We theoretically study the recently observed tunnel-barrier-enhanced dc voltage signals generated by magnetization precession in magnetic tunnel junctions. While the spin pumping is suppressed by the high tunneling impedance, two complimentary processes are predicted to result in a sizable voltage generation in ferromagnet (F)|insulator (I)|normal-metal (N) and F|I|F junctions, with one ferromagnet being resonantly excited. Magnetic dynamics in F|I|F systems induces a robust charge pumping, translating into voltage in open circuits. In addition, dynamics in a single ferromagnetic layer develops longitudinal spin accumulation inside the ferromagnet. A tunnel barrier then acts as a nonintrusive probe that converts the spin accumulation into a measurable voltage. Neither of the proposed mechanisms suffers from spin relaxation, which is typically fast on the scale of the exponentially slow tunneling rates. The longitudinal spin-accumulation buildup, however, is very sensitive to the phenomenological ingredients of the spin-relaxation picture.
We investigate a wafer scale tunnel junction fabrication method, where a plasma etched via through a dielectric layer covering bottom Al electrode defines the tunnel junction area. The ex-situ tunnel barrier is formed by oxidation of the bottom electrode in the junction area. Room temperature resistance mapping over a 150 mm wafer give local deviation values of the tunnel junction resistance that fall below 7.5 % with an average of 1.3 %. The deviation is further investigated by sub-1 K measurements of a device, which has one tunnel junction connected to four arrays consisting of N junctions (…
Spin dependent single electron tunneling in ferromagnetic double junctions is analysed theoretically in the limit of sequential tunneling. The influence of discrete energy spectrum of the central electrode (island)on the spin accumulation, spin fluctuations and tunnel magnetoresistance is analysed numerically in the case of a nonmagnetic island. It is shown that spin fluctuations are significant in magnetic as well as in nonmagnetic junctions.
We study the effect of strain on magnetic tunnel junctions (MTJ) induced by a diamond like carbon (DLC) film. The junction resistance as well as the tunnel magnetoresistance (TMR) reduces with the DLC film. Non-equilibrium Green's function quantum transport calculations show that the application of biaxial strain increases the conductance for both the parallel and anti-parallel configurations. However, the conductance for the minority channel and for the anti-parallel configuration is significantly more sensitive to strain, which drastically increases transmission through a MgO tunnel barrier,…
Magnetic tunnel junctions with wide band gap semiconductor ZnTe barrier were fabricated. A very low barrier height and sizable magnetoresistance were observed in the Fe/ZnTe/Fe junctions at room temperature. The nonlinear I-V characteristic curve confirmed the observed magnetoresistance is due to spin-dependent tunneling effect. Temperature dependent study indicated that the total conductance of the junction is dominated by direct tunneling, with only a small portion from the hopping conduction through the defect states inside the barrier.
We investigated spin dependent transport through Fe/GaAs/Fe tunnel junctions. The tunneling magnetoresistance effect (TMR) was probed for different types of Fe/GaAs interfaces. For interfaces cleaned by hydrogen plasma the TMR effect is increased and observable at room temperature. If an epitaxial Fe/GaAs(001) interface is involved, the tunnel junction exhibits a bias dependent inversion of the TMR effect. This is a first experimental signature for band structure effects at a Fe/GaAs interface and relevant for spin injection experiments.
