Double-barrier Magnetic Tunnel Junctions Research Articles

Enhanced thermal spin transfer in MgO-based double-barrier tunnel junctions

Based on atomic first principles, we predict enhanced thermal spin transfer (TST) effects and small switching temperature gradient in Fe MgO Fe MgO Fe double-barrier magnetic tunnel junctions (MTJs). At room temperature, temperature gradient ∼10 with ∼10 K nm−1 across barriers would be sufficient to switch the magnetic configurations circularly in a junction with 3 MgO atomic layers (L), which is about one order smaller than that in Fe MgO(3L) Fe MTJs. This temperature gradient is under the current experimental capability. The resonant quantum-well states in companion with resonant interfacial states are responsible for the enhancement. Moreover, a thermal induced ‘off’ state is found in a double-barrier MTJ.

Quantum size effects on spin-transfer torque in a double barrier magnetic tunnel junction with a nonmagnetic-metal (semiconductor) spacer

We study the quantum size effects of an MgO-based double barrier magnetic tunnel junction with a nonmagnetic-metal (DBMTJ-NM) (semiconductor (DBMTJ-SC)) spacer on the charge current and the spin-transfer torque (STT) components using non-equilibrium Green's function (NEGF) formalism. The results show oscillatory behavior due to the resonant tunneling effect depending on the structure parameters. We find that the charge current and the STT components in the DBMTJ-SC demonstrate the magnitude enhancement in comparison with the DBMTJ-NM. The bias dependence of the STT components in a DBMTJ-NM shows different behavior in comparison with spin valves and conventional MTJs. Therefore, by choosing a specific SC spacer with suitable thickness in a DBMTJ the charge current and the STT components significantly increase so that one can design a device with high STT and faster magnetization switching.

Effect of Deposition Conditions and Annealing Temperature on Tunnel Magnetoresistance and the Structure of MgO-Based Double-Barrier Magnetic Tunnel Junctions

Tunnel magnetoresistance (TMR) was measured in CoFeB–MgO-based double-barrier magnetic tunnel junctions. The variation of sputtering power density is used to control the relative B content of the middle electrode. MR ratios in both top and bottom junctions are suppressed with respect to the single-barrier case. While the bottom junction shows a saturation of the MR as a function of high-temperature annealing for all sputtering power densities, the top junction exhibits an increase in MR as a function of annealing temperature with higher values for higher sputtering power density. The suppression of high MR is attributed to a lack of strong crystallization in the middle electrode, which is confirmed by cross-sectional transmission electron microscopy. Slight crystallization of the middle electrode is achieved at the highest sputtering power density despite the fact that boron diffusion is suppressed due to the adjacent MgO tunnel barriers. Optimal deposition and postannealing conditions resulted in MR values of 140% and 80% for the top and bottom junctions, respectively.

Size Effect on Interlayer Coupling and Magnetoresistance Oscillation of Magnetic Tunnel Junction Embedded With Iron Nanoparticles

Here, we investigate the size effect of perpendicular-anisotropic double-barrier magnetic tunnel junction (MTJ) devices embedded with iron nanoparticles. A sputtering system in conjunction with the postannealing process is employed to prepare the sheet film and standard lithography techniques followed by the ion etching technique are used to fabricate the micrometer to submicrometer MTJ devices. A strong ferromagnetic coupling is observed as we reduce the size of the device to submicrometer scale, which is due to the reduction of magnetostatic energy of the device. Furthermore, a magnetoresistance (MR) oscillation is observed at room temperature while reducing the size of the device. MR peaks at low bias fields are believed to have magnon contributions, whereas the peaks observed at higher bias fields are responsible for phonon-assisted tunneling. Zero-bias anomalies are also observed and are more prominent in antiparallel states of the devices.

