Transport In Magnetic Tunnel Junctions Research Articles

Electron conduction mechanisms in magnetic tunnel junctions fabricated using amorphous Si-Zn-Sn-O as a low-resistive semiconducting barrier

CoFeB/ Si-Zn-Sn-O /CoFeB magnetic tunnel junctions (MTJs) have been fabricated using amorphous Si-Zn-Sn-O as a low-resistive semiconducting barrier. In the low bias voltage range (up to ∼0.2 V), direct tunneling is found to be the dominant transport mechanism in MTJs. Tunneling conduction is further verified by simulation of tunnel current density and differential conductance using Simmon’s and Brinkmann model, respectively. Simulated results provided valuable insights into the barrier properties, including interfacial barrier height, thickness, and barrier asymmetry. Above the direct tunneling regime, electron transport in MTJs is governed by Pool Frenkel emission, which possibly arises due to the presence of high-density localized tail states below the conduction band of amorphous Si-Zn-Sn-O. Tunnelling magnetoresistance value of MTJs is found to be very low, which is attributed to the presence of various inelastic conduction channels. The results of this study might be useful to explore the potential of amorphous Si-Zn-Sn-O for fabricating low-resistive MTJ based spintronic devices.

Efficient solution strategy to couple micromagnetic simulations with ballistic transport in magnetic tunnel junctions

We present a computationally efficient strategy that allows us to simulate magnetization switching driven by spin-transfer torque in magnetic tunnel junctions within a micromagnetic model coupled with a matrix-based nonequilibrium Green's function algorithm. Exemplary simulations for a realistic set of parameters are carried out and show switching times below $4\phantom{\rule{4.pt}{0ex}}\text{ns}$ for voltages above $300\phantom{\rule{4.pt}{0ex}}\text{mV}$ or around $2\ifmmode\times\else\texttimes\fi{}{10}^{10}\text{A}\phantom{\rule{0.16em}{0ex}}{\text{m}}^{\ensuremath{-}2}$ for the $\text{P}\ensuremath{\rightarrow}\text{AP}$ (parallel-to-antiparallel) direction. For $\text{AP}\ensuremath{\rightarrow}\text{P}$ switching, a trend reversal in the switching time is seen, i.e., the time for magnetization reversal first decreases with increasing bias voltage but then starts to rise again.

Spin-Injection Enhancements in van der Waals Magnetic Tunnel Junctions through Barrier Engineering

Magnetic tunnel junctions (MTJs) composed of different two-dimensional (2D) materials are promising for spin-valve applications. However, as the number of material combinations grows, guiding principles for heterostructure composition-property relationships become very desirable. The authors use first-principles calculations to provide design guidelines for spin-dependent transport in MTJs, based on the bulk properties of the constituent 2D materials, and find that low-work-function electrodes enable greater performance with CrI${}_{3}$ channels. This data-driven scheme offers a fast, systematic method to accelerate the design of spintronic devices based on 2D materials.

Coupled spin and charge drift-diffusion approach applied to magnetic tunnel junctions

A drift-diffusion approach to coupled spin and charge transport has been commonly applied to determine the spin-transfer torque acting on the magnetization in metallic valves. This approach, however, is not suitable to describe the predominant tunnel transport in magnetic tunnel junctions. In this work we present a coupled Finite Element solution to the spin and charge drift–diffusion equations. We demonstrate that by introducing a magnetization dependent resistivity one can successfully reproduce the resistance dependence on the magnetization orientation in the ferromagnetic layers. We then investigate the dependence of the resulting torques on system parameters, and show that the approach is able to reproduce the torque magnitude expected in a magnetic tunnel junction. As a unique set of equations is used for the entire structure, this constitutes an efficient Finite Element based approach to describe the magnetization dynamics in emerging spin-transfer torque memories.

Thermal Conductivity of Oxide Tunnel Barriers in Magnetic Tunnel Junctions Measured by Ultrafast Thermoreflectance and Magneto-Optic Kerr Effect Thermometry

