The influence of the combination of coherent and sequential tunneling on the tunneling magnetoresistance in a ferromagnet-metal-insulator-ferromagnet tunneling junction

We apply the spin-polarized free-electron model to theoretically study the tunneling magnetoresistance (TMR) in a ferromagnet-metal-insulator-ferromagnet tunneling junction. In the presence of the metallic layer, the tunneling of electrons for current flow through the junction can be divided into two components: the coherent tunneling and the sequential tunneling components. Our calculations show that the attenuated TMR oscillation is well fitted by a damped oscillation function with an exponential decay if only the coherent tunneling is considered. But, if the sequential tunneling is included in the calculations, the behavior of the attenuated TMR oscillation slightly deviates from the damped oscillation function.     

Dielectric barrier formation and tunneling magnetoresistance effect in strontium iron molybdate

A comparative X-ray diffraction study of the initial single-phase metal-oxide compound-strontium iron molybdate Sr₂FeMoO_{6 ? δ} (SFMO)-and that subjected to additional isothermal annealing shows that this heat treatment leads to the appearance of a SrMoO₄ (SMO) phase. Small-angle neutron scattering measurements indicate that the SMO phase forms a dielectric shell surrounding SFMO grains, which has a characteristic thickness of 2–4 nm and extends above 120 nm. The character of the temperature dependence of the electric resistance corresponds to the metal-type conduction in single-phase SFMO and changes to a semiconductor type in the material with SMO dielectric shells, which is evidence of a tunneling mechanism of charge transfer. This conclusion is confirmed by an increase in the absolute value of the negative magnetoresistance of SFMO due to the appearance of a tunneling magnetoresistance component of the same sign.     

Two-particle tunneling current in Josephson junctions

The contribution to the tunneling current of a Josephson junction from the Two-particle tunneling, to the 2nd order approximation in the barrier transparency, is investigated. Expressions for the current onset amplitudes corresponding to eV = _1,2 are given together with the full expressions for the voltage and the temperature dependencies of the two-particles current. The theory has ben developed to take also into account corrections due to depairing mechanisms, which lead to the smearing of the current singularities. Introducing a depairing parameter , which accounts for the probability of these processes, I–V curves in the voltage region of the onset of single and two particle current are obtained. It is shown that, though having the same functional dependence, spreading occurs over a voltage range of different widths. In particular, it is shown that the width of the single-particle structure is twice larger the one for the two-particle. A careful investigation of the I–V curves in the region 2 -eV is presented and some aspects of the interesting voltage region near ¦₁-₂¦ is discussed.     

Bulrush-like double perovskite: synthesis, tunneling magnetoresistance, and magnetocaloric effects

The recent observation of room temperature tunneling magnetoresistance (TMR) in half-metallic A2FeMoO6 (A = Ca, Sr, Ba) double perovskites, and their importance to the emerging field of spintronics has led to considerable effort being dedicated to detailed investigations of the physical and chemical properties of these materials. This article will present an review of our recent investigations covering the synthesis, structures, magnetic and transport properties of "bulrush-like" A2FeMoO6 (A = Sr, Ba). Utilizing the high shape anisotropy as well as the reactivity of A2FeMoO6 to water and a sonochemical technique, we managed to manipulate the properties of grain boundary barriers, and thus put forward a new approach for the enhancement of room temperature TMR. The magnetocaloric effects of A2FeMoO6 double perovskites will also be discussed.     

Spin-polarized current driven vortex-pair Switching in a magnetic ellipse

Micromagnetic simulations are performed to study the mechanism of vortex core reversal in a Permalloy elliptical element that contains two vortices with opposite polarities. A short current pulse is applied in the film plane along the short axis of the ellipse. The trajectories of the two vortex cores move either clockwise or anticlockwise, depending on the core polarization. Their reversal mechanisms of the two cores are the same through a creation-annihilation process of vortex-antivortex pair. By analyzing the partial energies of the sample we find that the core reversal occurs once the maximum local energy density reaches the threshold value (e.g., approximately 3.0 x 10(6) J/m3 for Permalloy). Interestingly, this energy threshold is a universal constant, irrespective of the applied current strength, vortex polarity and the sample size.     