Enhanced spin-torque in double tunnel junctions using a nonmagnetic-metal spacer

This study proposes an enhancement in the spin-transfer torque of a magnetic tunnel junction (MTJ) designed with double-barrier layer structure using a nonmagnetic metal spacer, as a replacement for the ferromagnetic material, which is traditionally used in these double-barrier stacks. Our calculation results show that the spin-transfer torque and charge current density of the proposed double-barrier MTJ can be as much as two orders of magnitude larger than the traditional double-barrier one. In other words, the proposed double-barrier MTJ has a spin-transfer torque that is three orders larger than that of the single-barrier stack. This improvement may be attributed to the quantum-well states that are formed in the nonmagnetic metal spacer and the resonant tunneling mechanism that exists throughout the system.

Long-Range Phase Coherence in Double-Barrier Magnetic Tunnel Junctions with a Large Thick Metallic Quantum Well

Double-barrier heterostructures are model systems for the study of electron tunneling and discrete energy levels in a quantum well (QW). Until now resonant tunneling phenomena in metallic QWs have been observed for limited thicknesses (1-2 nm) under which electron phase coherence is conserved. In the present study we show evidence of QW resonance states in Fe QWs up to 12 nm thick and at room temperature in fully epitaxial double MgAlO_{x} barrier magnetic tunnel junctions. The electron phase coherence displayed in this QW is of unprecedented quality because of a homogenous interface phase shift due to the small lattice mismatch at the Fe-MgAlO_{x} interface. The physical understanding of the critical role of interface strain on QW phase coherence will greatly promote the development of spin-dependent quantum resonant tunneling applications.

Modulation of spin transfer torque amplitude in double barrier magnetic tunnel junctions

Magnetization switching induced by spin transfer torque is used to write magnetic memories (Magnetic Random Access Memory, MRAM) but can be detrimental to the reading process. It would be quite convenient therefore to modulate the efficiency of spin transfer torque. A solution is adding an extra degree of freedom by using double barrier magnetic tunnel junctions with two spin-polarizers, with controllable relative magnetic alignment. We demonstrate, for these structures, that the amplitude of in-plane spin transfer torque on the middle free layer can be efficiently tuned via the magnetic configuration of the electrodes. Using the proposed design could thus pave the way towards more reliable read/write schemes for MRAM. Moreover, our results suggest an intriguing effect associated with the out-of-plane (field-like) spin transfer torque, which has to be further investigated.

Low frequency noise in asymmetric double barrier magnetic tunnel junctions with a top thin MgO layer**Project supported by the National Basic Research Program of China (Grant Nos. 2011CBA00106, 2012CB927400, 2010CB934401, and 2014AA032904), the National High Technology Research and Development Program of China (Grant No. 2014AA032904), and the National Natural Science Foundation of China (Grant Nos. 11434014 and 11104252).

Low frequency noise has been investigated at room temperature for asymmetric double barrier magnetic tunnel junctions (DBMTJs), where the coupling between the top and middle CoFeB layers is antiferromagnetic with a 0.8-nm thin top MgO barrier of the CoFeB/MgO/CoFe/CoFeB/MgO/CoFeB DBMTJ. At enough large bias, 1/f noise dominates the voltage noise power spectra in the low frequency region, and is conventionally characterized by the Hooge parameter αmag. With increasing external field, the top and bottom ferromagnetic layers are aligned by the field, and then the middle free layer rotates from antiparallel state (antiferromagnetic coupling between top and middle ferromagnetic layers) to parallel state. In this rotation process αmag and magnetoresistance-sensitivity-product show a linear dependence, consistent with the fluctuation dissipation relation. With the magnetic field applied at different angles (θ) to the easy axis of the free layer, the linear dependence persists while the intercept of the linear fit satisfies a cos(θ) dependence, similar to that for the magnetoresistance, suggesting intrinsic relation between magnetic losses and magnetoresistance.