We study the evolution of perpendicularly magnetized tunnel junctions under 300 to 400 $^{\circ}$C annealing. The hysteresis loops do not evolve much during annealing and they are not informative of the underlying structural evolutions. These evolutions are better revealed by the frequencies of the ferromagnetic resonance eigenmodes of the tunnel junction. Their modeling provides the exchange couplings and the layers' anisotropies within the stack which can serve as a diagnosis of the tunnel junction state after each annealing step. The anisotropies of the two CoFeB-based parts and the two Co/…
Recently, several groups have reported spin-dependent thermoelectric effects in magnetic tunnel junctions. In this paper, we present a setup for time-resolved measurements of thermovoltages and thermocurrents of a single micro- to nanometer-scaled tunnel junction. An electrically modulated diode laser is used to create a temperature gradient across the tunnel junction layer stack. This laser modulation technique enables the recording of time-dependent thermovoltage signals with a temporal resolution only limited by the preamplifier for the thermovoltage. So far, time-dependent thermovoltage co…
We study the tunneling magneto thermo power (TMTP) in CoFeB/MgO/CoFeB magnetic tunnel junction nanopillars. Thermal gradients across the junctions are generated by a micropatterned electric heater line. Thermo power voltages up to a few tens of \muV between the top and bottom contact of the nanopillars are measured which scale linearly with the applied heating power and hence with the applied temperature gradient. The thermo power signal varies by up to 10 \muV upon reversal of the relative magnetic configuration of the two CoFeB layers from parallel to antiparallel. This signal change corresp…
Usually, graphene is used in its horizontal directions to design novel concept devices. Here, we report a single electron tunneling diode based on quantum tunneling through a vertical graphene two-barrier junction. The junction is formed by positioning a scanning tunnelling microscopy (STM) tip above a graphene nanoribbon that was deposited on a graphite surface. Because of the asymmetry of the two-barrier junction, the electrons can unidirectional transfer from the tip to the graphene nanoribbon but not from the graphene to the tip. This result opens intriguing opportunities for designing new…
The effects of the spin-orbit interaction on the tunneling magnetoresistance of ferromagnet/semiconductor/normal metal tunnel junctions are investigated. Analytical expressions for the tunneling anisotropic magnetoresistance (TAMR) are derived within an approximation in which the dependence of the magnetoresistance on the magnetization orientation in the ferromagnet originates from the interference between Bychkov-Rashba and Dresselhaus spin-orbit couplings that appear at junction interfaces and in the tunneling region. We also investigate the transport properties of ferromagnet/semiconductor/…
We examine the spin torque (ST) response of magnetic tunnel junctions (MTJs) with ultra-thin MgO tunnel barrier layers to investigate the relationship between the spin-transfer torque and the tunnel magnetoresistance (TMR) under finite bias. We find that the spin torque per unit current exerted on the free layer decreases by less than 10% over a bias range where the TMR decreases by over 40%. We examine the implications of this result for various spin-polarized tunneling models and find that it is consistent with magnetic-state-dependent effective tunnel decay lengths.
We study the tunneling magneto thermopower and tunneling magneto thermocurrent of CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJ). The devices show a clear change of the thermoelectric properties upon reversal of the magnetisation of the CoFeB layers from parallel to the antiparallel orientation. When switching from parallel to antiparallel the thermopower increases by up to 55% where as the thermocurrent drops by 45%. These observations can be well explained by the Onsager relations taking into account the tunneling magneto resistance of the MTJ. These findings contrast previous studies on Al…
We have fabricated a spin-polarized tunneling device based on half metallic manganites incorporating $Ba_{2}LaNbO_{6}$ as insulating barrier. An anomalous bias dependence of tunnel magnetoresistance (TMR) has been observed, the first of its kind in a symmetric electrode tunnel junction with single insulating barrier. The bias dependence of TMR shows an extremely sharp zero bias anomaly, which can be considered as a demonstration of the drastic density of states variation around the Fermi level of the half metal. This serves as a strong evidence for the existence of minority spin tunneling stat…
We report measurements of magnetic switching and steady-state magnetic precession driven by spin-polarized currents in nanoscale magnetic tunnel junctions with low-resistance, < 5 Ohm-micron-squared, barriers. The current densities required for magnetic switching are similar to values for all-metallic spin-valve devices. In the tunnel junctions, spin-transfer-driven switching can occur at voltages that are high enough to quench the tunnel magnetoresistance, demonstrating that the current remains spin-polarized at these voltages.