Electromagnetic resonance tunneling in a single-negative sandwich structure

The electromagnetic wave tunneling phenomenon in a sandwich structure consisting of epsilon-negative (ENG), mu-negative (MNG), and epsilon-negative (ENG) media was investigated. Merging of resonance tunneling modes is demonstrated when the conjugate matched trilayer condition is satisfied. The resonance frequency is found to be independent of the thickness ratio of the matched trilayer structure. The resonance tunneling possesses particular angular-dependent and polarization-free properties. The electric fields corresponding to the frequencies of the resonance modes are found to be strongly localized at just one interface with low transmittance. The possible influence on resonance tunneling due to the losses from the single-negative materials is also investigated.     

Analysis of current conduction mechanism in CZTSSe/n-Si structure

In this study, Cu₂ZnSn(S,Se)₄ (CZTSSe) thin films were deposited by the single step thermal evaporation process using the sintered powder of CZTSSe on soda lime glass (SLG) and Si wafer substrates. The structural, optical, and electrical properties of deposited films were investigated. Current–voltage (I–V) in the temperature range of 250–350 K, capacitance–voltage(C–V) and conductance–voltage (G/w–V) measurements at room temperature were carried out to determine electrical properties of CZTSSe/n-Si structure. The forward bias I–V analysis based on thermionic emission (TE) showed barrier height inhomogeneity at the interface and thus, the conduction mechanism was modeled under the assumption of Gaussian distribution of barrier height. The mean barrier height \(({\bar {\Phi }_{B0}})\) and standard deviation \(({\sigma _0})\) at zero bias were obtained as 1.27 eV and 0.18 V, respectively. Moreover, Richardson constant was obtained as 120.46 A cm^?2 K^?2 via modified Richardson plot and the density of interface states (D_it) profile was determined using the data obtained from forward bias I–V measurements. In addition, by the results of frequency dependent C–V measurements, characteristics of the interface state density were calculated applying high-low frequency capacitance (C_HF ? C_LF) and Hill–Coleman methods.     

Spin transfer torque and tunneling magnetoresistance dependences on finite bias voltages and insulator barrier energy

We investigate the dependence of perpendicular and parallel spin transfer torque (STT) and tunneling magnetoresistance (TMR) on the insulator barrier energy of the magnetic tunnel junction (MTJ). We employed the single orbit tight binding model combined with the Keldysh non-equilibrium Green's function method in order to calculate the perpendicular and parallel STT and the TMR in the MTJ with finite bias voltages. The dependences of the STT and TMR on the insulator barrier energy are calculated for semi-infinite half metallic ferromagnetic electrodes. We find a perfect linear relation between the parallel STT and the tunneling current for a wide range of insulator barrier energy. Furthermore, the TMR also depends on the insulator barrier energy, contradicting Julliere's simple model.     

巨磁电阻多层膜结构的研究进展

归纳了巨磁电阻效应发现以来在各种合金体系中出现的巨磁电阻多层膜材料,对其性能和晶体结构进行了对比分析,着重从界面粗糙度、元素混合和晶体学织构等方面,评述了巨磁电阻多层膜结构的国内外研究现状,讨论了各种结构参数对巨磁电阻效应的影响。     

Effect of electron overheating on the tunneling current of a molecular transistor

The effect of overheating of the electron subsystem on the Coulomb blockade in a structure (molecular transistor) based on a metal cluster containing a finite number of atoms has been theoretically studied. The electron energy spectrum in such quantum grains of cylindrical and spherical shape has been calculated. An increase in the electron subsystem temperature in the cluster leads to vanishing of the current gap and pronounced smoothening of the quantum and Coulomb steps on the current-voltage characteristic of the structure, in agreement with experimental observations.     

