Spin-polarized Electron Tunneling Research Articles

Modulating negative magnetoresistance via inducing vacancy for regulates electron transport under magnetic ambient conditions

The introduction of external magnetic field is one of the most widely studied strategies for photogenerated carrier separation in recent years. However, magnetic materials have great disadvantages in terms of photocatalytic performance. Semiconductor materials with magnetic properties are also extremely rare. It is extremely peculiar to research the magnetism of semiconductor photocatalyst triggered by vacancy. Here, the first example of ferromagnetic Bi2S3 is achieved by controlled Bi vacancy (Bi2S3-VBi). The magnetism properties depends largely on the spin polarization of p electrons in S atoms, and its magnetic moment is mainly distributed on S atoms around cation vacancies. Under the induction of external magnetic field, the tunneling of spin-polarized electrons in S-2p orbital makes Bi2S3-VBi have negative magnetoresistance effect. The ferromagnetic arrangement of Bi2S3-VBi particles can reduce the resistivity and increase the tunneling of electrons. Meanwhile, Lorentz force generated by external magnetic field acts reversely on photogenerated electrons and holes, thus effectively inhibiting the recombination of carriers in photocatalyst. Moreover, the influence of Bi vacancy on Bi2S3 magnetism is investigated by DFT calculation. This study provides a new strategy for effective carrier separation in photocatalysis.

Thermoelectric Effects in Tunneling of Spin-Polarized Electrons in a Molecular Transistor

Thermal transmission in a molecular transistor with fully spin-polarized electrodes subjected to a temperature gradient is considered. The problem has been solved by using density matrix method in perturbation approach over small tunneling width. It has been found that due to the vibronic effects spintronic molecular transistor is characterized by negative differential thermoconductance. It has been demonstrated that in the dependence of thermopower $S$ on the detuning energy there is an increased number of points of change of the sign and magnitude of $S$ comparing with that of conventional molecular transistor. Optimal parameters, that provide the highest thermoelectric power at maximum efficiency $P_{me}$ for spintronic molecular transistor, have been found. The dependences of the figure of merit $ZT$ and $P_{me}$ on temperature and an external magnetic field have been calculated and the influence of Coulomb interaction on the thermolelectric properties has been studied. It has been revealed, that for non-zero Coulomb interaction more handy regime of thermoelectric device develops, characterized by continuous region of external magnetic fields, which provide high values of thermoelectric power.

Large spin-dependent tunneling magnetoresistance in Fe3O4/PET heterostructures developed at room temperature: A promising candidate for flexible and wearable spintronics

Half-metallic nanocrystalline magnetite (Fe3O4) thin films, with different thicknesses were developed on polyethylene-terephthalate (PET) substrates, by reactive sputtering at room temperature. Fe3O4 film (200-nm thick)/PET heterostructures possess superior electrical and magnetic characteristics, with a Verwey transition temperature (Tv) of ~122 K and a saturation magnetization (Ms) ~ 361 emu/cm3. Furthermore, the antiferromagnetic (AFM)-coupled antiphase boundaries (APBs) controlled the transport properties of the Fe3O4 thin films, due to the tunneling of spin-polarized electrons through the films. Very-high magnetoresistance (MR) value (−8.9%) were observed for H‖Film plane, constructed from Fe3O4 (200-nm thick)/PET when H values were below 60 kOe at 300 K. In addition, flexibility tests, to examine resistivity, M-H and MR, were performed using with 90° and 45° bent angles and cyclability experiments were implemented to validate the reproducibility of these characteristics. These outcomes demonstrated that Fe3O4/PET heterostructures may represent a promising candidate for flexible/wearable spintronics.

