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 well as electron incidence and spin-filtering by the barrier are described. We show that these mechanisms give rise to specific properties of spin transfer in tunnel junctions, very different from the case of metallic spin-valves. In the third part, the theoretical observable features of spin transfer in magnetic tunnel junctions are derived and the validity of these results is discussed and compared to recent experiments. To conclude this chapter, we study the mechanism of spin transfer in half-metallic tunnel junctions, expected to mimic MgO-based magnetic tunnel junctions.
Ferromagnetic spin-valves and tunneling junctions are crucial for spintronics applications and are one of the most fundamental spintronics devices. Motivated by the potential unique advantages of antiferromagnets for spintronics, we theoretically study here junctions built out of non-collinear antiferromagnets. We demonstrate a large and robust magnetoresistance and spin-transfer torque capable of ultrafast switching between parallel and anti-parallel states of the junction. In addition, we show that the non-collinear order results in a spin-transfer torque that is in several key aspects diffe…
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 $θ$.
A free electron description of spin-dependent tranport in magnetic tunnel junctions with non collinear magnetizations is presented. We investigate the origin of transverse spin density in tunnelling transport and the quantum interferences which give rise to oscillatory torques on the local magnetization. Spin transfer torque is also analyzed and an important bias asymmetry is found as well as a damped oscillatory behaviour. Furthermore, we investigate the influence of the s-d exchange coupling on torque in particular in the case of half-metallic MTJ in which the spin transfer torque is due to …
We theoretically study the current-induced spin torque in antiferromagnetic tunnel junctions, composed of two semi-infinite antiferromagnetic layers separated by a tunnel barrier, in both clean and disordered regimes. We find that the torque enabling the electrical manipulation of the Néel antiferromagnetic order parameter is out of plane $\sim {\bf n}\times{\bf p}$, while the torque competing with the antiferromagnetic exchange is in-plane $\sim {\bf n}\times({\bf p}\times{\bf n})$. Here, ${\bf { p}}$ and ${\bf { n}}$ are the Néel order parameter direction of the reference and free layers, re…
We study the adiabatic quantum pumping and quantum size effects of spin-torque in a magnetic tunnel junction within a scattering matrix approach. Quantum size effects are predicted in the presence of a dc bias as a function of the thickness of the normal metal layer inserted between two magnetic layers and of the fixed magnetic layer. In the presence of ac voltages, the results for the spin-torque show a peculiar magnification effect and advantages of spin-torque pumping in actual devices are also discussed.
Non-equilibrium spin transport in a magnetic tunnel junction comprising a single magnetic layer in the presence of interfacial spin-orbit interaction (SOI) is studied theoretically. The interfacial SOI generates a spin torque of the form {\bf T}=T_{||}{\bf M}x({\bf z}x{\bf M})+T_{\bot}{\bf z}x{\bf M}, even in the absence of an external spin polarizer. For thick and large tunnel barriers, the torque reduces to the perpendicular component, $T_{\bot}$, which can be electrically tuned by applying a voltage across the insulator. In the limit of thin and low tunnel barriers, the in-plane torque $T_{…
We study the current-induced torques in asymmetric magnetic tunnel junctions containing a conventional ferromagnet and a magnetic Weyl semimetal contact. The Weyl semimetal hosts chiral bulk states and topologically protected Fermi arc surface states which were found to govern the voltage behavior and efficiency of current-induced torques. We report how bulk chirality dictates the sign of the non-equilibrium torques acting on the ferromagnet and discuss the existence of large field-like torques acting on the magnetic Weyl semimetal which exceeds the theoretical maximum of conventional magnetic…
Spin transport in magnetic tunnel junctions comprising a single magnetic layer in the presence of interfacial spin-orbit interaction (SOI) is investigated theoretically. Due to the presence of interfacial SOI, a current-driven spin torque can be generated at the second order in SOI, even in the absence of an external spin polarizer. This torque possesses two components, in-plane and perpendicular to the plane of rotation, that can induce either current-driven {\em magnetization switching} from in-plane to out-of-plane configuration or {\em magnetization precessions}, similarly to Spin Transfer…
We study current-induced switching in magnetic tunnel junctions (MTJs) in the presence of a field-like spin-transfer torque and titled pinned-layer magnetization in the high current limit at finite temperature. We consider both the Slonczewski and field-like torques with coefficients $a_J$ and $b_J $, respectively. At finite temperatures, $σ= b_J/a_J = \pm1$ leads to a smaller mean switching time compared that with $σ= 0$. The reduction of switching time in the presence of the field-like term is due to the alignment effect (for $σ> 0$) and the initial torque effect.
