Double-barrier Magnetic Tunnel Junctions Research Articles

Magnon resonant tunneling effect in double-barrier insulating magnon junctions and magnon field effect transistor

We predict a magnonic resonant tunneling phenomenon in the double-barrier insulating magnon junctions of ${\text{FI}}_{1}/{\text{AFI}}_{1}/{\text{FI}}_{2}/{\text{AFI}}_{2}/{\text{FI}}_{3}$, where ${\text{FI}}_{(1,2,3)}$ and ${\text{AFI}}_{(1,2)}$ denote the ferromagnetic and antiferromagnetic insulating layers, respectively. Similar to electron tunneling in well-known double-barrier magnetic tunnel junctions, each antiferromagnetic insulator layer acts as an effective potential barrier for magnons due to its intrinsically large magnon-spectrum gap of the order of THz. Based on the magnon tunneling effect, we further propose a magnon field effect transistor that is capable of realizing a gate-tunable transmitted magnon flow by tuning the resonant tunneling via a gate electric field induced Dzyaloshinskii-Moriya interaction in the middle FI layer. The advantages of such transistors include their broadband frequency width ranging from GHz to THz at room temperature, high scalability, and intrinsic low dissipation without Joule heating loss.

Magnetic memory with a switchable reference layer

Spin-Transfer-Torque Magnetic Random Access Memory (STT-MRAM) is a memory which has bit cells made of magnetic tunnel junctions (MTJs), which comprise a storage switchable magnetic layer (“free layer”) and, typically, one thin insulating barrier and one stable magnetic layer providing reference spin polarization for read and write operations (“reference layer”). STT-MRAM may compete with conventional dynamic and static RAM on technological nodes below 22 nm, if its switching current is reduced. This goal may be achieved for MTJ, which has two insulating barriers and reference layers. Building such a double-barrier MTJ, however, faces tremendous material challenges. In this work, a new double-barrier MTJ design with a switchable reference layer is introduced. We show that its efficiency is similar to its counterpart with stable reference layers, but it is much easier to be built.

Symmetry broken spin reorientation transition in epitaxial MgO/Fe/MgO layers with competing anisotropies

The observation of perpendicular magnetic anisotropy (PMA) at MgO/Fe interfaces boosted the development of spintronic devices based on ultrathin ferromagnetic layers. Yet, magnetization reversal in the standard magnetic tunnel junctions (MTJs) with competing PMA and in-plane anisotropies remains unclear. Here we report on the field induced nonvolatile broken symmetry magnetization reorientation transition from the in-plane to the perpendicular (out of plane) state at temperatures below 50 K. The samples were 10 nm thick Fe in MgO/Fe(100)/MgO as stacking components of V/MgO/Fe/MgO/Fe/Co double barrier MTJs with an area of 20 × 20 μm2. Micromagnetic simulations with PMA and different second order anisotropies at the opposite Fe/MgO interfaces qualitatively reproduce the observed broken symmetry spin reorientation transition. Our findings open the possibilities to develop multistate epitaxial spintronics based on competing magnetic anisotropies.

Thermoelectric effects of resonant magnetic tunnel junctions

Thermoelectric effects of CoFeB/MgO/InP/MgO/CoFeB double-barrier magnetic tunnel junctions (DBMTJs) are presented in the linear response regime. The nonequilibrium Green’s function (NEGF) formalism is used within the effective mass approximation. There are descriptions of temperature and angular dependence for thermoelectric properties. The results show an enhancement in the DBMTJs thermoelectric compared to single-barrier MTJs due to the resonant tunneling effect through the DBMTJs. It is also found that the Seebeck coefficients may be increase in asymmetric DBMTJs. Effects of the temperature and magnetization alignments on the spin-dependent Seebeck coefficients are also described. Finally, the Seebeck coefficients are investigated considering the Rashba spin-orbit coupling at the insulator/semiconductor interfaces.