Inverse magnetoresistance has been observed in magnetic tunnel junctions with pinhole nanocontacts over a broad temperature range. The tunnel magnetoresistance undergoes a change of sign at higher bias and temperature. This phenomenon is attributed to the competition between the spin conserved ballistic transport through the pinhole contact where the transmission probability is close to unity and spin polarized tunneling across the insulating spacer with weak transmittivity.
Oscillatory tunneling magnetoresistance (TMR) as a function of spacer thickness is investigated theoretically for a magnetic tunnel junction with a nonmagnetic layer inserted between the tunnel barrier and the ferromagnetic layer. TMR is characterized in an analytical form, that is expressed with the transmission and reflection amplitudes of single interfaces at the Fermi level, and by the extremal wavevectors. Electronic structures with multiple bands are taken into account in the derivation characterizing the TMR, and the proposed analytical expression can be directly applied to real junctio…
The capacitance of MgO based magnetic tunnel junctions (MTJs) has been observed to be magnetic field dependent. We propose an equivalent circuit for the MTJs with a parallel-leaky capacitance (Cl) across the series combination of geometric and interfacial capacitance. The analysis of junctions with different tunneling magnetoresistance values suggests higher Cl for low TMR junctions. Using Cole-Cole plots the capacitive nature of MTJs is manifested. Fitting with Maxwell-Wagner capacitance model validates the RC parallel network model for MTJs and the extracted field dependent parameters match …
Direct and phonon-assisted tunneling currents in InAlGaAs-InGaAs bulk and double quantum well interband tunnel heterojunctions are simulated rigorously using the non-equilibrium Green's function formalism for coherent and dissipative quantum transport in combination with a simple two-band tight-binding model for the electronic structure. A realistic band profile and associated built-in electrostatic field is obtained via self-consistent coupling of the transport formalism to Poisson's equation. The model reproduces experimentally observed features in the current-voltage characteristics of the …
We investigate tunneling in metal-insulator-metal junctions employing few atomic layers of hexagonal boron nitride (hBN) as the insulating barrier. While the low-bias tunnel resistance increases nearly exponentially with barrier thickness, subtle features are seen in the current-voltage curves, indicating marked influence of the intrinsic defects present in the hBN insulator on the tunneling transport. In particular, single electron charging events are observed, which are more evident in thicker-barrier devices where direct tunneling is substantially low. Furthermore, we find that annealing th…
We report the observation of tunneling anisotropic magnetoresistance effect (TAMR) in the epitaxial metal-semiconductor system Fe/GaAs/Au. The observed two-fold anisotropy of the resistance can be switched by reversing the bias voltage, suggesting that the effect originates from the interference of the spin-orbit coupling at the interfaces. Corresponding model calculations reproduce the experimental findings very well.
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.
We study supercurrent and proximity vortices in a Josephson junction made of disordered surface states of a three-dimensional topological insulator with a proximity induced in-plane helical magnetization. In a regime where the rotation period of helical magnetization is larger than the junction width, we find supercurrent 0-π crossovers as a function of junction thickness, magnetization strength, and parameters inherent to the helical modulation and surface states. The supercurrent reversals are associated with proximity induced vortices, nucleated along the junction width, where the number of…
We theoretically investigate the angular and spin dependent transport in normal-metal/helical-multiferroic/ferromagnetic heterojunctions. We find a tunneling anisotropic magnetoresistance (TAMR) effect due to the spiral magnetic order in the tunnel junction and to an effective spin-orbit coupling induced by the topology of the localized magnetic moments in the multiferroic spacer. The predicted TAMR effect is efficiently controllable by an external electric field due to the magnetoelectric coupling.
Statistics of electron tunneling in normal tunnel junctions is studied analytically and numerically taking into account circuit (environment) effects. Full counting statistics, as well as full statistics of voltage and phase have been found for arbitrary times of observation. The theoretical analysis was based on the classical master equation, whereas the numerical simulations employed standard Monte-Carlo methods.
We calculate the contribution to the spin transfer torque from sequential tunneling through impurities in a magnetic tunnel junction. For a junction with weakly polarized ferromagnetic contacts, the torque is found to be in the plane spanned by the magnetizations of the ferromagnetic contacts and proportional to $\sin θ$, where $θ$ is the angle between the magnetic moments. If the polarization is larger, the torque acquires a significant out-of-plane component and a different dependence on $θ$.