We demonstrate the reduction of critical spin-transfer torque (STT) switching currents in Co-Fe-B/MgO based magnetic tunnel junctions (MTJ) with perpendicular magnetization anisotropy (PMA). The junctions yield tunnel magnetoresistance (TMR) ratios of up to 64% at 4 monolayer (ML) tunnel barrier thickness. In this paper, the reduction of the critical switching current density is studied. By optimizing the applied bias field during DC-STT measurements, ultra low critical switching current densities of less than 20 kA/cm$^2$, even down to 9 kA/cm$^2$, are found. With the reduced switching curren…
Subjecting a magnetic tunnel junction (MTJ) to a spin current and/or electric voltage induces magnetic precession, which can reciprocally pump current through the circuit. This results in an ac impedance, which is sensitive to the magnetic field applied to the MTJ. Measuring this impedance can be used to characterize the coupling between the magnetic free layer and the electric current as well as a read-out of the magnetic configuration of the MTJ.
We demonstrate that an all-antiferromagnetic tunnel junction with current perpendicular to the plane geometry can be used as an efficient spintronics device with potential high frequency operation. By using state-of-the-art density functional theory combined with quantum transport, we show that the Néel vector of the electrodes can be manipulated by spin-transfer torque. This is staggered over the two different magnetic sublattices and can generate dynamics and switching. At the same time the different magnetization states of the junction can be read by standard tunnelling magnetoresistance. C…
We report single-shot measurements of resistance versus time for thermally assisted spin-torque-driven switching in magnetic tunnel junctions. We achieve sufficient sensitivity to resolve the resistance signals leading up to switching, including the variations between individual switching events. Analyses of pre-switching thermal fluctuations allow detailed measurements of coherence times and variations in magnetization precession amplitude. We find that with a small in-plane hard-axis magnetic field the magnetization dynamics are more spatially coherent than for the case of zero field.
We study spin-dependent electron transport through a ferromagnetic-antiferromagnetic-normal metal tunneling junction subject to a voltage or temperature bias, in the absence of spin-orbit coupling. We derive microscopic formulas for various types of spin torque acting on the antiferromagnet as well as for charge and spin currents flowing through the junction. The obtained results are applicable in the limit of slow magnetization dynamics. We identify a parameter regime in which an unconventional damping-like torque can become comparable in magnitude to the equivalent of the conventional Sloncz…
There exists a significant challenge in developing efficient magnetic tunnel junctions with low write currents for non-volatile memory devices. With the aim of analysing potential materials for efficient current-operated magnetic junctions we have developed a multi-scale methodology combining the ab initio calculations of spin-transfer torque with large-scale time-dependent simulations using atomistic spin dynamics. In this work we introduce our multi-scale approach including a discussion on a number of possible mapping schemes the ab initio spin torques into the spin dynamics. We demonstrate …
We have investigated the current-induced spin transfer torque of a ferromagnet-insulator-ferromagnet tunnel junction by taking the spin-flip scatterings into account. It is found that the spin-flip scattering can induce an additional spin torque, enhancing the maximum of the spin torque and giving rise to an angular shift compared to the case when the spin-flip scatterings are neglected. The effects of the molecular fields of the left and right ferromagnets on the spin torque are also studied. It is found that $τ^{Rx}/I_{e}$ ($τ^{Rx}$ is the spin-transfer torque acting on the right ferromagnet…
We study ns scale spin-torque-induced switching in perpendicularly magnetized tunnel junctions (pMTJ). Although the switching voltages match with the macrospin instability threshold, the electrical signatures of the reversal indicate the presence of domain walls in junctions of various sizes. In the antiparallel (AP) to parallel (P) switching, a nucleation phase is followed by an irreversible flow of a wall through the sample at an average velocity of 40 m/s with back and forth oscillation movements indicating a Walker propagation regime. A model with a single-wall locally responding to the sp…
We simulate the spin torque-induced reversal of the magnetization in thin disks with perpendicular anisotropy at zero temperature. Disks typically smaller than 20 nm in diameter exhibit coherent reversal. A domain wall is involved in larger disks. We derive the critical diameter of this transition. Using a proper definition of the critical voltage, a macrospin model can account perfectly for the reversal dynamics when the reversal is coherent. The same critical voltage appears to match with the micromagnetics switching voltage regardless of the switching path.