Shot noise and tunneling magnetoresistance in disordered MgO-based double-barrier magnetic tunnel junctions: First-principles study

We report first-principles study of shot noise and tunneling magnetoresistance in Fe/MgO/Fe- /MgO/Fe double-barrier magnetic tunnel junctions (MTJs). We mainly investigate the effects of disordered interfacial oxygen vacancies and barrier asymmetry on the spin-dependent tunneling. It is found that interchannel scattering induced by interfacial oxygen vacancies can substantially enhance the tunneling conductance of the antiparallel magnetic configurations, and results in the dramatic decrease of tunneling magnetoresistance. Moreover, we find the interfacial disorder scattering favors the sub-Poissonian tunneling process. As a result, Fano factors of symmetric MTJs maintain at around 0.5, or are suppressed, while Fano factors of asymmetric MTJs can all be significantly suppressed, illustrating the important correlations in tunneling induced by interfacial disorders. Interchannel scattering induced by interfacial oxygen vacancies can effectively couple the electron to high-transmission channels, enhancing the transmission and reducing the shot noise. In comparison with interfacial disorder, middle-layer disordered Fe vacancies present limited modulation on the Fano factor. Increasing the asymmetry of barriers can quickly decrease high-transmission channels, and make the tunneling process Poissonian in double-barrier MTJs.

Quantum size effect in an Fe quantum well detected by resonant tunneling carriers injected from a p-type Ge semiconductor electrode

We report the clear observation of the quantum size effect in an Fe quantum well (QW) detected by resonant tunneling carriers injected from a p-type Ge semiconductor electrode in fully epitaxial double-barrier magnetic tunnel junctions, which are composed of Co/Fe/MgO/Fe QW/MgO/Ge:B grown on a p+-Ge(001) substrate. A large tunnel magnetoresistance (TMR) ratio up to 137% (237%), which is comparable to that in Fe/MgO/Fe, is obtained at 297 K (3.5 K). The quantum oscillations are clearly observed in the dI/dV–V and d2I/dV2–V curves of our devices, and the resonance voltages are in good agreement with the resonant levels calculated by the phase accumulation model. Following these oscillations, the TMR is modulated by the quantum size effect. Our results are promising for realizing future quantum spintronics devices based on semiconductor/metal hybrid heterostructures with advanced functionalities.

Temperature dependence of shot noise in double barrier magnetic tunnel junctions

Shot noise reveals spin dependent transport properties in a magnetic tunnel junction. We report measurement of shot noise in CoFeB/MgO/CoFeB/MgO/CoFeB double barrier magnetic tunnel junctions, which shows a strong temperature dependence. The Fano factor used to characterize shot noise increases with decreasing temperature. A sequential tunneling model can be used to account for these results, in which a larger Fano factor results from larger spin relaxation length at lower temperatures.

Dependences of the Tunnel Magnetoresistance and Spin Transfer Torque on the Sizes and Concentration of Nanoparticles in Magnetic Tunnel Junctions

Dependences of the tunnel magnetoresistance and in-plane component of the spin transfer torque on the applied voltage in a magnetic tunnel junction have been calculated in the approximation of ballistic transport of conduction electrons through an insulating layer with embedded magnetic or nonmagnetic nanoparticles. A single-barrier magnetic tunnel junction with a nanoparticle embedded in an insulator forms a double-barrier magnetic tunnel junction. It has been shown that the in-plane component of the spin transfer torque in the double-barrier magnetic tunnel junction can be higher than that in the single-barrier one at the same thickness of the insulating layer. The calculations show that nanoparticles embedded in the tunnel junction increase the probability of tunneling of electrons, create resonance conditions, and ensure the quantization of the conductance in contrast to the tunnel junction without nanoparticles. The calculated dependences of the tunnel magnetoresistance correspond to experimental data demonstrating peak anomalies and suppression of the maximum magnetoresistances at low voltages.