1. Introduction 2. Description of the environment 3. Electron tunneling rates for single tunnel junctions 4. Examples of electromagnetic environments 5. Tunneling rates in Josephson junctions 6. Double junction and single electron transistor 7. Microscopic foundation
A ferroelectric tunnel barrier in between two ferromagnetic electrodes (multiferroic tunnel junction, MFTJ), is one of the most promising concepts for future microelectronic devices. In parallel, Hafnia based ferroelectrics are showing great potential for device miniaturization down to the nanoscale. Here we utilize ferroelectric Hf0.5Zr0.5O2 (HZO) with thickness of only 2 nm, epitaxially grown on La0.7Sr0.3MnO3 (LSMO) ferromagnetic electrodes, as a large band-gap insulating barrier integrated in MFTJs with cobalt top electrodes. As previously reported for other MFTJs with similar electrodes, …
Micrometer-sized Al/AlO$_{x}$/Y tunnel junctions were fabricated by the electron-beam lithography technique. The thin ($\approx$ 1.5--2 nm thickness) insulating AlO$_{x}$ layer was grown on top of the Al base electrode by O$_{2}$ glow discharge. The zero-bias conductances $G(T)$ and the current-voltage characteristics of the junctions were measured in a wide temperature range 1.5--300 K\@. In addition to the direct tunneling conduction mechanism observed in low-$G$ junctions, high-$G$ junctions reveal a distinct charge transport process which manifests the thermally fluctuation-induced tunneli…
This chapter presents a review on spin transfer torque in magnetic tunnel junctions. In the first part, we propose an overview of experimental and theoretical studies addressing current-induced magnetization excitations in magnetic tunnel junctions. The most significant results are presented and the main observable characteristics are discussed. A description of the mechanism of spin transfer in ferromagnets is finally proposed. In the second part, a quantum description of spin transport in magnetic tunnel junctions with amorphous barrier is developed. The role of spin-dependent reflections as…
We study influence of image forces on conductance of ferroelectric tunnel junctions. We show that the influence of image forces is twofold: i) they enhance the electro-resistance effect due to polarization hysteresis in symmetric tunnel junctions at non-zero bias and ii) they produce the electro-resistance effect due to hysteresis of dielectric permittivity of ferroelectric barrier. We study dependence of ferroelectric tunnel junction conductance on temperature and show that image forces lead to strong conductance variation with temperature.
The effects of the spin-orbit interaction on the tunneling magnetoresistance of magnetic tunnel junctions are investigated. A model in which the experimentally observed two-fold symmetry of the anisotropic tunneling magnetoresistance (TAMR) originates from the interference between Dresselhaus and Bychkov-Rashba spin-orbit couplings is formulated. Bias induced changes of the Bychkov-Rashba spin-orbit coupling strength can result in an inversion of the TAMR. The theoretical calculations are in good agreement with the TAMR experimentally observed in epitaxial Fe/GaAs/Au tunnel junctions.
We propose a model that describes current-in-plane tunneling transport in double barrier magnetic tunnel junctions in diffusive regime. Our study shows that specific features appear in double junctions that are described by introducing two typical length scales. The model may be used to measure the magnetoresistance and the resistance area product of both barriers in unpatterned stacks of double barrier magnetic tunnel junctions.