Spin-dependent charge transport in magnetic tunnel junctions (MTJs) can be manipulated by a temperature gradient, which can be utilized for spintronic and spin caloritronic applications. Evaluation of the thermally induced phenomena requires knowledge of the temperature differences across the oxide tunnel barrier adjacent to the ferromagnetic (FM) leads. However, it is challenging to accurately measure thermal properties of an oxide tunnel barrier consisting of only a few atomic layers. In this work, we experimentally interrogate the temperature evolutions in Ru/oxide/FM/seed/MgO (oxide=MgO, MgAl2O4; FM=Co, CoFeB; seed=Pt, Ta) structures having perpendicular magnetic anisotropy using ultrafast thermometry. The Ru layer is optically thick and heated by ultrafast laser pulses; the subsequent temperature changes are monitored using thermoreflectance of Ru and magneto-optic Kerr effect (MOKE) of the FM layers. We independently measure the response times of Co and CoFeB magnetism using quadratic MOKE and obtain {\tau}em=0.2 ps for Co and 2 ps for CoFeB. These time scales are much shorter than the time scale of heat transport through the oxide tunnel barrier, which occurs at 10-3000 ps. We determine effective thermal conductivities of MgO and MgAl2O4 tunnel barriers in the range of 0.4-0.6 W m-1 K-1, comparable to an estimate of the series conductance of the Ru/oxide and oxide/FM interfaces and an order of magnitude smaller than the thermal conductivity of MgO thin films. We find that the electron-phonon thermal conductance near the tunnel barrier is only a factor of 5-12 larger than the thermal conductance of the oxide tunnel barrier. Therefore, the drop in the electronic temperature is approximately 20-30% larger than the drop in the phonon temperature across the tunnel barrier.

Robustness of Voltage-induced Magnetocapacitance

One of the most important achievements in the field of spintronics is the development of magnetic tunnel junctions (MTJs). MTJs exhibit a large tunneling magnetoresistance (TMR). However, TMR is strongly dependent on biasing voltage, generally, decreasing with applying bias. The rapid decay of TMR was a major deficiency of MTJs. Here we report a new phenomenon at room temperature, in which the tunneling magnetocapacitance (TMC) increases with biasing voltage in an MTJ system based on Co40Fe40B20/MgO/Co40Fe40B20. We have observed a maximum TMC value of 102% under appropriate biasing, which is the largest voltage-induced TMC effect ever reported for MTJs. We have found excellent agreement between theory and experiment for the bipolar biasing regions using Debye-Fröhlich model combined with quartic barrier approximation and spin-dependent drift-diffusion model. Based on our calculation, we predict that the voltage-induced TMC ratio could reach 1100% in MTJs with a corresponding TMR value of 604%. Our work has provided a new understanding on the voltage-induced AC spin-dependent transport in MTJs. The results reported here may open a novel pathway for spintronics applications, e.g., non-volatile memories and spin logic circuits.

Spin-dependent electronic transport characteristics in Fe4N/BiFeO3/Fe4N perpendicular magnetic tunnel junctions

Perpendicular magnetic tunnel junctions (MTJs) have attracted increasing attention owing to the low energy consumption and wide application prospects. Herewith, against Julliere's formula, an inverse tunnel magnetoresistance (TMR) appears in tetragonal Fe4N/BiFeO3/Fe4N perpendicular MTJs, which is attributed to the binding between the interface resonant tunneling state and central (bordered) hot spots. Especially, antiferromagnetic BiFeO3 shows an extra spin-polarized resonant state in the barrier, which provides a magnetic-barrier factor to affect the tunneling transport in MTJs. Meanwhile, due to the spin-polarized transport in Fe4N/BiFeO3/Fe4N MTJs, the sign of TMR can be tuned by the applied bias. The tunable TMR and resonant magnetic barrier effect pave the way for clarifying the tunneling transport in other junctions and spintronic devices.

Tunnel magnetoresistance and spin transfer torque in magnetic tunnel junction with embedded nanoparticles

The theoretical model of spin-dependent transport in magnetic tunnel junctions (MTJ) containing magnetic or non-magnetic nanoparticle is developed. The dependences of tunnel magnetoresistance (TMR) and in-plane component of spin transfer torque (STT) on the applied voltage for various sizes of nanoparticles of the order of the mean free path of the conduction electron are calculated. The calculation is performed in the approximation of the ballistic transport of conduction electrons through the insulating layers of the MTJ and the nanoparticles.

First principle study on TMR effect in A-MgO-A (A = Fe, Co and Ni) magnetic tunnel junction

• The role of electrode materials in TMR devices have investigated using DFT with Non-Equilibrium Green's Function (NEGF). • TMR effect is existing in Co-MgO-Co, Fe-MgO-Fe and Ni-MgO-Ni systems. • Iron electrodes have very high TMR efficiency than nickel and cobalt electrode. Abstract Magnetic tunnel junctions are used in storage devices. Electron transport in magnetic tunnel junctions highly dependents upon spin states of electron in ferromagnetic material. Iron, cobalt and nickel are room temperature ferromagnetic materials, magnetic tunnel junctions have been constructed using these materials as electrode and MgO as insulating layer (Co-MgO-Co, Fe-MgO-Fe and Ni-MgO-Ni). Total density of states, projected density of states and transmission coefficient have been calculated using non-equilibrium Green’s function (NEGF) with density functional theory (DFT) to analyze the spin transport properties of these devices. To study the TMR efficiency of these devices, TMR percentage at zero bias conductance was calculated based on optimistic and pessimistic. Our result shows that, iron ferromagnetic electrodes have very high TMR efficiency than nickel and cobalt electrode.