Middle-layer ferromagnetism-induced transition of the tunnel magneto-resistance in double-barrier magnetic tunnel junctions: A non-equilibrium Green's function study

Using the non-equilibrium Green's function modeling of the transport characteristics of tunnel devices, we have found the middle-layer ferromagnetism-induced transition of the tunnel magneto-resistance in double-barrier magnetic tunnel junctions. It is observed from our study that even a weak ferromagnetism of the middle metallic layers is capable of promoting resonant tunnel magneto-resistance in these devices and the strength of the ferromagnetism is found to have strong influence on the bias dependence of the resonant tunnel magneto-resistance. The spin-up and spin-down currents flow in opposite directions for certain band occupancies and at certain bias voltage ranges when there is antiferromagnetic coupling between the electrodes of the tunnel junction. Resonant tunnel magneto-resistance occurs when the net current (sum of spin-up and spin-down currents) becomes very small at situations mentioned above. We have further studied the influence of band occupation of the electrode layers and the many-body interactions present in the electrode region on the spin current and magneto-resistance of these devices.

Resonant Magnetoresistance in Asymmetric Double-Barrier Magnetic Tunnel Junctions

Resonant tunneling is studied theoretically for the planar asymmetrical double-barrier magnetic tunnel junction (DMTJ) when a dc bias field is applied. The spin-polarized conductance and tunnel magnetoresistance (TMR) through the DMTJ have been calculated. In DMTJ nanostructure the magnetization of middle ferromagnetic metal layer can be aligned parallel or antiparallel with respect to the fixed magnetizations of the top and bottom ferromagnetic electrodes. Analytical expression for the transmission coefficient of the DMTJ is received, which is expressed through the single-barrier transmission coefficients taking into account the voltage drop on each barrier and spin degrees of freedom of the electron. The dependencies of the tunnel conductance and TMR on the applied voltage have been calculated for the case of resonant transmission.

Crystallization characteristics of a middle CoFeB layer in a double MgO barrier magnetic tunnel junction

The crystallization characteristics of a middle CoFeB free layer in a magnetic tunnel junction (MTJ) with double MgO barriers were investigated by tunneling magnetoresistance (TMR) measurements of patterned cells across an 8-inch wafer. The MTJ structure was designed to have two CoFeB free layers and one bottom pinned layer, separated by MgO tunnel barriers. The observed resistance showed three types of TMR curves depending on the crystallization of the middle CoFeB layer. From the analysis of TMR curves, coherent crystallization of the middle CoFeB layer with the top and bottom MgO barriers was found to occur non-uniformly: About 80% of the MTJ cells in the wafer exhibited coherent crystallization of the middle CoFeB layers with the bottom MgO tunnel barrier, while others had coherent crystallization with the top MgO tunnel barrier or both barriers. This non-uniform crystallization of the middle CoFeB layer in a double MTJ was also clearly observed in tunneling electron microscopy images. Thus, control of the crystallization of the middle CoFeB layer is important for optimizing the MTJ with double MgO barriers, and especially for the fabrication of double barrier MTJ on a large area substrate.

Effect of thermal annealing on the ac impedance of Co(75)/Al2O3(2.3)/Co(5.0)/Al2O3(2.3)/Co(50) double-barrier MTJs

Double-barrier magnetic tunnel junctions (DBMTJs) were prepared from Co(75 nm)/Al2O3(2.3 nm)/Co(5 nm)/Al2O3(2.3 nm)/Co(50 nm) sputtering pentalayer films. The ac electrical properties of as-deposited DBMTJs and those annealed in a vacuum at 100–350 °C for 30 min were then investigated using a complex impedance spectroscopic technique. The ac impedance responses as a function of annealing temperature were further analyzed based on Maxwell's layered dielectric barrier and Maxwell–Wagner capacitor models after considering the DBMTJs as having double-capacitor-type structures. The effect of thermal annealing on the ac transport behavior of the DBMTJs was interpreted by examining the equivalent electric circuits fitted to Nyquist plots of each different sample. The effects were found to be due to changes in the structural characteristics in both bulk and interface morphologies of Co and Al2O3 layers. The structural morphology determined the different ac transport modes that occurred in the DBMTJs.