A multi-level Anderson model is employed to simulate the system of a nanostructure tunnel junction with any number of one-particle energy levels. The tunneling current, including both shell-tunneling and shell-filling cases, is theoretically investigated via the nonequilibrium Green's function method. We obtain a closed form for the spectral function, which is used to analyze the complicated tunneling current spectra of a quantum dot or molecule embedded in a double-barrier junction. We also show that negative differential conductance can be observed in a quantum dot tunnel junction when the C…
We show that the S/F/S Josephson $φ_0$-junction permits detection of macroscopic quantum tunneling and quantum oscillation of the magnetic moment by measuring the ac voltage across the junction. Exact expression for the tunnel splitting renormalized by the interaction with the superconducting order parameter is obtained. It is demonstrated that magnetic tunneling may become frozen at a sufficiently large $φ_0$. The quality factor of quantum oscillations of the magnetic moment due to finite Ohmic resistance of the junction is computed. It is shown that magnetic tunneling rate in the $φ_0$-junct…
We report a complementary metal oxide semiconductor (CMOS) technology compatible ferroelectric tunnel junction memristor grown directly on top of a Silicon substrate using a scandium doped aluminum nitride as the ferroelectric layer.
Spin filter tunnel junctions are based on selective tunneling of up and down spin electrons controlled through exchange splitting of the band structure of a ferromagnetic insulator. Therefore, spin filter efficiency can be tuned by adjusting exchange strength of the tunnel barrier. We have observed that magnetic field and bias voltage (current) can be used to regulate exchange strength and consequently spin-filter efficiency in tunnel junctions with ferromagnetic DyN and GdN tunnel barrier. In tunnel junctions with DyN barrier we obtained $\sim$37$\%$ spin polarization of tunneling electrons a…
The Ueda-Guinea model of a dissipative tunnel junction is investigated. This model accounts for final state effects associated with single-electron tunneling. A quantum phase transition emerges, marking a boundary between insulating (Coulomb blockade) and conducting phases. The system is analyzed by large-N techniques, self-consistent harmonic approximation, and Monte Carlo methods.
Thermally induced spin-dependent transport across magnetic tunnel junctions is theoretically investigated. We analyze the thermal analog of Slonczewski's model (as well as its limiting case---Julliere's model) of tunneling magnetoresistance and obtain analytical expressions for the junction thermopower and the tunneling magneto-thermopower (TMT). The analytical model is tested numerically for the special case of an Al$_2$O$_3$-based MTJ, for which we analyze the dependence of the thermopower and TMT on the relative magnetization orientations, as well as on the barrier height and thickness. We …
The influence of ballistic channels superimposed on tunneling conduction channels in magnetic tunnel junctions has been studied in a manganese oxide based tunneling device. Inversion of magnetoresistance has been observed in magnetic tunnel junctions with pinhole nanocontacts over a broad temperature range. The tunnel magnetoresistance undergoes a change of sign at higher bias and temperature. This phenomenon is attributed to the parallel conduction channels consisting of spin conserved ballistic transport through the pinhole contact where the transmission probability is close to unity and spi…
The paper investigates theoretically effects of noise on low-bias parts of $IV$ curves of tunnel junctions. The analysis starts from the effect of shot noise from an additional (noise) junction on the Coulomb blockaded Josephson junction in high-impedance environment. Asymmetry of shot noise characterized by its odd moments results in asymmetry of the $IV$ curve of the Josephson junction. At high currents through the noise junction the $IV$ curve is very sensitive to electron counting statistics. The theory is generalized on another type of noise (phase noise of a monochromatic AC input), on a…
Udalov and Beloborodov in the recent papers [Phys. Rev. B 95, 134106 (2017); Phys. Rev. B 96, 125425 (2017)] report the strong influence of image forces on the conductance of ferroelectric tunnel junctions. In particular, the authors state that there is enhancement of the electroresistance effect due to polarization hysteresis in symmetric tunnel junctions at nonzero bias. This conjecture seems to be a breakthrough --- the common knowledge is that the considerable effect, linear over voltage bias, takes place only in NONsymmetric junctions. We show that the influence of image forces on the con…
