Electrical conductance properties for magnetic tunnel junctions with MgO barriers

We measured inelastic electron tunneling (IET) spectra and conductance for MgO tunneling magnetoresistance (TMR) films to obtain information on the ferromagnetic/barrier layer interface. The IET spectra showed the difference between amorphous and crystalline structures in the barrier. In the magnetic tunnel junction (MTJ) with a crystalline barrier the IET spectra indicated an Mg-O phonon peak at a low bias voltage by measurement with a parallel magnetization configuration. On the other hand, no peak was observed in the MTJ with an amorphous barrier.     

Bias voltage effect on electron tunneling across a junction with a ferroelectric-ferromagnetic two-phase composite barrier

The effect of bias voltage on electron tunneling across a junction with a ferroelectric-ferromagnetic composite barrier is investigated theoretically. Because of the inversion symmetry breaking of the spontaneous ferroelectric polarization, bias voltage dependence of the electron tunneling shows significant differences between the positive bias and the negative one. The differences of spin filtering or tunnel magnetoresistance increase with the increasing absolute value of bias voltage. Such direction preferred electron tunneling is found intimately related with the unusual asymmetry of the electrical potential profile in two-phase composite barrier and provides a unique change to realize rectifying functions in spintronics.     

Adsorbate motions induced by vibrational mode coupling

Adsorbate motions are discussed with a primary attention focused on the coupling between a vibrational mode excited by ultrafast laser heated hot-electrons or by inelastic tunneling electrons with scanning tunneling microscope and the reaction coordinate (RC) mode. Recent experimental results have demonstrated an efficient reaction pathways involving an indirect excitation of a frustrated translational mode, rather than its direct excitation for adsorbate hopping on surfaces. Elementary processes are briefly described for hopping of CO molecules on a laser heated stepped Pt surface, where excitation of the frustrated rotation mode has been found to plays an indispensable. Calculation of the inelastic tunneling current (ITC) for excitation of the C-O stretch mode of a CO molecule is combined with a theory of anharmonic mode coupling to activate the frustrated translation mode above the barrier. The hopping rate as a function of the bias voltage agrees with the experimental result. An unified theory of single-, and two-electron processes for ITC-induced motions induced by an indirect excitation of the RC-mode via mode coupling is also applied to reproduce a crossover from hopping to desorption of a single NH₃ molecule on Cu(1 0 0) with an increase in the tunneling current.     

Enhanced magnetotransport at high bias in quasimagnetic tunnel junctions with EuS spin-filter barriers

In quasimagnetic tunnel junctions with a EuS spin-filter tunnel barrier between Al and Co electrodes, we observed large magnetoresistance (MR). The bias dependence shows an abrupt increase of MR ratio in high bias voltage, which is contrary to conventional magnetic tunnel junctions. This behavior can be understood as due to Fowler-Nordheim tunneling through the fully spin-polarized EuS conduction band. The I-V characteristics and bias dependence of MR calculated using tunneling theory show excellent agreement with experiment.     

Effect of inelastic electron-phonon interaction on tunneling magnetoresistance through ferromagnetic-organic molecule-ferromagnetic junction

We have studied the effect of inelastic electron-phonon interaction on the spin-dependent transport properties of a molecule, trans-polyacetylene (trans-PA), as a molecular bridge sandwiched between two ferromagnetic (FM) electrodes. The work is based on a tight-binding Hamiltonian model within the framework of a generalized Green’s function technique and relies on the Landauer-Büttiker formalism as the basis for studying the current-voltage characteristic of this system. We use the wide-band approximation for FM electrodes. It is shown that due to inelastic interactions, the spin currents increase in a finite value of voltage and tunnel magnetoresistance (TMR) decreases compared with TMR obtained in the absence of phonons.     

Tunneling magnetoresistance of the double spin-filter junctions at nonzero bias

A recent theoretical estimation indicated that the NM/FI/FI/NM double spin-filter junction (DSFJ, here the NM and FI represent the nonmagnetic electrode and the ferromagnetic insulator (semiconductor) spacer, respectively) could have very high tunneling magnetoresistance (TMR) at zero bias. To meet the requirement in research and application of the magnetoresistance devices, we have calculated the dependences of tunneling magnetoresistance of DSFJ on the bias (voltage), the thicknesses of ferromagnetic insulators (semiconductors) and the average barrier height. Our results show that except its very high value, the TMR of DSFJ does not decrease monotonously and rapidly with rising bias, but increase slowly at first and decrease then after having reached a maximum value. This feature is in distinct contrast to the ordinary magnetic tunnel junction FM/NI/FM (FM and NI denote the ferromagnetic electrode and the nonmagnetic insulator (semiconductor) spacer, respectively), and is of benefit to the use of DSFJ as a magnetoresistance device.     

