Double-barrier Tunneling Research Articles

Oscillations of tunnel magnetoresistance induced by spin-wave excitations in ferromagnet-ferromagnet-ferromagnet double-barrier tunnel junctions

The possibility of oscillations of the tunnel conductance and magnetoresistance induced by spin wave excitations in a ferromagnet-ferromagnet-ferromagnet double-barrier tunnel junction, when the magnetizations of the two side ferromagnets are aligned antiparallel to that of the middle ferromagnet, is investigated in a self-consistent manner by means of Keldysh nonequilibrium Green's function method. It has been found that owing to the $s\text{\ensuremath{-}}d$ exchange interactions between conduction electrons and the spin density induced by spin accumulation in the middle ferromagnet, the differential conductance and the tunnel magnetoresistance indeed oscillate with the increase of bias voltage, being qualitatively consistent with the phenomenon that is observed recently in experiments. The effects of magnon modes, the energy levels of electrons, as well as the molecular field in the central ferromagnet on the oscillatory transport property of the system are also discussed.

Dynamically modulated tunneling for multipurpose electron devices: Application to THz frequency multiplication

The development of THz electron devices by coupling time-dependent electron quantum transport and electromagnetism is studied. A novel proposal for a frequency multiplier that generates a 1 THz harmonic from a 200 GHz input signal is described in detail with numerical simulations. The proposed electron device is a nanoscale double-gate field-effect transistor with a double barrier tunneling structure. Its functionality is based on dynamically modulated tunneling: a gate voltage forces the oscillation of the double barrier at frequencies comparable to the inverse of the electron transit time. This example points out the viability of using driven tunneling phenomena to develop room-temperature electron device for THz applications that can reduce the cost, the size and the complexity of present prototypes.

Simple models for electron and spin transport in barrier–conductor–barrier devices

We use two simple models, a phase accumulation model for quantum well (QW) resonances and a two-current circuit model, to study electron and spin transport in nanoscale systems consisting of barrier–conductor–barrier structures. Such structures include double barrier tunnel junctions, and long chain molecules with weak contacts to two electrodes. The phase accumulation model is applied to QW states in ultrathin films and nanodots sandwiched between two barriers. This model is used to illustrate the difference in the QW confinement effect between thin films and nanodots, and justifies circuit model which describes sequential tunneling. The circuit model for spin transport leads to a generalization of the well-known Julliere’s formula and accounts for the conductivity mismatch. The circuit model is also applied to organic light-emitting diodes and yields a simple formula for the quantum efficiency. The results yield excellent agreement with experiment.

Transparent organic light-emitting diodes using resonant tunneling double barrier structures

A semitransparent cathode of indium tin oxide (ITO)/Ag/ITO was developed as a resonant tunneling double barrier structure for transparent organic light-emitting diodes. A weak negative differential resistance was observed in devices using a 100nm thick ITO/Ag/ITO layer as a cathode in combination with a thin LiF∕Al layer. The current injection of devices was dominated by resonant tunneling, which induced no luminance at low voltage. This was achieved by employing an e-beam evaporated ITO/Ag/ITO cathode due to the double quantum barriers of ITO and the quantum well of Ag. The authors also applied the multilayer cathode to small molecule devices, which showed the same resonant tunneling currents.