Switching of the vacuum-arc discharge direct current

Interruption of the direct current of a vacuum-arc discharge (VAD) is analyzed with three methods: the connection of a shunt circuit with a capacitor to the discharge chamber, the overlaying of a pulse nonhomogeneous axisymmetric magnetic field upon the discharge, and the combined action of the magnetic field and the shunt circuit. The probability of interruption of the current I = 500 A as a function of the magnetic field induction is obtained for various shunt capacitances within the range 25 ≤ C ≤ 2500 microfarad (μF). It is shown that the probability of interruption for a given magnetic field induction increases as the capacitance is raised and for the given capacitance it increases as the induction is increased. The magnitude of the magnetic system current I _m to ensure the current interruption with the probability P = 1 is obtained as a function of the shunt capacitance. The impact of the shunt circuit parameters upon the arc current is analyzed. The time-domain plot of the arc current for the shunt capacitor C = 2500 μF was estimated. Its results agree with the experiment. It is shown that the combined use of the magnetic field and the capacitive shunt circuit is an effective method of VAD direct current interruption.     

Insertion of a specular reflective and transmissive nano-oxide layer into giant magnetoresistance spin-valve structure

We have studied the influence of the insertion of a nano-oxide layer (NOL) into a magnetic GMR spin-valve. It was found that the spin-valve with NOL has a higher GMR ratio than that of the normal spin-valve without NOL. Naturally formed NOL without vacuum break shows a uniform layer, which effectively suppresses the current shunt, resulting in the reduction of the sheet resistance of GMR. The NOL spin-valve also shows a lower interlayer coupling (Hin) than that of the optimal normal spin-valve, which is consistent with AFM measurement showing lower roughness of NOL formed CoFe surface. Based on the advantage of NOL, we succeeded in lowering Hin while maintaining GMR ratio by insertion of NOL inside the CoFe free layer, where the free layer consists of CoFe/NOL/CoFe/NOL/Capping layer.     

Spin-transfer switching current distribution and reduction in magnetic tunneling junction-based structures

Spin transfer switching current distribution within a cell and switching current reduction were studied at room temperature for magnetic tunnel junction-based structures with resistance area product (RA) ranged from 10 to 30 /spl Omega/-/spl mu/m/sup 2/ and TMR of 15%-30%. These were patterned into current perpendicular to plane configured nanopillars having elliptical cross sections of area /spl sim/0.02 /spl mu/m/sup 2/. The width of the critical current distribution (sigma/average of distribution), measured using 30 ms current pulse, was found to be 3% for cells with thermal factor (KuV/k/sub B/T) of 65. An analytical expression for probability density function p(I/I/sub c0/) was derived considering a thermally activated spin transfer model, which supports the experimental observation that the thermal factor is the most significant parameter in determining the within-cell critical current distribution. Spin-transfer switching current reduction was investigated through enhancing effective spin polarization factor /spl eta//sub eff/ in magnetic tunnel junction-based dual spin filter (DSF) structures. The intrinsic switching current density (J/sub c0/) was estimated by extrapolating experimental data of critical current density (J/sub c/) versus pulse width (/spl tau/), to a pulse width of 1 ns. A reduction in intrinsic switching current density for a dual spin filter (DSF: Ta/PtMn/CoFe/Ru/CoFeB/Al2O3/CoFeB/spacer/CoFe/PtMn/Ta) was observed compared to single magnetic tunnel junctions (MTJ: Ta/PtMn/CoFe/Ru/CoFeB/Al2O3/CoFeB/Ta). J/sub c/ at /spl tau/ of 1 ns (/spl sim/J/sub c0/) for the MTJ and DSF samples were 7/spl times/10/sup 6/ and 2.2/spl times/10/sup 6/ A/cm/sup 2/, respectively, for identical free layers. Thus, a significant enhancement of the spin transfer switching efficiency is seen for DSF structure compared to the single MTJ case.     

