Single Tunnel Junction Research Articles

Field detection in single and double barrier MgO magnetic tunnel junction sensors

Single and double barrier MgO based magnetic tunnel junction sensors were processed with area of 200μm2 and small aspect ratios (<2). These sensors present resistance area products ranging from 10to70kΩμm2 and tunneling magnetoresistance (TMR) values of up to 160%. Sensor linearization was mostly obtained by thinning the CoFeB free layer to 15.5A, close to the free layer transition from ferromagnetic to superparamagnetic behavior. Three different thicknesses were studied for the free layer CoFeB: 1.55, 1.8, and 3nm. The CoFeB thickness decrease required for the linearization implies a loss in TMR. Field detection range of ∼80pT∕Hz0.5 was obtained for both double and single barrier sensors (at 500KHz). Out of the 1∕f regime, the detection range decreases to ∼2pT∕Hz0.5 for the single barrier sensor. The field detection dependence on bias voltage was also studied at 500KHz and outside the 1∕f dominated regime.

Large oscillating tunnel magnetoresistance in ferromagnetic graphene single tunnel junction

Spin-polarized transports of relativistic electrons through graphene-based ferromagnet/insulator/ferromagnet (FG/IG/FG) single junctions have been investigated theoretically. Large oscillating tunnel magnetoresistance (TMR) has been found in monolayer and bilayer FG/IG/FG junctions. The oscillating amplitudes of TMR do not decrease with the increase of the thickness and the height of barrier, in contrast to the exponential decay in conventional ferromagnet/insulator/ferromagnet single junction. The physical origin for such a phenomenon has also been analyzed. It is anticipated to apply such a phenomenon to design the spin-polarized electron device based on the graphene materials.

Transmission electron microscopy study on the polycrystalline CoFeB∕MgO∕CoFeB based magnetic tunnel junction showing a high tunneling magnetoresistance, predicted in single crystal magnetic tunnel junction

We carried out a detailed transmission electron microscopy study on the polycrystalline CoFeB∕MgO∕CoFeB based magnetic tunnel junction, which shows high magnetoresistance predicted theoretically only in single crystal magnetic tunnel junction. Previously it has been suggested that the crystallinity and degree of texture of MgO tunnel barrier play a crucial role in the polycrystalline system. Here we suggest, on top of the crystallinity issue of MgO, that a grain-to-grain epitaxy of CoFeB∕MgO∕CoFeB satisfies the prerequisite for the coherent tunneling for the giant tunneling magnetoresistance theoretically predicted.

Interlayer Exchange Coupling in Single Crystal Magnetic Tunnel Junctions Studied by Electron Holography

AbstractIon irradiation has been performed on epitaxial Fe/MgO/Fe magnetic tunnel junctions exhibiting antiferromagnetic exchange coupling. Electron holography and VSM experiments show that the amplitude and the nature of the coupling are modified by the irradiation. We demonstrate that the roughness induced by the irradiation at the Fe/MgO interfaces destroys the AF coupling. The biquadratic coupling experimentally observed in the irradiated samples is the result of the competition between local antiferromagnetic and ferromagnetic coupling. For the largest irradiation doses, the net coupling becomes ferromagnetic.

Shot Noise in Magnetic Tunnel Junctions: Evidence for Sequential Tunneling

We report the experimental observation of sub-Poissonian shot noise in single magnetic tunnel junctions, indicating the importance of tunneling via impurity levels inside the tunnel barrier. For junctions with weak zero-bias anomaly in conductance, the Fano factor (normalized shot noise) depends on the magnetic configuration being enhanced for antiparallel alignment of the ferromagnetic electrodes. We propose a model of sequential tunneling through nonmagnetic and paramagnetic impurity levels inside the tunnel barrier to qualitatively explain the observations.

Writing properties of clad lines in a toggle MRAM

We examined the magnetic and writing properties of clad writing lines for magnetic random access memory (MRAM) devices. After fabricating them, we applied a magnetic field of 1.2 T to them at room temperature. We call this operation initialization. Initialization reduced the flop current deviation of the single magnetic tunnel junction toggle cells. We fabricated a 4-Mbit toggle memory array with the clad lines, using initialization. It exhibited a large operating margin and a reduced switching current.

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.

Six-Layer Process for the Fabrication of<tex>$rm Nb/Al-rm AlO_x/rm Nb$</tex>Josephson Junction Devices

A six-layer process is described for the fabrication of Josephson junction devices and Superconducting QUantum Interference Devices (SQUIDs) on silicon wafers up to 150 mm in diameter. The Nb/Al-AlO/sub x//Nb trilayers are sputter-deposited in a load-locked vacuum chamber with base pressure in the low 10/sup -7/ Pa (10/sup -9/ Torr) range and a very low leak up rate of 5/spl times/10/sup -6/ Pa/min (3.5/spl times/10/sup -5/ mTorr/min). The barrier process is optimized for a critical current density of 100 A/cm/sup 2/, and all resistors are fabricated from palladium with a sheet resistivity of 1 /spl Omega//sq. A charge dissipative dielectric layer is used in order to reduce the probability of damage caused by electrostatic charge build-up and discharge. The trilayer films are wet etched, while the subsequent metal and dielectric films are patterned using fluorine-based reactive ion etch, Ar ion milling, or lift-off techniques. This process is currently being used to fabricate dc SQUIDs for various instrumentation applications, integrated SQUID magnetometers and planar gradiometers, and single tunnel junction detectors.