Voltage-driven spin transfer torques in magnetic tunnel junctions provide an outstanding tool to design advanced spin-based devices for memory and reprogrammable logic applications. The non-linear voltage dependence of the torque has a direct impact on current-driven magnetization dynamics and on devices performances. After a brief overview of the progress made to date in the theoretical description of the spin torque in tunnel junctions, I present different ways to alter and control the bias dependence of both components of the spin torque. Engineering the junction (barrier and electrodes) st…
We analyse the appearance of a mechanical torque that acts on a chiral molecule: a single-stranded DNA, in which the spin-orbit interaction is expected to induce a spin-selectivity effect. The mechanical torque is shown to appear as a result of the non-conservation of the spin current in the presence of the spin-orbit interaction. Adopting a simple microscopic model Hamiltonian for a chiral molecule connected to source and drain leads, and accounting for the mechanical torque acting on the chiral molecule as the back action on the electrons traversing the molecule, we derive the spin continuit…
The dependence of tunneling magnetoresistance and spin-transfer torque in FeCo/MgO/FeCo tunnel junctions on the Co concentration and the bias voltage are investigated ab initio. We find that the tunneling magnetoresistance decreases with the Co concentration in contradiction with previous calculations but in agreement with recent experiments. This dependence is explained from bulk properties of the alloys. By using a realistic description of the disorder in the alloys we can show that even small amounts of disorder lead to a drastic drop in the tunneling magnetoresistance. This provides a quan…
The phenomenon of spin transfer torque (STT) has attracted a great deal of interests due to its promising prospects in practical spintronic devices. In this paper, we report a theoretical investigation of STT in a noncollinear magnetic tunnel junction under ac modulation based on the nonequilibrium Green's function formalism, and derive a closed-formulation for predicting the time-averaged STT. Using this formulation, the ac STT of a carbon-nanotube-based magnetic tunnel junction is analyzed. Under ac modulation, the low-bias linear (quadratic) dependence of the in-plane (out-of-plane) torque …
GaMnAs-based magnetic tunnel junction (MTJ) devices are characterized by in-plane and perpendicular-toplane magnetotransport at low temperatures. Perpendicular-to-plane transport reveals the typical tunneling magnetotransport (TMR) signal. Interestingly, a similar TMR signature is observed in the in-plane transport signal. Here, low-ohmic shunting of the MTJ by the top contact results in significant perpendicular-to-plane current paths. This effect allows the determination of TMR ratios of MTJs based on a simplified in-plane measurement. However, the same effect can lead to an inaccurate deter…
Future applications of spin-orbit torque will require new mechanisms to improve the efficiency for switching nanoscale magnetic tunnel junctions (MTJs), while also controlling the magnetic dynamics to achieve fast, nanosecond scale performance with low write error rates. Here we demonstrate a strategy to simultaneously enhance the interfacial magnetic anisotropy energy and suppress interfacial spin memory loss by introducing sub-atomic and monatomic layers of Hf at the top and bottom interfaces of the ferromagnetic free layer of an in-plane magnetized three-terminal MTJ device. When combined w…
The thermal spin-transfer torque (TSTT) is an effect to switch the magnetic free layer in a magnetic tunnel junction by a temperature gradient only. We present ab initio calculations of the TSTT. In particular, we discuss the influence of magnetic layer composition by considering $\text{Fe}_\text{x}\text{Co}_{\text{1-x}}$ alloys. Further, we compare the TSTT to the bias voltage driven STT and discuss the requirements for a possible thermal switching. For example, only for very thin barriers of 3 monolayers MgO a thermal switching is imaginable. However, even for such a thin barrier the TSTT is…
We propose the use of resonant tunneling as a route to enhance the spin-transfer torque switching characteristics of magnetic tunnel junctions. The proposed device structure is a resonant tunneling magnetic tunnel junction based on a MgO-semiconductor heterostructure sandwiched between a fixed magnet and a free magnet. Using the non-equilibrium Green's function formalism coupled self consistently with the Landau-Lifshitz-Gilbert-Slonczewski equation, we demonstrate enhanced tunnel magneto-resistance characteristics as well as lower switching voltages in comparison with traditional trilayer dev…
We report on a time-domain study of parametric synchronization in a magnetic tunnel junction based spin torque nano-oscillator (STNO). Time-domain measurements of the instantaneous frequency ($f_{i}$) of a parametrically synchronized STNO show random short-term unlocking of the STNO signal for low injected radio-frequency (RF) power, which cannot be revealed in time-averaged frequency domain measurements. Macrospin simulations reproduce the experimental results and reveal that the random unlocking during synchronization is driven by thermal fluctuations. We show that by using a high injected R…