Tunnel magnetoresistance oscillations in a ferromagnet-insulator-ferromagnet system

A model of spin-dependent transport of electrons through a ferromagnet-insulator-ferromagnet structure is developed. It takes into account the image forces, tunnel barrier parameters, and effective masses of an electron tunneling in the barrier and in the ferromagnetic electrode in the free electron approximation. Calculations for an iron-aluminum oxide-iron structure show that, with an increase in the bias voltage, the tunnel magnetoresistance decreases monotonically and then breaks into damped oscillations caused by the interference of the electrons’ wave functions in the conduction region of the potential barrier. The image forces increase the tunnel magnetoresistance by two or three times.     

Qualitative analysis of spin-dependent tunneling in a ferromagnetic metal-insulator-ferromagnetic metal junction

A qualitative analysis of spin-dependent tunneling in ferromagnetic metal-insulator-ferromagnetic metal junctions is performed using the WKB approximation and a parabolic band model. It is shown that, as distinct from other tunneling characteristics, only electrons moving at large angles in the plane of the tunnel barrier contribute to the magnetoresistance. The cause of the rapid decrease in the junction magnetoresistance upon applying a bias voltage across the junction is ascertained. It is shown that this cause is attributed to the mirror character of tunneling and remains valid within the framework of more complicated models.     

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.     

Inelastic effect of electron–phonon interactions in tunnelling magnetoresistance of a single metallofullerene

The effects of elastic and inelastic electron–phonon interactions on current–voltage characteristic and tunnelling magnetoresistance (TMR) of Li@C₅₉X (X = N, B) molecule that is coupled to two ferromagnetic electrodes was investigated using the non-equilibrium Green's function (NEGF) method. Our results by taking also into consideration spin degrees of freedom (excluding spin-mixing effects) indicate that the presence of inelastic electron–phonon interaction polaron formation increases current and shifts the TMR behaviour to higher values. Also, an increase of two orders of magnitude observed in current for Li@C₅₉B compared to C₆₀.     

Absence of zero-bias anomaly in spin-polarized vacuum tunneling in Co(0001)

In a joint experimental and theoretical study, we investigate the bias-voltage dependence of the tunnel magnetoresistance (TMR) through a vacuum barrier. The TMR observed by spin-polarized scanning tunneling microscopy between an amorphous magnetic tip and a Co(0001) sample is almost independent of the bias voltage at large tip-sample separations. Whereas qualitative understanding is achieved by means of the electronic surface structure of Co, the experimental findings are compared quantitatively with bias-voltage dependent first-principles calculations for ballistic tunneling. At small tip-sample separations, a pronounced minimum in the experimental TMR was found at +200 mV bias.     

Reversal of spin polarization in Fe/GaAs (001) driven by resonant surface states: first-principles calculations

A minority-spin resonant state at the Fe/GaAs(001) interface is predicted to reverse the spin polarization with the voltage bias of electrons transmitted across this interface. Using a Green's function approach within the local spin-density approximation, we calculate the spin-dependent current in a Fe/GaAs/Cu tunnel junction as a function of the applied bias voltage. We find a change in sign of the spin polarization of tunneling electrons with bias voltage due to the interface minority-spin resonance. This result explains recent experimental data on spin injection in Fe/GaAs contacts and on tunneling magnetoresistance in Fe/GaAs/Fe magnetic tunnel junctions.     

An experimental test of symmetry selection rules in inelastic electron tunneling spectroscopy (IETS)

The inelastic electron tunneling spectrum of a highly symmetric molecule, anthracene, is presented. An analysis of the spectrum shows that the vibrational frequencies as measured by tunneling agree well with optical measurements. The high degree of symmetry of the molecule allows separation of the infrared, Raman, and optically inactive modes. The infrared and Raman active modes have comparable intensities; the optically inactive modes are much weaker. Analysis of these intensities within the framework of existing theories suggests that the coupling of tunneling electrons to the induced dipole moment of molecules is large and cannot be neglected.     

Electron transport in granular amorphous silicon dioxide films with ferromagnetic nanoparticles placed in a magnetic field

Electron transport in amorphous silicon dioxide films with embedded nanoparticles (Co, Nb, Ta) was studied. The mean number of localized states in the interparticle tunneling channel was derived from the temperature dependence of conductivity for various grain concentrations under the assumption of the electron transport being governed by resonance tunneling in a chain of localized states between grains. To confirm the assumption of the inelastic character of tunneling, the dependences of the magnetoresistance on grain concentration, temperature, and magnetic field were studied. Accepting the single-orbital model, where the intergrain tunneling magnetoresistance is determined by s-s tunneling, it was found that the existence of weakly split localized states in the tunneling channel results in a lack of magnetoresistance saturation in strong magnetic fields. The combined effect of a decrease in the s-s tunneling coefficient and of growth in the probability of inelastic electron spin scattering with increasing length of the chain of localized states between particles in which the electron is tunneling accounts for the characteristic temperature-concentration dependences of the magnetoresistance. The experimental observation of these features provides an argument for the electron transport in a-SiO₂(Co,Nb,Ta) structures being governed by inelastic resonance tunneling through intergrain localized states.     