Optical and transport studies of single molecule tunnel junctions based on self-assembled monolayers

We have fabricated a variety of novel molecular tunnel junctions based on self-assembled-monolayers (SAM) of two-component solid-state mixtures of molecular wires (1,4-methane benzene-dithiol; Me-BDT with two thiol anchoring groups), and molecular insulator spacers (1-pentanethiol; PT with one thiol anchoring group) at different concentration ratios, r of wires/spacers, which were sandwiched between two metallic electrodes such as gold and cobalt. FTIR spectroscopy and surface titration were used, respectively, to verify the formation of covalent bonds with the electrodes, and obtain the number of active molecular wires in the device. The electrical transport properties of the SAM devices were studied as a function of (i) r-value, (ii) temperatures, and (iii) different electrodes, via the conductance and differential conductance spectra. The measurements were used to analyze the Me-BDT density of states near the electrode Fermi level, and the properties of the interface barriers. We measured the Me-BDT single molecule resistance at low bias and gold electrodes to be 6×10 9 Ω. We also determine the energy difference, Δ between the Me-BDT HOMO level and the gold Fermi level to be about 1.8 eV. In addition we also found that the temperature dependence of the SAM devices with r<10 −4 is much weaker than that of the pure PT device (or r=0), showing a small interface barrier.

Flux-gate like 2D magnetometer based on a single magnetic tunnel junction

The pick-up coil of a conventional flux-gate sensor has been replaced by a magnetic tunnel junction. With this respect, the magnetic properties of tunnel junctions have been optimized to be relevant to flux-gate operation i.e. low-coercitive field and reproducibility of the dynamic switchings. Furthermore, the use of an alternative two-dimensional magnetic driving field extends the use of the flux-gate like sensor to make a 2D magnetometer with a single magnetic tunnel junction without any cross sensitivity. A first demonstration is given with sensitivity above 500 V/T and a noise level evaluated to 2 with an optimized magnetoresistive response.

The simulation of single-charging effects in the programming characteristics of nanocrystalmemories

We present a Monte Carlo simulation and modelling method for single-electron nanocrystalmemories including the nanocrystal inter-dot effect. A nanocrystal memory is modelled as anetwork of modified single-tunnel junctions for its programme operation andas a network of channel resistances for simulating drain current to extract itsthreshold voltage. Additionally, in each modelling, capacitive coupling betweeneach geometrical element was determined using a device simulator. The resultsshow that nanocrystal memory parameters are closely related to both static anddynamic programming characteristics, especially in the single-charging property.When the inter-dot spacing is large, a step-like single-charging feature becomessalient, but the threshold voltage shift between each state decreases. In contrast,when the dots are spatially closer, the threshold voltage shift between each stateincreases, while the step-like shape vanishes and resembles that of conventional flashmemories. From the dynamic viewpoint, a closer spacing of dots makes it easier tocharge the nanocrystal memory quantum dots more uniformly. The presentedmethod will be helpful for the design of single-electron nanocrystal memories.

Monte Carlo Simulation of Single-Electron Nanocrystal Memories

We present a Monte Carlo simulation method for single-electron nanocrystal memories including the nanocrystal interdot effect, and show the simulation results obtained. A nanocrystal memory was modeled as a network of modified single-tunnel junctions for its program operation and as a network of channel resistances for simulating drain current to extract its threshold voltage. The results show that, in programming characteristics, device parameters have mutual trade-off relationships in the clearness of steplike single-charging features and in the magnitude of threshold voltage shifts. The presented method will be helpful for the design of single-electron nanocrystal memories.

Intrinsic thermally compensated field sensor based on single magnetic tunnel junctions

The association of an antiferromagnetic material with a ferromagnetic material in an exchange coupled bilayer is used as a detection layer in a tunnel magnetoresistive sensor. The magnetic response is shown to be reversible and linear depending on the field range to be measured. Both tunnel magnetoresistance and exchange field decrease linearly with the temperature and lead to temperature independent sensor sensibility.