Typical measurements of nanowire devices rely on end-to-end measurements to reveal mesoscopic phenomena such as quantized conductance or Coulomb blockade. However, creating nanoscale tunnel junctions allows one to directly measure other properties such as the density of states or electronic energy distribution functions. In this paper, we demonstrate how to realize uniform tunnel junctions on InSb nanowires, where the low invasiveness preserves ballistic transport in the nanowires. The utility of the tunnel junctions is demonstrated via measurements using a superconducting tunneling probe, whi…
We report about the fabrication and analysis of the properties of Cr/CrO_x/Cr tunnel junctions and SET transistors, prepared by different variants of direct-writing multilayer technique. In all cases, the CrO_x tunnel barriers were formed in air under ambient conditions. From the experiments on single junctions, values for the effective barrier height and thickness were derived. For the Cr/CrO_x/Cr SET transistors we achieved minimal junction areas of 17 x 60 nm^2 using a scanning transmission electron microscope for the e-beam exposure on Si_3N_4 membrane substrate. We discuss the electrical …
Significant scientific and technological progress in the field of spintronics is based on trilayer magnetic tunnel junction devices which principally rely on the physics of single barrier tunneling. While technologically relevant devices have been prototyped, the physics of single barrier tunneling poses ultimate limitations on the performance of magnetic tunnel junction devices. Here, we propose a fresh route toward high performance magnetic tunnel junctions by making electronic analogs of optical phenomena such as anti-reflections and Fabry-Pèrot resonances. The devices we propose feature an…
We describe the concept and experimental demonstration of primary thermometry based on a four probe measurement of a single tunnel junction embedded within four arrays of junctions. We show that in this configuration random sample specific and environment-related errors can be avoided. This method relates temperature directly to Boltzmann constant, which will form the basis of the definition of temperature and realization of official temperature scales in the future.
We theoretically investigate the spin--wave spin current induced by the spin Seebeck effect in magnonic spin tunneling junctions (MSTJs) for arbitrary magnetization directions. We show that the MSTJ functions as a \textit{tunable} spin Seebeck diode in which the tunneling spin current can be turned on and off with high efficiency by controlling the magnetization direction.
We investigate the micromechanical exfoliation and van der Waals (vdW) assembly of ferromagnetic layered dichalcogenide Fe0.25TaS2. The vdW interlayer coupling at the Fe-intercalated plane of Fe0.25TaS2 allows exfoliation of flakes. A vdW junction between the cleaved crystal surfaces is constructed by dry transfer method. We observe tunnel magnetoresistance in the resulting junction under an external magnetic field applied perpendicular to the plane, demonstrating spin-polarized tunneling between the ferromagnetic layered material through the vdW junction.
We have investigated the mechanism at the origin of the infra-red radiation emitted by a biased tunnel junction by detecting photons at frequencies $ν<eV/h$. To address this regime, the bias voltage $V$ exceeds one volt and the potential profile of the tunnel barrier is driven far from its equilibrium state. As a consequence, the $I(V)$ characteristic of the junction is strongly nonlinear. At optical frequencies, the transport through the junction cannot be simply expressed in term of the dc current and the current fluctuations are no longer described by the fluctuation-dissipation relation. T…
Tunneling hetero-junctions (THJs) usually induce confined states at the regions close to the tunnel junction which significantly affect their transport properties. Accurate numerical modeling of such effects requires combining the non-equilibrium coherent quantum transport through tunnel junction, as well as the quasi-equilibrium statistics arising from the strong scattering in the induced quantum wells. In this work, a novel atomistic model is proposed to include both effects: the strong scattering in the regions around THJ and the coherent tunneling. The new model matches reasonably well wit…
We propose tunnel junctions of a Hall bar and a superconducting lead, for observing Cooper-pair tunneling into singlet fractional quantum Hall edge states. These tunnel junctions provide a natural means of extracting precise information of the spin polarization and the filling factor of the state. The low energy regime of one of the set-ups is governed by a novel quantum entangled fixed point.