Bistability of single 1,5 cyclooctadiene molecules on Si(001)

The adsorption and current-induced bistability of single 1,5 cyclooctadiene molecules on Si(001) were studied in ultrahigh vacuum by low-temperature scanning tunneling microscopy (STM). After a dosage of ≈0.05 L at room temperature followed by cooling to the measuring temperature of 7 K, we find that the cyclic alkene molecule preferably adsorbs in the bridge structure with both C=C double bonds reacting with two adjacent Si dimers via [2+2] cycloaddition reaction. The time-dependent current measured upon tunneling through the adsorbed molecule at fixed STM tip height displays a switching between two current levels with the same mean residence time in each level. Higher bias and/or reduced tip height—and therefore higher current—increase the switching rate, suggesting that the reversible switching is due to inelastic electron tunneling. The observed bistability is interpreted as a dynamic interconversion between two degenerate conformations of the adsorbed molecule.     

Single-molecule reaction and characterization by vibrational excitation

Controlled chemical reaction of single trans-2-butene molecules on the Pd(110) surface was realized by dosing tunneling electrons from the tip of a scanning tunneling microscope at 4.7 K. The reaction product was identified as a 1,3-butadiene molecule by inelastic electron tunneling spectroscopy. Threshold voltage for the reaction is approximately 365 mV, which coincides with the vibrational excitation of the C-H stretching mode. The reaction was ascertained to be caused by C-H bond dissociation by multiple vibrational excitations of the C-H stretching mode via inelastic electron tunneling process.     

温度和电子有效质量对隧穿磁致电阻偏压依赖关系的影响

本文研究温度和势垒中的电子有效质量对铁磁隧道结中隧穿磁致电阻(TMR)的偏压依赖和变号行为的影响。所求得的TMR随温度上升明显减小的结果与实验一致。除了前文中指出的势垒高度(Ф)以外, 还发现垒中电子的有效质量(mB)是物理上控制TMR变号的另一个重要因素。相应于TMR变号的临界电压(VC)将随Ф升高而增大, 但随mB增大而减小。此外, 零偏压的TMR和临界电压VC将因温度升高而减小。作者希望上述理论结果将有助于实验研究。     

分子电子器件简化模型的电子透射谱的计算

基于分子线耦合到电极的构成特点,采用简化的非对称多势垒连续隧穿模型模拟复合分子器件偏压下的电子隧穿过程,推导电子透射谱的解析表达式,同时计算垒宽、垒距、垒高、电子有效质量和所加偏压等参数与透射系数的关系,结果发现：当电子的能量为某些值时,出现明显的共振隧穿,且透射系数对这些参数的变化非常敏感,这表明可以通过适当的控制方式（如改变复合分子组成、构型等）来修改分子电子器件的输运性质. 关键词： 分子器件 非对称势垒模型 电子透射谱 共振隧穿     

Tunneling through two single molecular magnets in series arrangement junction in parallel/antiparallel situations

We investigate the spin-dependent tunneling transport in a heterostructure with two single molecular magnets (SMMs). The tunneling magnetoresistance (TMR) and negative differential conductance due to the strong resonant tunneling in the junction are demonstrated by the master equation approach. At low bias voltage, the device presents low/high resistant states with the initial states of the single molecular magnets parallel/antiparallel. Strong Coulomb repulsive interaction suppresses the current greatly in antiparallel situation. At high voltage, the middle system containing two SMMs tends to be non-polarized, and acts like ordinary quantum dots.     

Resonant tunneling through electronic trapping states in thin MgO magnetic junctions

We report an inelastic electron tunneling spectroscopy study on MgO magnetic junctions with thin barriers (0.85-1.35 nm). Inelastic electron tunneling spectroscopy reveals resonant electronic trapping within the barrier for voltages V>0.15 V. These trapping features are associated with defects in the barrier crystalline structure, as confirmed by high-resolution transmission electron microscopy. Such defects are responsible for resonant tunneling due to energy levels that are formed in the barrier. A model was applied to determine the average location and energy level of the traps, indicating that they are mostly located in the middle of the MgO barrier, in accordance with the high-resolution transmission electron microscopy data and trap-assisted tunneling conductance theory. Evidence of the influence of trapping on the voltage dependence of tunnel magnetoresistance is shown.