CoFe/NiFeSiB/CoFe 자유층을 갖는 이중장벽 자기터널접합의 바이어스전압 의존특성

이 연구에서는 Ta 45/Ru 9.5/IrMn 10/CoFe <TEX>$3/AlO_x$</TEX>/자유층/<TEX>$AlO_x$</TEX>/CoFe 7/IrMn 10/Ru 60(nm) 구조를 갖는 이중장벽 자기터널접합(double-barrier magnetic tunnel junction: DMTJ)를 다루었다. 자유층은 <TEX>$Ni_{16}Fe_{62}Si_8B_{14}\;7nm$</TEX>, <TEX>$Co_{90}Fe_{10}(fcc)$</TEX> 7 nm 및 <TEX>$CoFet_1$</TEX>/NiFeSiB <TEX>$t_2$</TEX>/CoFe <TEX>$t_1$</TEX>으로 구성하였으며 두께 <TEX>$t_1,\;t_2$</TEX>는 변화시켰다. 즉 TMR비와 RA를 개선하기 위하여 부분적으로 CoFe층을 대체할 수 있는 비정질 NiFeSiB층이 혼합된 자유층 CoFe/NiFeSiB/CoFe을 갖는 DMTJ를 연구하였다. NiFeSiB(<TEX>$t_1=0,\;t_2=7$</TEX>)만의 자유층을 갖는 DMTJ는 터널자기저항(TMR)비 28%, 면적-저항곱(RA) <TEX>$86k{\Omega}{\mu}m^2$</TEX>, 보자력(<TEX>$H_c$</TEX>) 11 Oe 및 층간 결합장(<TEX>$H_i$</TEX>) 20 Oe를 나타내었다. <TEX>$t_1=1.5,\;t_2=4$</TEX>인 경우의 하이브리드 DMTJ는 TMR비 30%, RA <TEX>$68k{\Omega}{\mu}m^2$</TEX> 및 <TEX>$H_c\;11\;Oe$</TEX>를 가졌으나 <TEX>$H_i$</TEX>는 37 Oe로 증가하였다. 원자현미경(AFM)과 투과전자현미경(TEM)측정을 통하여 NiFeSiB층 두께가 감소하면 <TEX>$H_i$</TEX>가 증가하는 것을 확인하였다. 비정질 NiFeSiB층이 두꺼워지면 보통 계면의 기복을 유도하는 원주형성장(columnar growth)를 지연시키는데 유효하였다. 그러나 NiFeSiB층이 얇으면 표면거칠기는 증가하고 전자기적 Neel 결합 때문에 Hi는 커졌다. The typical double-barrier magnetic tunnel junction (DMTJ) structure examined in this paper consists of a Ta 45/Ru 9.5/IrMn 10/CoFe7/<TEX>$AlO_x$</TEX>/free layer/AlO/CoFe 7/IrMn 10/Ru 60 (nm). The free layer consists of an <TEX>$Ni_{16}Fe_{62}Si_8B_{14}$</TEX> 7 nm, <TEX>$Co_{90}Fe_{10}$</TEX> (fcc) 7 nm, or CoFe <TEX>$t_1$</TEX>/NiFeSiB <TEX>$t_2$</TEX>/CoFe <TEX>$t_1$</TEX> layer in which the thicknesses <TEX>$t_1$</TEX> and <TEX>$t_2$</TEX> are varied. The DMTJ with an NiFeSiB-free layer had a tunneling magnetoresistance (TMR) of 28%, an area-resistance product (RA) of <TEX>$86\;k{\Omega}{\mu}m^2$</TEX>, a coercivity (<TEX>$H_c$</TEX>) of 11 Oe, and an interlayer coupling field (<TEX>$H_i$</TEX>) of 20 Oe. To improve the TMR ratio and RA, a DMTJ comprising an amorphous NiFeSiB layer that could partially substitute for the CoFe free layer was investigated. This hybrid DMTJ had a TMR of 30%, an RA of <TEX>$68\;k{\Omega}{\mu}m^2$</TEX>, and a of 11 Oe, but an increased of 37 Oe. We confirmed by atomic force microscopy and transmission electron microscopy that increased as the thickness of NiFeSiB decreased. When the amorphous NiFeSiB layer was thick, it was effective in retarding the columnar growth which usually induces a wavy interface. However, if the NiFeSiB layer was thin, the roughness was increased and became large because of the magnetostatic <TEX>$N{\acute{e}}el$</TEX> coupling.

I - V characteristics of single electron tunneling from symmetric and asymmetric double-barrier tunneling junctions

I - V characteristics of single electron tunneling from a symmetric and an asymmetric double-barrier tunneling junction (DBTJ) were examined. A single Au nanoparticle was trapped in nanogap whose size was precisely controlled using a combination of electron beam lithography and molecular ruler technique. Though the symmetric junction showed a monotonic rise with a bias beyond the Coulomb gap voltage, the asymmetric junction showed Coulomb staircases. The capacitance of the junction estimated from the fitting curves using the Coulomb conventional theory was consistent with the capacitance calculated from the observed structure. The authors quantitatively found the correlation between the electrical and structural properties of DBTJ.