Large magneto-Seebeck effect in magnetic tunnel junctions with half-metallic Heusler electrodes

Spin caloritronics studies the interplay between charge-, heat- and spin-currents, which are initiated by temperature gradients in magnetic nanostructures. A plethora of new phenomena has been discovered that promises, e.g., to make wasted heat in electronic devices useable or to provide new read-out mechanisms for information. However, only few materials have been studied so far with Seebeck voltages of only some microvolt, which hampers applications. Here, we demonstrate that half-metallic Heusler compounds are hot candidates for enhancing spin-dependent thermoelectric effects. This becomes evident when considering the asymmetry of the spin-split density of electronic states around the Fermi level that determines the spin-dependent thermoelectric transport in magnetic tunnel junctions. We identify Co2FeAl and Co2FeSi Heusler compounds as ideal due to their energy gaps in the minority density of states, and demonstrate devices with substantially larger Seebeck voltages and tunnel magneto-Seebeck effect ratios than the commonly used Co-Fe-B-based junctions.

Tunneling anisotropic magnetoresistance driven by magnetic phase transition

The independent control of two magnetic electrodes and spin-coherent transport in magnetic tunnel junctions are strictly required for tunneling magnetoresistance, while junctions with only one ferromagnetic electrode exhibit tunneling anisotropic magnetoresistance dependent on the anisotropic density of states with no room temperature performance so far. Here, we report an alternative approach to obtaining tunneling anisotropic magnetoresistance in α′-FeRh-based junctions driven by the magnetic phase transition of α′-FeRh and resultantly large variation of the density of states in the vicinity of MgO tunneling barrier, referred to as phase transition tunneling anisotropic magnetoresistance. The junctions with only one α′-FeRh magnetic electrode show a magnetoresistance ratio up to 20% at room temperature. Both the polarity and magnitude of the phase transition tunneling anisotropic magnetoresistance can be modulated by interfacial engineering at the α′-FeRh/MgO interface. Besides the fundamental significance, our finding might add a different dimension to magnetic random access memory and antiferromagnet spintronics.

Influence of spin-orbit interaction within the insulating barrier on the electron transport in magnetic tunnel junctions

We present a theory of the anisotropy of tunneling magnetoresistance (ATMR) phenomenon in magnetic tunnel junctions (MTJs) attributed to Rashba spin-orbit interaction in the insulating barrier. ATMR represents the difference of tunnel magnetoresistance (TMR) amplitude measured with in-plane and out-of-plane magnetic configurations. It is demonstrated that within the spin-polarized free-electron model the change of conductance associated with the ATMR is exactly twice the change of conductance measured at full saturation (i.e., in parallel configuration of magnetizations) between in-plane and out-of-plane configuration, i.e., the tunneling anisotropic magnetoresistance (TAMR). Both ATMR and TAMR are closely related to the TMR amplitude and spin-orbit constant. The predicted ATMR phenomenon is confirmed experimentally, showing a few percent value in the case of the widely studied CoFeB/MgO/CoFeB based MTJ.

Enhancing TMR and spin-filtration by using out-of-plane graphene insulating barrier in MTJs.

First principles investigations are performed to understand the spin-polarized transport in Magnetic Tunnel Junctions (MTJs) consisting of an out-of-plane graphene sheet as a barrier in between two CrO2 Half-Metallic-Ferromagnetic (HMF) electrodes. Upon comparison of the results with the results of in-plane graphene based MTJs reported in the past, it is observed that out-of-plane structures offer a high TMR of ∼100% and the transport phenomenon is tunneling since there are no transmission states near the Fermi level. However, in in-plane structures, the transport phenomenon cannot be tunneling since there are a significant number of transmission states near the Fermi level, although a high Magneto Resistance (MR) of ∼90% is observed. Both the TMR and Spin Injection Efficiency η (Spin-Filtration) are higher in out-of-plane structures in comparison to in-plane structures, which is due to the graphene sheet acting as a perfect barrier in out-of-plane structures, which results in negligible spin down current (I↓) in both the Parallel Configuration (PC) and Antiparallel Configuration (APC).

Spin Torque Oscillator for High Performance Magnetic Memory

A study on spin transfer torque switching in a magnetic tunnel junction with perpendicular magnetic anisotropy is presented. The switching current can be strongly reduced under a spin torque oscillator (STO), and its use in addition to the conventional transport in magnetic tunnel junctions (MTJ) should be considered. The reduction of the switching current from the parallel state to the antiparallel state is greater than in the opposite direction, thus minimizing the asymmetry of the resistance versus current in the hysteresis loop. This reduction of both switching current and asymmetry under a spin torque oscillator occurs only during the writing process and does not affect the thermal stability of the free layer.