This letter presents experiments on junctions fabricated by a new technique that enables the use of high quality aluminum oxide tunnel barriers with normal metal electrodes at low temperatures. Inverse proximity effect is applied to diminish the superconductivity of an aluminum dot through a clean lateral connection to a normal metal electrode. To demonstrate the effectiveness of this method, fully normal-state single electron transistors (SET) and normal metal-insulator-superconductor (NIS) junctions applying proximized Al junctions were fabricated. The transport characteristics of the juncti…
Temperature dependence of the tunnel magnetoresistance (TMR) was calculated in range of the quantum-ballistic model in the magnetic tunnel junctions (MTJs) with embedded nanoparticles (NPs). The electron tunnel transport through NP was simulated in range of double barrier approach, which was integrated into the model of the magnetic point-like contact. The resonant TMR conditions and temperature impact were explored in detail. Moreover, the possible reasons of the temperature induced resonant conditions were discussed in the range of the lead-tunneling cell-lead model near Kondo temperature. W…
Spin-coherent quantum transport in carbon nanotube magnetic tunnel junctions was investigated theoretically. A spin-valve effect is found for metallic, armchair tubes, with a magneto-conductance ratio ranging up to 20%. Because of the finite length of the nanotube junctions, transport is dominated by resonant transmission. The magnetic tunnel junctions are found to have distinctly different transport behavior depending on whether or not the length of the tubes is commensurate with a 3N+1 rule, with N the number of basic carbon repeat units along the nanotube length.
Organic molecules have recently revolutionized ways to create new spintronic devices. Despite intense studies, the statistics of tunneling electrons through organic barriers remains unclear. Here we investigate conductance and shot noise in magnetic tunnel junctions with PTCDA barriers a few nm thick. For junctions in the electron tunneling regime, with magnetoresistance ratios between 10 and 40\%, we observe superpoissonian shot noise. The Fano factor exceeds in 1.5-2 times the maximum values reported for magnetic tunnel junctions with inorganic barriers, indicating spin dependent bunching in…
We demonstrate a local strain sensing method for nanostructures based on metallic Al tunnel junctions with AlOx barriers. The junctions were fabricated on top of a thin silicon nitride membrane, which was actuated with an AFM tip attached to a stiff cantilever. A large relative change in the tunneling resistance in response to the applied strain (gauge factor) was observed, up to a value 37. This facilitates local static strain variation measurements down to ~10^{-7}.
In this contribution, we calculate the spin-dependent ballistic and coherent transport through epitaxial Fe/ZnSe (001) simple and double magnetic tunnel junctions with two different interface terminations: Zn-terminated and Se-terminated. The electronic structure of the junctions is modeled by a second-nearest neighbors {\it spd} tight-binding Hamiltonian parametrized to {\it ab initio} calculated band structures, while the conductances and the tunneling magnetoresistance are calculated within Landauer's formalism. The calculations are done at zero bias voltage and as a function of energy. We …
We derive an expression for the spin-current through a tunnel barrier in terms of many-body Green's functions. The spin current has two contributions. One can be associated with angular-momentum transfer by spin-polarized charge currents crossing the junction. If there are magnetic moments on both sides of the tunnel junction, due to spin accumulation or ferromagnetic ordering, then there is a second contribution related to the exchange coupling between the moments.
The Tunneling Anisotropic Magneto-Thermopower (TAMT) and the Tunneling Anisotropic Spin-Seebeck (TASS) effects are studied for a magnetic tunnel junction (MTJ) composed of a ferromagnetic electrode, a zinc-blende semiconductor and a normal metal. We develop a theoretical model for describing the dependence of a thermally induced tunneling current across the MTJ on the in-plane orientation of the magnetization in the ferromagnetic layer. The model accounts for the specific Bychkov-Rashba and Dresselhaus spin-orbit interactions present in these systems, which are responsible for the $C_{2v}$ sym…
We study current fluctuations in tunnel junctions driven by a voltage source. The voltage is applied to the tunneling element via an impedance providing an electromagnetic environment of the junction. We use circuit theory to relate the fluctuations of the current flowing in the leads of the junction with the voltage fluctuations generated by the environmental impedance and the fluctuations of the tunneling current. The spectrum of current fluctuations is found to consist of three parts: a term arising from the environmental Johnson-Nyquist noise, a term due to the shot noise of the tunneling …