We use pulsed inductive microwave magnetometry to study the precessional magnetization dynamics of the free layer in CoFeB/MgO/CoFeB based magnetic tunnelling junction stacks with varying MgO barrier thickness. From the field dependence of the precession frequency we are able to derive the uniaxial anisotropy energy and the exchange coupling between the free and the pinned layer. Furthermore the field dependence of the effective damping parameter is derived. Below a certain threshold barrier thickness we observe an increased effective damping for antiparallel orientation of free and pinned lay…
A three-dimensional self-consistent spin transport model is developed, which includes both tunnelling transport, as well as metallic transport. Using the spin accumulation computed either side of a tunnel barrier, spin torques are obtained, and it is shown the model reproduces both damping-like and field-like spin-transfer torques, with the expected sinusoidal angular dependence, and inverse ferromagnetic layer thickness dependence. An explicit solution to the drift-diffusion model is derived, which allows analysing the effect of both the reference and free layer thickness on the spin-transfer…
Voltage-driven spin transfer torque in a magnetic tunnel junction comprising magnetic insulating electrodes is studied theoretically. In contrast with the conventional magnetic tunnel junctions comprising transition metal ferromagnets, the spin transfer torque presents unconventional bias dependencies, related to the presence of spin-dependent Fowler-Nordheim tunneling processes. In particular, we find that (i) the out-of-plane torque generally dominates the in-plane torque, (ii) out-of-plane torque and in-plane torque are symmetric and asymmetric at low bias voltage, respectively, and (iii) b…
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 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.
The responses of the electrical current and the current-induced spin transfer torque (CISTT) to an ac bias in addition to a dc bias in a magnetic tunnel junction are investigated by means of the time-dependent nonquilibrium Green function technique. The time-averaged current (time-averaged CISTT) is formulated in the form of a summation of dc current (dc CISTT) multiplied by products of Bessel functions with the energy levels shifted by $m\hbar ω_{0}$. The tunneling current can be viewed as to happen between the photonic sidebands of the two ferromagnets. The electrons can pass through the bar…
The influence of structural asymmetries (barrier height and exchange splitting), as well as inelastic scattering (magnons and phonons) on the bias dependence of the spin transfer torque in a magnetic tunnel junction is studied theoretically using the free electron model. We show that they modify the "conventional" bias dependence of the spin transfer torque, together with the bias dependence of the conductance. In particular, both structural asymmetries and bulk (inelastic) scattering add {\em antisymmetric} terms to the perpendicular torque ($\propto V$ and $\propto j_e|V|$), while the interf…
Spin-polarized charge-currents induce magnetic tunnel junction (MTJ) switching by virtue of spin-transfer-torque (STT). Recently, by taking advantage of the spin dependent thermoelectric properties of magnetic materials, novel means of generating spin-currents from temperature gradients, and their associated thermal spin torques (TSTs) have been proposed, but so far these TSTs have not been large enough to influence MTJ switching. Here we demonstrate significant TSTs in MTJs by generating large temperature gradients across ultrathin MgO tunnel barriers that considerably affect the switching fi…
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
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…
We calculate the spin-transfer torque in Fe/MgO/Fe tunnel junctions and compare the results to those for all-metallic junctions. We show that the spin-transfer torque is interfacial in the ferromagnetic layer to a greater degree than in all-metallic junctions. This result originates in the half metallic behavior of Fe for the $Δ_1$ states at the Brillouin zone center; in contrast to all-metallic structures, dephasing does not play an important role. We further show that it is possible to get a component of the torque that is out of the plane of the magnetizations and that is linear in the bias…
We demonstrate a simple network-analyzer technique to make quantitative measurements of the bias dependence of spin torque in a magnetic tunnel junction. We apply a microwave current to exert an oscillating spin torque near the ferromagnetic resonance frequency of the tunnel junction's free layer. This produces an oscillating resistance that, together with an applied direct current, generates a microwave signal that we measure with the network analyzer. An analysis of the resonant response yields the strength and direction of the spin torque at non-zero bias. We compare to measurements of the …
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.