Low-current blocking temperature writing of double barrier magnetic random access memory cells

A magnetic random access memory cell architecture is fabricated where the pinned layer is reversed by heating above a reduced blocking temperature with a current pulse crossing the junction, and cooled under an external applied field (word line), minimizing half-select switching of nonaddressed bits. In order to improve Joule heating and increase breakdown voltage, a double barrier structure was used, with a common antiferromagnetic layer (60 Å MnIr) two pinned 30 Å CoFe layers, and two free layers incorporating nanooxide structures. The blocking temperature was reduced to 120 °C. A TMR of 25% was achieved for both single barrier and double barrier tunnel junctions with resistance×area products of ∼40 Ω×μm2 and ∼280 Ω×μm2, respectively. Pinned layer writing allows the definition of a three-state memory, requiring, however, a destructive read out. A significant improvement of writing efficiency is observed with the double barrier structure. A 10 ns current pulse of 9 mA/μm2 is sufficient to heat the double barrier junctions above the blocking temperature and induce pinned layer switching.

Nb/NiCu bilayers in single and stacked superconductive tunnel junctions: preliminary results

Abstract We present preliminary experimental results concerning both single and stacked tunnel junctions in which one of the electrodes was formed by a superconductor/ferromagnet (S/F) bi-layer. In particular, in the stacked configuration a Nb/NiCu bi-layer was used as the intermediate electrode, and it was probed by tunneling on both sides. Tunnel junctions have been characterized in terms of current–voltage characteristics (IVC), and differential conductance. Preliminary steady-state injection–detection measurements performed in the stacked devices at T=4.2 K are also presented and discussed.

スピン二重トンネル接合の磁気抵抗効果

Magnetic double tunnel junctions (DTJs) have been investigated to achieve weak bias voltage dependence and to obtain large magnetoresistance (MR) ratio at several hundred mV. IrMn/Co90Fe10/AlOx/Co90Fe10/AlOx/Co90Fe10/IrMn structures were prepared as dual-spin-valve-type DTJs, which possess the middle ferromagnetic layers consisting of both continuous and discontinuous Co90Fe10 films. For the DTJs with continuous middle Co90Fe10 layers, the bias voltage value which MR ratio reduces by half (Vh) was about 880 mV, while that of single tunnel junctions (STJs) was about 450 mV. For the DTJs with discontinuous middle Co90Fe10 layers, it was found that the resistance increased remarkably and the magnetic field response decreased in MR curves. The discontinuous Co90Fe10 layers showed superparamagnetic behavior in magnetization curves.

Fabrication of metallic tunnel junctions for the scanning single electron transistor atomic force microscope

Metallic tunnel junctions are important in the formation of high temperature single electron devices, which can act as the ultimate electrometer. We present a method for the fabrication of highly defined metallic tunnel junctions based on the step-edge cutoff process. Fabrication involves conventional electron beam lithography and lift-off of metallic thin films. Junctions scaling down to 50 nm linewidth have been achieved. The devices show a spread in impedance at low bias ranging from less than 10 MΩ to more than 100 GΩ. We have investigated the behavior of thin metallic films across a step forming a single tunnel junction. In the case of palladium we find that grain growth during deposition can give rise to multiple junctions across a single step. We illustrate this using one particular example of a junction, where isolated grains form a spontaneous Coulomb blockaded island with a charging energy of 20 meV at 77 K. The single and double junction fabrication process is compatible with our generic atomic force microscope probe technology, which has been shown to be capable of defining ultrasmall metallic structures on cantilevers and pyramidal tips. We demonstrate this by the fabrication of a lithographically defined device on a silicon cantilever with an integrated Ti/Au thin-film strain gauge.

Tunneling into multiwalled carbon nanotubes: coulomb blockade and the Fano resonance.

Tunneling spectroscopy measurements of single tunnel junctions formed between multiwalled carbon nanotubes (MWNTs) and a normal metal are reported. Intrinsic Coulomb interactions in the MWNTs give rise to a strong zero-bias suppression of a tunneling density of states that can be fitted numerically to the environmental quantum-fluctuation theory. An asymmetric conductance anomaly near zero bias is found at low temperatures and interpreted as Fano resonance in the strong tunneling regime.

TMR of double spin-valve type AF/FM/I/FM/I/FM/AF magnetic tunneling junctions

An unusually large enhancement of TMR at 77 K was observed in double barrier tunnel junctions (DBTJ). This is explained with extended Julliere's model which yields a twice larger TMR value. When the spin coherence length is much smaller at higher temperature, DBTJ is shown to work as a series of two single barrier tunnel junctions.

Mesoscopic phenomena in nano-porous alumina films: single nano-tunnel junctions connected to Ni-nanowires and carbon nanotubes

We report on two mesoscopic phenomena observable in nano-porous alumina films. One is Coulomb blockade associated with weak localization in an array of single nano-tunnel junctions/multi-walled carbon nanotubes (MWNTs), and the other is an abrupt magnetoresistance (MR) jump in Ni-nanowire system. Weak localization yields CB in single tunnel junction system by acting as high impedance external electromagnetic environment, following phase correlation theory. The CB is very sensitive to phase modulation in the MWNTs. An abrupt MR jump is associated with magnetic domain walls. Its correlation with macroscopic quantum tunneling and weak localization is discussed.