Influence of a ZnO layer on Seebeck coefficients in asymmetric double-barrier tunnel junctions

The Seebeck coefficients are studied in asymmetric double-barrier magnetic tunnel junctions (DBMTJs) with the structures as CoFeB/MgO/ZnO/MgO/LSMO. In the linear response regime, oscillatory behavior is observed with change in the ZnO layer thickness because of the resonant tunneling in the DBMTJs. Effects of average temperature and magnetizations orientation are also discussed. It is found that the Seebeck coefficients are greater in the DBMTJs than asymmetric single-barrier MTJs (SBMTJs). Therefore, it is to be achieved the greater Seebeck coefficients by choosing a proper thickness for the ZnO layer in the proposed asymmetric DBMTJs.

Spin-Dependent Thermoelectric Effects in Double-Barrier Magnetic Tunnel Junctions with a Non-magnetic Metal Spacer

Spin-dependent thermoelectric (TE) effects are studied in MgO-based double-barrier magnetic tunnel junctions with a non-magnetic metal spacer (DBMTJs-NM) in the linear response regime. Using non-equilibrium Green’s function (NEGF) formalism, the results show resonant peaks at specific thicknesses of the NM spacer because of the resonant tunneling effect through the DBMTJs-NM. Effects of average temperature and magnetization alignment are also described. Therefore, thermally charge and spin voltage lead to large spin-dependent TE effects by the DBMTJs-NM structures.

Effects of Fe insertion in CoFeB in the middle electrode on the properties of MgO double-barrier magnetic tunnel junctions

We successfully improved the tunnel magnetoresistance ratio (TMR) of an MgO/CoFeB/MgO-based double-barrier magnetic tunnel junction (DMTJ) by inserting ultra-thin Fe into a CoFeB middle electrode. The Fe played as an absorber of B diffused out of the CoFeB and thereby facilitated the crystallization of the CoFeB at the interface. Changes in resistivity of the middle electrode with Fe insertion calculated by the modified current-in-plane tunneling method suggested that they can be explained by the formation of Fe borides, which is ascribed to the fact that Fe indeed promoted the diffusion of B by providing places for the B diffusion. As a result, Fe insertion improved the (001) texture of the middle electrode and thereby increased the TMR of the junction due to enhanced coherent tunneling. However, when inserted Fe was thick, the tunnel properties of DMTJs were degraded because of the absence of strong (001) texture of MgO. Materials and their thickness for a middle electrode are key parameters to successful application of DMTJs for spin devices.

Huge spin-transfer torque in a magnetic tunnel junction by a superlattice barrier

Huge spin-transfer torque (STT) in a magnetic tunnel junction (MTJ) achieved by superlattice barrier composed of alternate layers of a nonmagnetic metal and an insulator is proposed. The magnitude of the STT depends on the number of cells in the superlattice barrier and the nonmagnetic metal layer's thickness. The result shows that the STT of the novel superlattice-barrier MTJ can reach values up to four orders of magnitude greater than those of traditional single-barrier stacks based on three cells superlattice by designing the nonmagnetic metal layer's thickness. In addition, the spin-transfer torque of the proposed MTJ can also be thousands of magnitude greater than those of traditional double-barrier MTJs.

Enhancement of Thermopower by Structural Asymmetry in Double-Barrier Tunnel Junctions

Thermopower is studied in double-barrier magnetic tunnel junctions (DBMTJs) with a non-magnetic metal (NM) spacer by considering structural asymmetry—related to the different thicknesses of the MgO insulators—in the linear response regime. Using non-equilibrium Green’s function (NEGF) formalism, it is found that the thermopower is larger in the case with thicker left insulator than the other cases which is related to the voltage drop within the insulator layer. The thermopower has shown its maximum value as large as S = 482μ V/K in the anti-parallel configuration of the magnetizations and T = 300K. Therefore, it is possible to achieve the large thermopower in the thermoelectric devices based on MTJs.