Temperature plays an important role in spin torque switching of magnetic tunnel junctions causing magnetization fluctuations that decrease the switching voltage but also introduce switching errors. Here we present a systematic study of the temperature dependence of the spin torque switching probability of state-of-the-art perpendicular magnetic tunnel junction nanopillars (40 to 60 nm in diameter) from room temperature down to 4 K, sampling up to a million switching events. The junction temperature at the switching voltage---obtained from the thermally assisted spin torque switching model---sa…
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…
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…
We present an implementation of the steady state Keldysh approach in a Green's function multiple scattering scheme to calculate the non-equilibrium spin density. This density is used to obtain the spin transfer torque in junctions showing the magnetoresistance effect. We use our implementation to study the spin transfer torque in metallic Co/Cu/Co junctions.
Understanding the magnetization dynamics induced by spin transfer torques in perpendicularly magnetized magnetic tunnel junction nanopillars and its dependence on material parameters is critical to optimizing device performance. Here we present a micromagnetic study of spin-torque switching in a disk-shaped element as a function of the free layer's exchange constant and disk diameter. The switching is shown to generally occur by 1) growth of the magnetization precession amplitude in the element center; 2) an instability in which the reversing region moves to the disk edge, forming a magnetic d…
Spin transport in magnetic tunnel junctions in the presence of spin diffusion is considered theoretically. Combining ballistic tunneling across the barrier and diffusive transport in the electrodes, we solve the spin dynamics equation in the metallic layers. We show that spin diffusion mixes the transverse spin current components and dramatically modifies the bias dependence of the effective spin transfer torque. This leads to a significant linear bias dependence of the out-of-plane torque, as well as a non-conventional thickness dependence of both spin torque components.
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…
Using a first-principles noncollinear wave-function-matching method, we studied the spin-transfer torques (STTs) in CoFe/Mg-B-O/CoFe(001) magnetic tunnel junctions (MTJs), where three different types of B-doped MgO in the spacer are considered, including B atoms replacing Mg atoms (Mg$_3$BO$_4$), B atoms replacing O atoms (Mg$_4$BO$_3$), and B atoms occupying interstitial positions (Mg$_4$BO$_4$) in MgO. A strong asymmetric angular dependence of STT can be obtained both in ballistic CoFe/Mg$_3$BO$_4$ and CoFe/Mg$_4$BO$_4$ based MTJs, whereas a nearly symmetric STT curve is observed in the junc…
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…
In this work we demonstrate explicit analytical expressions for both charge and spin currents which constitute the 2x2 spinor in magnetic tunnel junctions with noncollinear magnetizations under applied voltage. The calculations have been performed within the free electron model in the framework of the Keldysh formalism and WKB approximation. We demonstrate that spin/charge currents and spin transfer torques are all explicitly expressed through only three irreducible quantities, without further approximations. The conditions and mechanisms of deviation from the conventional sine angular depende…
The spin-torque in magnetic tunnel junctions possesses two components that both depend on the applied voltage. Here, we develop a new method for the accurate extraction of this bias-dependence from experiments over large voltage ranges. We study several junctions with different magnetic layer structures of the top electrode. Our results obtained on junctions with symmetric CoFeB electrodes agree well with theoretical calculations. The bias-dependences of asymmetric samples, with top electrodes containing NiFe, however, are twisted compared to the quadratic form generally assumed. Our measureme…
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…