Magnetoreflection and Kerr Effect in La2/3Ba1/3MnO3 Films with a Variant Structure

A correlation between the effect of magnetoreflection of natural light and tunnel magnetoresistance has been found in La2/3Ba1/3MnO3 films with a variant structure grown on ZrO2(Y2O3) substrates. It is shown that the magnetoreflection effect and giant magnetoresistance in these films are maximum in the region of magnetic ordering near room temperature (TC ≈ 295 K). Magnetoreflection spectra of a La2/3Ba1/3MnO3 film with a variant structure are formed via the same mechanisms as for films without a variant structure and can be described within the theory of the magnetorefractive effect. The field and temperature dependences of the magnetoreflection exhibit the presence of an additional low-temperature contribution to the reflection of a La2/3Ba1/3MnO3 film due to the tunneling of spin-polarized electrons through structural domain boundaries.

Магнитоотражение и эффект Керра в пленках La-=SUB=-2/3-=/SUB=-Ba-=SUB=-1/3-=/SUB=-MnO-=SUB=-3-=/SUB=- с вариантной структурой

A correlation was found between the magnetoreflection of natural light and tunneling magnetoresistance in La2 / 3Ba1 / 3MnO3 films with a variant structure grown on ZrO2 (Y2O3) substrates. It was shown that the magnetoreflection as well as the colossal magnetoresistance, is maximum in the region of magnetic ordering near room temperature (Tc ~ 295 K) of the films. The magnetoreflection spectra of a La2 / 3Ba1 / 3MnO3 film with the variant structure are formed by the same mechanisms as in the case of films without the variant structure and can be described in terms of the theory of the magnetorefractive effect. The field and temperature dependences of magnetoreflection demonstrate the presence of an additional low-temperature contribution to the reflection of the La2 / 3Ba1 / 3MnO3 film due to tunneling of spin-polarized electrons through the boundaries of structural domains.

Interface driven electrical and magneto-transport properties of (100 – x)% La0.7Sr0.3MnO3– x% Paraffin wax (0 ≤ x ≤ 1) hybrid nanocomposites

We report electrical and magneto-transport properties of La0.7Sr0.3MnO3(LSMO)/Paraffin wax nanocomposites prepared through low temperature chemical pyrophoric reaction process. A core shell structure is expected to be formed having a ferromagnetic LSMO core and a spin disordered shell of paraffin wax. Enhanced charging energy due to coulomb blockade contribution with increasing paraffin wax is observed for the composites attributing enhanced intergranular distance between LSMO grains. Effective spin-polarized tunneling of conduction electrons resulting in sharp low field magnetoresistance (MR) of the composites is due to reordering of spin disorder at paraffin wax shell with magnetic field. Moreover, observation of enhanced magnetoresistive hysteresis with decrease in temperature has been attributed to increased blocked states at LSMO grain surface. Calculated surface spin susceptibility (χb) has been found to follow similar behavior of MR with temperature indicating surface magnetization dependent MR. Fitting of magnetoconductance versus field curves reveal increase of Para to ferro transition temperature, TC and χb indicate higher antiferromagnetic coupling of shell spins for composite samples than pristine LSMO above 50 K. χb also indicate systematic decrease in surface spin disorder of the composite samples with respect to pure LSMO which is expected to be due to the probable covalent bond formation between Paraffin wax molecules and surface atoms of LSMO nanoparticles.

The shot noise in quasi one-dimensional magnetic tunnel junctions

Based on the free electron approximation, the shot noise of spin-polarized electron tunneling ferromagnetic metal /insulator/ferromagnetic metal (FM/I/FM) tunnel junctions is investigated. The calculated results show that the Fano factors for electrons with different spin orientations increase with the insulator thickness. When the thickness of insulator is small, the Fano factors for up and down spin electrons decrease slowly with the increase of incident electron energy. For large insulator thickness, the electrons Fano factors for different spin directions are the same when the incident electron energy is located in the low energy region, but decrease sharply and appear resonance when the incident electron energy is located in the high energy region. In addition, with the changes of the angle of magnetic moment in two ferromagnetic layers, the electrons Fano factors for different spin directions show obvious separation characteristics. When the magnetic moments in two ferromagnetic layers are antiparallel, the Fano factors tend to same. At the same time, with the increase of the thickness of the insulator and the incident electron energy, the phase variations of between the tunneling conductances and the Fano factors are always opposite.