Crossover from Kondo-assisted suppression to co-tunneling enhancement of tunneling magnetoresistance via ferromagnetic nanodots in MgO tunnel barriers

The dependence of the tunneling magnetoresistance (TMR) of planar magnetic tunnel junctions on the size of magnetic nanodots incorporated within MgO tunnel barriers is explored. At low temperatures, in the Coulomb blockade (CB) regime, for smaller nanodots the conductance of the junction is increased at low bias consistent with Kondo-assisted tunneling and the TMR is suppressed. For slightly larger nanodots but within the CB regime, the TMR is enhanced at low bias, consistent with co-tunneling. Magnetic tunnel junctions (MTJ) exhibit giant magnetoresistance in small magnetic fields that arises from the flow of spin-polarized current through an ultrathin tunnel barrier separating two magnetic electrodes. The current through an MTJ device depends on the magnetic orientation of the electrodes and is typically higher when the electrode moments are parallel than when they are antiparallel. It has recently been demonstrated that the spin polarization of the tunneling current can be greatly enhanced by using crystalline tunnel barriers formed from MgO as compared with conventional amorphous barriers formed from alumina, due to spin filtering across the MgO layer. The magneto-transport properties of magnetic granular alloys and magnetic tunnel junction devices with magnetic nanodots embedded in amorphous dielectric matrices, and tunnel barriers, respectively, have been studied by several groups, but no systematic studies of the dependence on these properties on the nanodot size have been made.     

Monte Carlo study of the ion-induced electron current tunneling through a metal-insulator-metal junction

A Monte Carlo program is developed to investigate the kinetically excited electrons passing through a realistic Ag-Al₂O₃-Al junction when Ar⁺ ions impact on the top Ag layer. The program includes excitation of the target electrons (by projectile ions, recoiling target atoms and fast primary electrons) and subsequent transport of these excited electrons from Ag to bottom Al layer of the metal-insulator-metal (MIM) junction. The calculated tunneling electron yield is consistent with the recently reported experimental results. The simulation, however, enables the calculation of partial tunneling electron yields of the electrons excited by the projectile ions, recoil atoms and cascade electrons, the depth distribution of the electron excitation points in the MIM junction and energy distribution of the tunneling electrons. Our calculation showed that the electrons excited by fast cascade electrons are the major contributor to the tunneling electron yield while the direct contribution of projectile ions to tunneling electron yield is evident only at the projectile energies greater than 10 keV. The tunneling electrons have their origin close to the bottom end of the Ag layer and bulk of the tunneling electrons have energies around 2 eV.     

Theory of the cos ø term in the Josephson tunneling current

The current-phase relation for the Josephson tunnel junction is investigated by using the time-dependent Ginzburg-Landau (TDGL) equations. The cos ø term is shown to have a negative coefficient and the same order of magnitude as the phase-independent term. The effect of the tunneling barrier is taken into account in terms of the boundary conditions of the order parameter and the electric potential at the junction; the new boundary conditions are derived by generalizing de Gennes' discussions for the dc Josephson effect to the time-dependent phenomena. The TDGL equations are solved on the assumption that the transmission coefficient across the junction is small enough and that the processes involved are quasi-stationary. It is concluded that the spatial and temporal variations of various quantities are of vital importance to an understanding of the nonequilibrium characteristics in the tunnel junction.     

Anisotropic magnetoresistance and planar hall effect in GaAs structure with Mn-delta-doped layer

Anisotropic magnetoresistance and the planar Hall effect are discovered and studied in a GaAs structure containing a single layer delta-doped with manganese to its total content equivalent to 0.18 monolayer. The Mn-delta-doped structures were obtained by combining the methods of metalorganic chemical vapor deposition (MOCVD) in a hydride system and laser ablation of solid targets in a common technological cycle. The character of the dependence of the Hall resistance on the magnetic field is indicative of the presence of a planar cubic magnetic anisotropy.     

影响巨磁电阻钙钛矿锰氧化合物居里温度的结构分析

测定了La1-xAx(MnB）O3型巨磁电阻钙钛矿化合物在不同组成时的居里温度，利用A位离子对晶格能的贡献和B位离子极化力的变化分别对A和B位离子掺杂及A，B位离子组成同时变化时钙钛矿的居里温度进行了分析，结果与实验一致。