Insight into spin transport in oxide heterostructures from interface-resolved magnetic mapping.

At interfaces between complex oxides, electronic, orbital and magnetic reconstructions may produce states of matter absent from the materials involved, offering novel possibilities for electronic and spintronic devices. Here we show that magnetic reconstruction has a strong influence on the interfacial spin selectivity, a key parameter controlling spin transport in magnetic tunnel junctions. In epitaxial heterostructures combining layers of antiferromagnetic LaFeO(3) (LFO) and ferromagnetic La(0.7)Sr(0.3)MnO(3) (LSMO), we find that a net magnetic moment is induced in the first few unit planes of LFO near the interface with LSMO. Using X-ray photoemission electron microscopy, we show that the ferromagnetic domain structure of the manganite electrodes is imprinted into the antiferromagnetic tunnel barrier, endowing it with spin selectivity. Finally, we find that the spin arrangement resulting from coexisting ferromagnetic and antiferromagnetic interactions strongly influences the tunnel magnetoresistance of LSMO/LFO/LSMO junctions through competing spin-polarization and spin-filtering effects.

Controlling the spin-torque efficiency with ferroelectric barriers

Nonequilibrium spin-dependent transport in magnetic tunnel junctions comprising a ferroelectric barrier is theoretically investigated. The exact solutions of the free electron Schr\"odinger equation for electron tunneling in the presence of interfacial screening are obtained by combining Bessel and Airy functions. We demonstrate that the spin transfer torque efficiency, and more generally the bias dependence of tunneling magneto- and electroresistance, can be controlled by switching the ferroelectric polarization of the barrier. In particular, the critical voltage at which the in-plane torque changes sign can be strongly enhanced or reduced depending on the direction of the ferroelectric polarization of the barrier. This effect provides a supplementary way to electrically control the current-driven dynamic states of the magnetization and related magnetic noise in spin transfer devices.

A Two-Band Tight-Binding Model of Spin-Polarized Transport in Magnetic Tunnel Junctions

The main features determining spin-dependent transport in magnetic tunnel junctions are well appreciated now mainly due to ab initio calculations. Nevertheless, it seems useful to have a comparatively simple model which reproduces the salient characteristics of spin-dependent transport in magnetic multilayers. We present two-band tight-binding model of magnetic tunnel junction with the layers of arbitrary thickness and non-collinear magnetization configuration. The model accounts for different symmetry of the tunneling electrons and their mixing on the interfaces. As an illustration for double-barrier structure we calculate I-V curves and spin-transfer torque. The calculations demonstrate the resonant character of the I-V dependences. Also, the significant difference in the magnitude of the torque acting on the various planes of the same magnetic layer is found.

Spin manipulations through electrical and thermoelectrical transport in magnetic tunnel junctions

A brief review is presented, which includes the direct current, alternate current, electrical and thermoelectrical transport as well as spin transfer effect in a variety of spin-based nanostructures such as the magnetic tunnel junction (MTJ), ferromagnet(FM)-quantum dot (QD)/FM-FM, double barrier MTJ, FM-marginal Fermi liquid-FM, FM-unconventional superconductor-FM (FUSF), quantum ring and optical spin-field-effect transistor. The magnetoresistances in those structures, spin accumulation effect in FM-QD-FM and FUSF systems, spin injection and spin filter into semiconductor, spin transfer effect, photon-assisted spin transport, magnonassisted tunneling, electron-electron interaction effect on spin transport, laser-controlled spin dynamics, and thermoelectrical spin transport are discussed.

Spin transfer torque with spin diffusion in magnetic tunnel junctions

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.

Charge and Spin Transport in Magnetic Tunnel Junctions: Microscopic Theory

We study the charge and spin currents passing through a magnetic tunnel junction (MTJ) on the basis of a tight-binding model. The currents are evaluated perturbatively with respect to the tunnel Hamiltonian. The charge current has the form $A[\bm M_1(t)\times\dot{\bm M}_1(t)]\cdot\bm M_2+B\dot{\bm M}_1(t)\cdot\bm M_2$, where $\bm M_1(t)$ and $\bm M_2$ denote the directions of the magnetization in the free layer and fixed layer, respectively. The constant $A$ vanishes when one or both layers are insulators, {while the constant $B$ disappears when both layers are insulators or the same ferromagnets.} The first term in the expression for charge current represents dissipation driven by the effective electric field induced by the dynamic magnetization. In addition, from an investigation of the spin current, we obtain the microscopic expression for the enhanced Gilbert damping constant $\varDelta \alpha$. We show that $\varDelta\alpha$ is proportional to the tunnel conductance and depends on the bias voltage.