Transport signatures of surface potentials on three-dimensional topological insulators

The spin-momentum locked nature of the robust surface states of three dimensional topological insulators (3D TI) make them promising candidates for spintronics applications. Surface potentials which respect time reversal symmetry can exist at the surface between a 3D TI and the trivial vacuum. These potentials can distort the spin texture of the surface states while retaining their gapless nature. In this work, the effect of all such surface potentials on the spin textures is studied. Since, a tunnel magnetoresistance signal carries the information of the spin texture, it is proposed that spin-polarized tunneling of electrons to a 3D TI surface can be used to uniquely identify the surface potentials and quantitatively characterize them.

Non-adiabatic small polaron hopping and the effect of spin-polarized tunneling in electron-doped Ca0.85Nd0.15MnO3

The electron-doped manganite Ca0.85Nd0.15MnO3 has been investigated through magnetic and electrical transport studies. It shows insulating behavior down to the lowest temperature (5 K) measurement even on application of high magnetic field (up to 15 T) indicating a robust insulating behavior. Small polaron hopping and variable range hopping conduction have been observed at high- and low-temperature region, respectively. High-T conductivity of the sample follows non-adiabatic hopping conduction mechanism. Magnetic field-dependent resistivity has been thoroughly examined in terms of a phenomenological model taking into account the spin-polarized tunneling (SPT) of the conduction electrons. In order to explore the nature of the temperature-dependent magnetoresistance (MR), we have separated the contribution related to SPT and spin fluctuation basing on the model. Scaled isothermal MR data collapse into two curves separating the charge order temperature (TCO), which indicates the existence of general formalism of MR and also suggests temperature-independent behavior of the reduced MR.

Tunnel magnetoresistance with atomically thin two-dimensional hexagonal boron nitride barriers

The two-dimensional atomically thin insulator hexagonal boron nitride (h-BN) constitutes a new paradigm in tunnel based devices. A large band gap, along with its atomically flat nature without dangling bonds or interface trap states, makes it an ideal candidate for tunnel spin transport in spintronic devices. Here, we demonstrate the tunneling of spin-polarized electrons through large area monolayer h-BN prepared by chemical vapor deposition in magnetic tunnel junctions. In ferromagnet/h-BN/ferromagnet heterostructures fabricated on a chip scale, we show tunnel magnetoresistance at room temperature. Measurements at different bias voltages and on multiple devices with different ferromagnetic electrodes establish the spin polarized tunneling using h-BN barriers. These results open the way for integration of 2D monolayer insulating barriers in active spintronic devices and circuits operating at ambient temperature, and for further exploration of their properties and prospects.

Cluster altered magnetic and transport properties in Ge1−x−yMnxEuyTe

Magnetic and transport properties of Ge1−x−yMnxEuyTe crystals with chemical compositions 0.041 ≤ x ≤ 0.092 and 0.010 ≤ y ≤ 0.043 are studied. Ferromagnetic order is observed at 150 < T < 160 K. Aggregation of magnetic ions into clusters is found to be the source of almost constant, composition independent Curie temperatures in our samples. Magnetotransport studies show that below 25 K there is a negative magnetoresistance, which is not linear and has a minimum and above 60 K the magnetoresistance is positive and linear. Negative magnetoresistance detected at T < 25 K is found to be due to a tunneling of spin-polarized electrons between ferromagnetic clusters. A linear positive magnetoresistance is identified to be a geometrical effect related to the presence of ferromagnetic clusters inside the semiconductor matrix. The product of the polarization constant (P) and the inter-grain exchange constant (J), JP, varies between about 0.13 meV and 0.99 meV. A strong anomalous Hall effect is observed for T ≤ TC, where TC is the Curie temperature, with coefficients RS independent of temperature. The scaling analysis of the AHE leads to a conclusion that this effect is due to a skew scattering mechanism.