Spin tunnelling phenomena in single-crystal magnetic tunnel junction systems

A brief theoretical review points out the specific aspects of electronic transport insingle-crystal magnetic tunnel junctions employing bcc(100) Fe electrodes and a MgO(100)insulating barrier. The theoretical predictions are compared to the experimental reality inboth equilibrium and out-of-equilibrium regimes. For extremely small MgO thickness, weillustrate that the equilibrium tunnel transport in Fe/MgO/Fe systems leads toantiferromagnetic interactions. Artificial antiferromagnetic systems based on coupling byspin polarized tunnelling have been elaborated and studied. In the out-of-equilibriumregime and for large MgO barrier thickness, the tunnel transport validates specificspin filtering effects in terms of symmetry of the electronic Bloch function andsymmetry-dependent wavefunction attenuation in the single-crystal barrier. Within thisframework, we explain the experimental giant tunnel magnetoresistive effects at roomtemperature, up to 180%, measured in our simple or double barrier tunnel junctionsystems. Moreover, we illustrate that the magneto-transport properties of the junctionsmay be skilfully engineered by adjusting the interfacial chemical and electronic structure.

Electron transport across capped Au nanoclusters adsorbed in different configurations on highly oriented pyrolytic graphite substrate using scanning tunneling microscopy / spectroscopy

Comparison of electron transport across dodecanethiol capped Au nanoclusters adsorbed in different locations viz. in an agglomeration, on isolated particle either on the terrace or at the step edge of highly oriented pyrolytic graphite using scanning tunneling microscopy / spectroscopy is carried out. Current-distance measurements on these different locations shows different regimes suggesting different mechanisms for electron transport. Current-voltage measurements at these locations have been carried out and the results are discussed in the light of orthodox theory for electron transport in double-barrier tunnel junction.

Nanoscale voltage tunable tunnel rectifier by gold nanostructures embedded in SiO2

Conductive-atomic force microscopy is used to electrically characterize a nanometric metal-SiO2–Si (n+) system with Au nanocluster inclusions in SiO2. The system shows a marked rectifying behavior at room temperature with a threshold voltage function of the cluster size. This behavior is interpreted crossing physical considerations on metal-oxide-semiconductor structure and on double barrier tunnel junction device. The system fabricated and analyzed is proposed as a possible basic component for nanoelectronic circuits working at room temperature.

Verification of Au Nanodot Size Dependence on Coulomb Step Width by Non-contact Atomic-force Spectroscopy

We demonstrate single electron counting on an alkanethiol-protected Au nanodot in a double-barrier tunneling structure by noncontact atomic-force spectroscopy (nc-AFS). The Coulomb step width dependence on the Au nanodot diameter is observed. Evaluation of fractional charge Q 0 and contact potential difference by nc-AFS reveals a V d -independent voltage shift due to Q 0 .

Spin transfer torque in double barrier magnetic tunnel junctions

We present tight-binding calculations of the current-induced spin torque in double-barrier magnetic tunnel junctions (DBMTJ) using the non-equilibrium Keldysh formalism. The components of the local spin torque in the central ferromagnetic region, parallel, T ∥ , and perpendicular, T ⊥ , to the interface are strongly enhanced for certain thickness of the middle layer. The underlying mechanism for the spin torque enhancement is the relative position of the majority and minority quantum well states within the bias energy window around the Fermi level. Both T ∥ and T ⊥ exhibit oscillatory behavior in the middle FM region.

Structural and electrical characterization of gold nanoclusters in thin SiO2 films: Realization of a nanoscale tunnel rectifier

Conductive atomic force microscopy has been used to measure the I-V characteristics of nanometric Au clusters embedded in a SiO2 film prepared by sputter deposition and low temperature annealing. Highly local asymmetric rectifier I-V characteristics were evidenced and modelled in terms of electrical transport through an asymmetric double barrier tunnel junction SiO2/Au cluster/SiO2. The threshold voltage depends strongly on the cluster size and barrier thickness according to the model given.