Modified analytical method for evaluation of unpatterned double-barrier magnetic tunnel junctions

A current-in-plane tunneling measurement is a superb solution for evaluating a magnetic tunnel junction's properties because it does not require the series of patterning processes in a clean room that have frustrated many researchers because of the damage and side effects frequently induced by these processes. We found that previously proposed current-in-plane tunneling analysis for double-barrier magnetic tunnel junctions (DMTJs) accurately predicts their electrical properties but often fails to provide other detailed properties of DMTJs. Here, we propose and demonstrate a modified analytic method that can provide an excellent estimate of the electrical and magnetic properties of DMTJs by considering the intermediate magnetization state between parallel and antiparallel states, which has previously been ignored. We found that considering this intermediate state is necessary and sufficient for evaluating the tunneling properties. We also prove that our method is valid even for DMTJs with two identical barriers and even without knowledge of any of the initial properties of their layers, a challenging task for previous methods. We believe that our analytic method yields the correct results when evaluating the properties of DMTJs and will be particularly useful for those who cannot access the well-maintained clean-room facilities needed to make tunnel patterns.

Spin-dependent transport in double-barrier magnetic tunnel junction with Dresselhaus spin–orbit interaction

We have theoretically investigated the effect of Dresselhaus spin–orbit coupling on spin-transport properties of ferromagnet/insulator/semiconductor/insulator/ferromagnet (Fe/Al2O3/SM/Al2O3/Fe) heterostructure. Based on the two band model and nearly-free-electron apptoximation, tunnel current and magnetoresistance are calculated as a function of bias voltage. Our calculations are carried out for three different semiconductors such as InAs, AlAs and InP. It is shown that the Dresselhaus spin–orbit interaction, in general, leads to enhancement of tunneling magnetoresistance. It is also shown that it strongly depends on the thickness of layers, bias voltage and electron effective mass in semiconductor layers.

Tunnel magnetoresistance in asymmetric double-barrier magnetic tunnel junctions

Abstract The spin-polarized tunnel conductance and tunnel magnetoresistance (TMR) through a planar asymmetric double-barrier magnetic tunnel junction (DBMTJ) have been calculated using quasi-classical model. In DBMTJ nanostructure the magnetization of middle ferromagnetic metal layer can be aligned parallel or antiparallel with respect to the fixed magnetizations of the top and bottom ferromagnetic electrodes. The transmission coefficients of an electron to pass through the barriers have been calculated in terms of quantum mechanics. The dependencies of tunnel conductance and TMR on the applied voltage have been calculated in case of non-resonant transmission. Estimated in the framework of our model, the difference between the spin-channels conductances at low voltages was found relatively large. This gives rise to very high magnitude of TMR.

Size Effect of Inserted Iron Nanoparticles in the MgO-Based Double Barrier Magnetic Tunnel Junction

The transport properties of stacked CoFeB (1.86)/MgO (0.2)/Fe-nanoparticles ( tFe)/MgO (0.5)/CoFeB(0.6) double barrier magnetic tunnel junctions have been characterized. In this investigation, stacked film is prepared by magnetron sputter system, annealed at 250 °, and patterned into a micron-scale device using standard electron beam lithography in conjunction with two-angle ion beam etching. First, the structure, magnetic and electrical properties are examined by using transmission electron microscopy, alternating gradient magnetometers, and lock-in techniques. Stacked film having 1.5 nm lateral size of iron nanoparticles (sample S1) exhibits an in-plane magnetic anisotropy, nonetheless the stacked film having 2 nm lateral size of iron nanoparticles (sample S5) possesses out-of-plane magnetic anisotropy. In addition, the micron scaled S1 device displays not only a normal in-plane magnetoresistance (MR) curve but also an anomalous out-of-plane MR behavior, whereas the S5 device only barely shows magnetic response in the in-plane MR. A hysteresis-free linear region in out-of-plane MR curve has shown field sensitivity of ~ 36 Ω/Oe that is promising for magnetic field sensor applications.