Spin injection and transport in organic spin-valves based on fullerene C60

We report the fabrication and magnetoresistance (MR) of the La0.67Sr0.33MnO3/C60/Co spin valves. The introduction of 1.5nm AlOx barrier between the C60 layer and cobalt top electrode prevents effectively interfacial diffusion and Co penetration, and thus an appreciable positive MR (as large as 3.65%) at room temperature was exhibited for the devices in the thickness range (5–40nm) of C60 studied. Possible mechanisms on the MR polarity are proposed. Furthermore, based on the temperature- and thickness- dependent MR and I–V characteristics, we have obtained clear evidences that the MR of C60-based spin-valves originates from the tunneling of spin-polarized electrons at the low thickness of C60, however at the larger thickness (>20nm) the electrons are injected into and subsequently hopping transport within the C60 spacer.

On the valve nature of a monolayer of aligned molecular magnets in tunneling spin-polarized electrons: Towards organic molecular spintronics

We form a monolayer of magnetic organic molecules and immobilize their moments pointing either upwards or downwards with respect to the substrate through an electrostatic-binding process. Such a monolayer is probed with a scanning tunneling microscope tip, which is also magnetized with the magnetization vector pointing towards (or away from) apex of the tip. From spin-polarized tunneling current, we show that the current was higher when magnetization vectors of the tip and molecules were parallel as compared to that when they were anti-parallel. We show that for tunneling of spin-polarized electrons, aligned organic molecular magnets can act as a valve.

Low-temperature resistance minimum in granular hole-doped cobaltites

It is for the first time that in three-dimensional granular cobaltites (La1−x Srx)1−yAgyCoO3 with a grain size of about 1 μm and replacement ratios x = 0.35 and y = 0, 0.05 a “metal–nonmetal” conductivity transition was observed, presumably associated with the AFM ordering of granule moments. This transition manifests itself as a minimum in the temperature dependence of resistance of the samples. An explanation of the nature of the minimum is proposed, based on the intragranular mechanism of electron correlation on the basis of the Zener double-exchange and on the intergranular mechanism of spin-polarized electron tunneling between the nearest neighbors under the antiferromagnetic exchange interaction of granule moments. Numerical calculation of the system conductance is made via the model based on summation of intragranular phase conductivities of the electronic system and on the computation of the total impedance of the system as a sum of resistances of individual granules with a given resistive contribution of intergranular tunneling. It is found that the external magnetic field of up to 10 T does not affect the depth of the minimum and its position on the temperature scale, suggesting that the intergranular antiferromagnetic interaction is stable to external magnetic fields.

Double metal-insulator transitions and low-temperature resistivity minimum in La<sub>1/3</sub>Nd<sub>1/3</sub>Ba<sub>1/3</sub>MnO<sub>3</sub> manganite

La 1/3 Nd 1/3 Ba 1/3 MnO 3 polycrystalline bulk samples in single phase form with colossal magnetoresistant effect were synthesized in air atmosphere by the conventional solid phase reaction process for ceramic, and their fundamental magnetoelectronic properties were discussed. X-ray diffraction pattern shows that the manganite substituted with two kinds of rare earth element has a cubic perovskite structure, instead of a rhombohedral. The temperature dependence curve of magnetization reveals the existence of spin ice due to the competition of ferromagnetism and antiferromagnetism in the manganite containing ions of Mn 3+ and Mn 4+ . The melting point of spin ice shifts toward low temperature with the increase of applied field. At H = 40 kOe, the phenomenon of spin freezing disappears. At room temperature, above the Currie temperature (~250 K), the hysterisis loop shows the behavior of parasitic ferromagnetism likely contributed by the fluctuation of lattice distortion. Double metal to insulator transitions, both strongly field dependent, were observed in the temperature dependence curve of resistivity. The resistivity peak near the Currie temperature is oriented from the intrinsic contribution of La 1/3 Nd 1/3 Ba 1/3 MnO 3 crystal, while the other peak at ~190 K sensitive to both applied field and sintering time is likely to be related to the tunneling of spin-polarized electrons at grain boundaries. An anomalous minimum of electrical resistivity at ~40 K reveals the existence of local magnetic disorders. By a simulation with the traditional least square method, we believe that the anomalous minimum fits the Kondo theory for diluted magnetic alloy.