Interfacial mixing in double-barrier magnetic tunnel junctions with amorphous NiFeSiB layers

Double-barrier magnetic tunnel junctions (DMTJs) comprising Ta 45/Ru 9.5/IrMn 10/CoFe 7/AlO x /free layer (CoFe 4/NiFeSiB 2/CoFe 4, CoFe 10, or NiFeSiB 10)/AlO x /CoFe 7/IrMn 10/Ru 60 (nm) have been examined with an emphasis given on understanding the interfacial mixing effects. The DMTJ, consisted of NiFeSiB, shows low switching field and low bias voltage dependence because the amorphous NiFeSiB has lower M S (=800 emu/cm 3) and offers smoother interfaces than polycrystalline CoFe. An interesting feature observed in the CoFe/NiFeSiB/CoFe sandwich free layered DMTJ is the presence of a wavy MR transfer curve at high-resistance region. Because the polycrystalline CoFe usually grows into a columnar structure, diamagnetic CoSi, paramagnetic FeSi, and/or diamagnetic CoB might have been formed during the sputter-deposition process. By employing electron energy loss spectrometry (EELS) and Auger electron spectroscopy (AES), we were able to confirm that Si and B atoms were arranged evenly in the top and bottom portions of AlO x /CoFe interfaces. This means that the interfacial mixing resulted in a distorted magnetization reversal process.

Influence of annealing on the bias voltage dependence of tunneling magnetoresistance in MgO double-barrier magnetic tunnel junctions with CoFeB electrodes

Double-barrier magnetic tunnel junctions with two MgO barriers and three CoFeB layers exhibiting tunneling magnetoresistance (TMR) values of more than 100% were fabricated. The bias voltage dependence of the TMR ratio is highly asymmetric after annealing at low temperatures, indicating dissimilar CoFeB∕MgO interfaces. The TMR effect decays very slowly for positive bias and is only reduced to half of its maximum value at V1∕2=1.88V when the junctions are processed at 200°C. The largest output voltage, 0.62V, is obtained after annealing at 300°C, a temperature that combines high TMR ratios with a considerable asymmetric bias dependence.

Bias Voltage Dependence of Magnetic Tunnel Junctions Comprising Double Barriers and CoFe/NiFeSiB/CoFe Free Layer

The typical double-barrier magnetic tunnel junction (DMTJ) structure examined in this paper consists of a Ta 45/Ru 9.5/IrMn 10/CoFe 7/AlOx/free layer/AlOx/CoFe 7/IrMn 10/Ru 60 (nm). The free layer consists of an Ni16Fe62Si8 B14 7 nm, Co90Fe10 (fcc) 7 nm, or CoFe t1/NiFeSiB t2/CoFe t1 layer in which the thicknesses t1 and t2 are varied. The DMTJ with an NiFeSiB-free layer had a tunneling magnetoresistance (TMR) of 28%, an area-resistance product (RA) of 86 k Omega mum2, a coercivity (Hc) of 11 Oe, and an interlayer coupling field (Hi) of 20 Oe. To improve the TMR ratio and RA, a DMTJ comprising an amorphous NiFeSiB layer that could partially substitute for the CoFe free layer was investigated. This hybrid DMTJ had a TMR of 30%, an RA of 68 k Omegamu m2, and a Hc of 11 Oe, but an increased Hi of 37 Oe. We confirmed by atomic force microscopy and transmission electron microscopy that Hi increased as the thickness of NiFeSiB decreased. When the amorphous NiFeSiB layer was thick, it was effective in retarding the columnar growth which usually induces a wavy interface. However, if the NiFeSiB layer was thin, the roughness was increased and Hi became large because of the magnetostatic Neacuteel coupling

Mechanism of enhanced rectification in unimolecular Borromean ring devices

We have studied charge transport through individual Borromean Ring complexes, both with and without anchor groups, in gated double barrier tunneling junctions (DBTJs) formed using the electrical breakjunction technique on gold nanowires. While common single molecule device characteristics can be observed with either form of the Borromean Rings, the complexes with anchor groups show strong rectification of conduction in a relatively high percentage of samples. We present our data along with a simple model underlining the mechanism by which the arrangement and composition of the weakly bonding anchor groups attached to the electroactive element may promote a device configuration resulting in rectification.

