boundaries.

In-plane electric current is induced by tunneling of spin-polarized carriers.

It has been shown that tunneling of spin-polarized electrons through a semiconductor barrier is accompanied by generation of an electric current in the plane of the interfaces. The direction of this interface current is determined by the spin orientation of the electrons and symmetry properties of the barrier; in particular, the current reverses its direction if the spin orientation changes the sign. Microscopic origin of such a "tunneling spin-galvanic" effect is the spin-orbit coupling-induced dependence of the barrier transparency on the spin orientation and the wave vector of electrons.

Room temperature ultrahigh magnetoresistance in La 0.67Sr 0.33MnO 3 thin films with ordered nanometer structure

We report the preparation and investigation of ferromagnetic La 0.67Sr 0.33MnO 3 (LSMO) thin films with orthogonally (along (110) and (1ı̄0)) distributed nanometer-scale cracks. When the density of cracks reaches ∼10 4 cm −1, the magnetoresistance MR=( R H− R 0)/ R 0>−30% was observed in the magnetic fields ≤200 Oe and temperature range from 5 to >300 K. The discussion about such unique feature will be done based on the spin-polarized electron tunneling and spin scattering.

Tuning negative and positive magnetoresistances by variation of spin-polarized electron transfer into π-conjugated polymers

A series of polyparaphenyl derivatives with different conductivities have been synthesized to fabricate three kinds of polymer-embedded La0.7Ca0.3MnO3 (LCMO) composites by mixing different weight fractions of polymers and LCMO. X-ray diffraction and Fourier transform infrared spectra show the coexistence of the LCMO particles and polymers and no chemical reactions between each other. By adjusting the conductivity and π electron polarization of polymers, spin-polarized electron transfer from the surfaces of LCMO magnetic particles through the interfacial coupling into polymers can be tuned, leading to the tunable negative and positive magnetoresistances in these composites. This abnormal positive MR can be mainly attributed to the spin-polarized electron tunneling weakening and magnetic scattering enhancement on polarized π electrons through the LCMO/polymer interfaces.