Spin-dependent resonant quantum tunneling between magnetic nanoparticles on a macroscopic length scale

Macroscopic quantum phenomena are common features observed in superconductors, superfluid helium, and Bose-Einstein condensates. However, most of quantum transport studies are based on a small number of dots and are not in long-range electron transport length scale. Here we show that spin-dependent resonant quantum tunneling is achieved in the macroscopic length scale (a few millimeters) corresponding to an array of up to ${10}^{4}$ junctions in a series consisting of Co nanoparticles embedded in an oxygen-deficient TiO${}_{2}$ matrix. This phenomenon is observed by magnetoresistance measurements at 5 K in a Coulomb blockade regime. We further present a model based on resonant spin-polarized quantum tunneling of electrons of Co particles. It occurs through resonant continuous spin-polarized defect band states located near the Fermi level of the defective TiO${}_{2}$, which acts as a magnetic tunnel barrier. These results might be potentially useful for future designs of spintronic quantum devices.

Magneto-transport and magnetic properties of (1– x)La 0.7Ca 0.3MnO 3+ xAl 2O 3 composites

We report magneto-transport and magnetic properties of (1– x)La 0.7Ca 0.3MnO 3+ xAl 2O 3 composites synthesized through a solid-state reaction method combined with a high energy milling method. Most interestingly, the effective magnetic anisotropy is found to decrease with increase in the non-magnetic insulating Al 2O 3 phase fraction in the composites. In addition, we observed that the magnitude of low-field magnetoresistance arising from spin-polarized tunneling of conduction electrons, as well as that of high-field magnetoresistance, displays a Curie–Weiss law-like behavior. Finally, we found that the temperature dependence of low and high-field magnetoresistance is controlled predominantly by the nature of temperature response of surface magnetization of the particles.

Tunneling magnetoresistance in Fe/MgO granular multilayers

We have investigated the transport and magnetotransport properties in Fe/MgO multilayers around the Fe percolation threshold as a function of the temperature and the nominal thickness of iron layer (t). Electrical resistivity measurements allowed us to disclose the charge transport mechanisms involved, which are closely related to the degree of discontinuity in the Fe layers. The samples with Fe thickness below percolation threshold (t∼0.8 nm) exhibit isotropic magnetoresistance (MR), which can be understood considering spin-polarized electron tunneling between nanometer-sized, superparamagnetic Fe grains. The MR ratio increases with decreasing temperature from ∼3% at room temperature to ∼10% at 30 K. The temperature dependence of MR can be explained satisfactorily in terms of a modified Mitani’s model.

Influence of Al 2O 3 on low-field spin-polarized tunneling magnetoresistance of (1 − x) La 0.7Ca 0.3MnO 3 + x Al 2O 3 composites

In this study we report the effect of Al 2O 3 on the low field magnetoresistance ( LFMR) of (1 − x) La 0.7Ca 0.3MnO 3 + x Al 2O 3 composite synthesized through a solid-state reaction method combined with an energy milling method. Based upon a spin-polarized tunneling of conduction electrons at the grain boundaries, we have proposed a phenomenological model to explain the observed electrical transport behavior over the whole temperature range (5 300 K), especially the gradual drop of metal-insulator transition temperature ( T p = T max) as a function of increasing Al 2O 3 content, while the ferromagnetic–paramagnetic transition temperature ( T C) remains almost constant ( T C = 250 K).

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.