Tunneling processes in asymmetric double barrier magnetic tunnel junctions with a thin top MgO layer

Dynamic conductance dI/dV and inelastic electron tunneling spectroscopy (IETS) d2I/dV2 have been measured at different temperatures for double barrier magnetic tunnel junctions with a thin top MgO layer. The resistance in the antiparallel state exhibits a normal tunnel-like behavior, while the resistance in the parallel state shows metallic-like transport, indicating the presence of pinholes in the thin top MgO layer. Three IETS peaks are the zero-bias anomaly, interface magnons, and barrier phonons in both the parallel and antiparallel states. The zero-bias anomaly is the strongest peak in the parallel state and its intensity decreases with temperature. The magnon has the largest intensity in the antiparallel state and its intensity also decreases with temperature. The origins of the dips and peaks in the dI/dV-V curve are also discussed.

ChemInform Abstract: MgO(001) Barrier Based Magnetic Tunnel Junctions and Their Device Applications

AbstractReview: 210 refs.

MgO-Based Double Barrier Magnetic Tunnel Junctions With Synthetic Antiferromagnetic Free Layer

CoFeB/Ru/CoFeB has been used as the middle free layer in MgO-based double barrier magnetic tunnel junctions (DBMTJs). The tunneling magnetoresistance (TMR) ratio, V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sub> (bias voltage at half maximum TMR ratio) and V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">out</sub> (output voltage, defined as V multiplied by TMR ratio) have been investigated as a function of annealing temperature (T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</sub> ) and Ru thickness (t <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Ru</sub> ) in the free layer. Magnetization data reveal that the two CoFeB layers in CoFeB/Ru/CoFeB are antiferromagnetically coupled. Compared with V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sub> of only 0.31 V for single barrier MTJs (SBMTJs) annealed at 375°C, V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sub> for DBMTJs is up to 0.66 V. By increasing Ta, TMR ratio first increases, reaching the highest critical value, and then decreases. The highest TMR ratio is 181% with t <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Ru</sub> = 0.6 nm, which is much higher than that obtained in the DBMTJs with the pure CoFeB as the free layer. The improved TMR ratio is mainly due to the relatively thorough crystallization of the CoFeB layers in the free layer. The thermal annealing has been proven to be an effective method to remove the dissimilarity of the top and bottom CoFeB/MgO interfaces. V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">out</sub> in the positive and negative voltage branches follows the same trend as that of TMR ratio with T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</sub> .

Shot Noise in Epitaxial Double-Barrier Magnetic Tunnel Junctions

We demonstrate that shot noise in Fe/MgO/Fe/MgO/Fe double-barrier magnetic tunnel junctions is determined by the magnetic configuration of the junction—the P-state with the magnetic moments of all three ferromagnetic layers aligned parallel, the AP1 state with the central electrode magnetized opposite to its neighbors, and two different AP2 states with the magnetic moment of the upper or bottom electrode aligned opposite to the other two. We also show that the asymmetry between both MgO barriers is another important factor which affects the shot noise. Some voltages present an enhancement in both conductance and shot noise, which indicates that resonant tunneling through quantum well states formed in the middle magnetic layer is taking place. When the junctions are heated to 60 K, the resonance tunneling anomalies in the shot noise smear out, but they survive in the differential conductance. On the other hand, at low voltages (below 200 mV) and low temperatures (below 4 K) the shot noise tends to decrease, probably due to multi-step tunneling via localized defect states in the tunnel MgO barrier. The theoretical model of sequential tunneling proposed for this system, which takes into account spin relaxation, successfully describes the experimental observations in the bias range between 200 mV and 500 mV, where the influence of tunneling through barrier defects and resonant states inside the central electrode is negligible.