Probing Spin-Flip Scattering in Ballistic Nanosystems

Because spin-flip length is longer than the electron mean-free path in a metal, past studies of spin-flip scattering are limited to the diffusive regime. We propose to use a magnetic double barrier tunnel junction to study spin-flip scattering in the nanometer sized spacer layer near the ballistic limit. We extract the voltage and temperature dependence of the spin-flip conductance Gs in the spacer layer from magnetoresistance measurements. In addition to spin scattering information including the mean-free path (70 nm) and the spin-flip length (1.0-2.6 microm) at 4.2 K, this technique also yields information on the density of states and quantum well resonance in the spacer layer.

Bias voltage and temperature dependence of magneto-electric properties in double-barrier magnetic tunnel junction with amorphous Co-Fe-B electrodes

Bias voltage and temperature dependence of magneto-electric properties in double-barrier magnetic tunnel junctions(DBMTJs) with a structure of [IrMn/CoFe/Ru/CoFeB]/Al-O/CoFeB/Al-O/[CoFeB/Ru/CoFe/IrMn], have been investigated. The DBMTJs show a large tunnel magnetoresistance (TMR) ratio of up to 57.6%, a high V1/2 value of 1.26 V and small switching field Hc of 9.5 Oe at room temperature (RT). The TMR reaches the maximum at 30 K, about 89.0%, and decreases slightly from 30 to 4.2 K. A novel zero-bias anomaly (ZBA) in the P state is found and is temperature dependent, more sharply at low temperature, whereas a normal ZBA exists in the AP state. These effects are ascribed to magnon-, phonon- and impurity-assisted tunneling, and variation of density of states. The DBMTJ with a large TMR ratio, a high V1/2, and small switching field Hc is promising for developing the future spin electronic devices.

Absence of enhanced superconductivity in double-barrier superconducting tunnel junctions: Measurements of lateral electric transport in the middle normal-metal layer

The lateral conductivity of the middle Al layer of a $\mathrm{Nb}∕\mathrm{Al}∕\mathrm{Al}{\mathrm{O}}_{x}∕\mathrm{Al}∕\mathrm{Al}{\mathrm{O}}_{x}∕\mathrm{Al}∕\mathrm{Nb}$ (SINIS) multiterminal device is studied as a function of the current injected perpendicular to the layers in a regime where a gap-difference-like feature is observed in the current-voltage characteristic. The response of the Al layer does not confirm an earlier reported observation of superconductivity in the Al far above its transition temperature [Blamire et al., Phys. Rev. Lett. 66, 220 (1991)].

Bias voltage dependence of magnetic tunnel junctions comprising double barriers and amorphous NiFeSiB layers

A double-barrier magnetic tunnel junction (DMTJ) comprising an amorphous ferromagnetic NiFeSiB was investigated to reduce bias voltage dependence of tunneling magnetoresistance (TMR) ratio. The typical DMTJ structures were Ta45∕Ru9.5∕IrMn10∕CoFe7∕AlOx∕free layer7∕AlOx∕CoFe7∕IrMn10∕Ru60 (in nanometers). Various free layers such as CoFe 7, NiFeSiB 7, CoFe3.5∕NiFeSiB3.5, and NiFeSiB3.5∕CoFe3.5 were prepared and compared. The NiFeSiB-used DMTJ shows a low root-mean-square surface roughness of 0.17 nm, a resistance of about 860Ω, a Vh (voltage where the TMR ratio becomes half of its nonbiased value) of 1.1 V, and a high junction breakdown voltage of 2.0 V. The DMTJ with an amorphous NiFeSiB free layer offers smooth surface roughness resulting in reduced interlayer coupling field and bias voltage dependence.