Enhancement of Seebeck Coefficients by Resonant Tunneling Effect

The Seebeck coefficients in an MgO-based double-barrier magnetic tunnel junction (DBMTJ) with a semiconductor (e.g., Ge) spacer are studied using non-equilibrium Green’s function formalism in the linear response regime. The DBMTJ results show a magnitude enhancement compared with a single-barrier MTJ (SBMTJ) at the specific thicknesses of the semiconductor spacer due to the resonant tunneling effect through the DBMTJ structure. With an increase of the average temperature of the junctions, the Seebeck coefficients also increase and they are at a maximum in the anti-parallel configuration of the magnetizations. Therefore, it is possible to achieve large Seebeck coefficients using a DBMTJ compared with a conventional SBMTJ structure.

Resonant enhancement of thermal spin-transfer torque in asymmetric double-barrier tunnel junctions

Charge current and thermal spin-transfer torque (TSTT) in asymmetric double-barrier magnetic tunnel junctions (DBMTJs) with a non-magnetic metal (NM) spacer as CoFeB/MgO/NM/MgO/LSMO are studied by non-equilibrium Green’s function (NEGF) formalism. It is found that the results show oscillatory behavior with an increase of the NM spacer thickness due to the resonant tunneling effect through the DBMTJs structures. About the dependence of the temperature difference and angle between the magnetizations, the charge current and TSTT are larger at the specific thicknesses (resonant positions) of the NM spacer than the others. The results also illustrate a magnitude enhancement for asymmetric DBMTJs compared with asymmetric single-barrier MTJs (SBMTJs). Therefore, it is possible to achieve a large TSTT for magnetization dynamics by new asymmetric DBMTJs.

Reliability enhancement due to in-situ post-oxidation of sputtered MgO barrier in double MgO barrier magnetic tunnel junction

We have investigated the effects of in-situ post-oxidation (PO) of a sputtered MgO barrier in a double-MgO-barrier magnetic tunnel junction (MTJ) and found that the short error rate was significantly reduced, the magnetoresistance (MR) ratio was increased approximately 18%, and the endurance lifetime was extend. In addition, we found that the distribution of breakdown number (a measure of endurance) exhibits trimodal characteristics, which indicates competition between extrinsic and intrinsic failures. This improvement in reliability might be related to the suppression of Fe and Co diffusion to the MgO barrier, as revealed by electron energy-loss spectroscopy (EELS) analysis.

Charge and Spin Thermopower in Double-Barrier Magnetic Tunnel Junctions with a Semiconductor Spacer

The charge and spin thermopower in double-barrier magnetic tunnel junctions with a semiconductor (e.g. GaAs) spacer (DBMTJs-SC) are studied using non-equilibrium Green’s function (NEGF) formalism in the linear response regime. The results show oscillatory behavior with an increase of the SC spacer thickness due to the resonant tunneling effect through the DBMTJs-SC. Effects of the average temperature of the junctions and the spin-splitting of the ferromagnetic (FM) contacts on the charge and spin thermopower are also described. It is found that the results show a magnitude enhancement at the specific thicknesses of the SC spacer (resonant positions) compared with the other thicknesses.

Temperature Gradient Effects on Spin Torque in Double Barrier Magnetic Tunnel Junctions with a Non-magnetic Metal Spacer

Temperature gradient effects on spin torque (ST) are studied in MgO-based double-barrier magnetic tunnel junctions (DBMTJs) with a non-magnetic metal (NM) spacer. Using non-equilibrium Green’s function (NEGF) formalism, it is found that the thermal ST (TST) show oscillatory behavior with an increase of theNM spacer thickness. The results also show a magnitude enhancement of the TST in a DBMTJ compared with a single-barrier MTJ (SBMTJ). Such behavior is related to the existence of the quantum well states inside the NM spacer and resonant tunneling effect. Therefore, a faster magnetization switching is possible by TST in a DBMTJ without any bias voltage.

Resonant spin-transfer torque in asymmetric double barrier magnetic tunnel junctions (MTJs)

The substitution effect of a Ferro-magnet (FM) electrode by a half-metallic FM material La0.7Sr0.3MnO3 (LSMO) on charge current and spin-transfer torque (STT) components is studied in MgO-based double barrier magnetic tunnel junctions (DBMTJs) with a middle non-magnetic metal (NM) layer. Using non-equilibrium Green’s function (NEGF) formalism, it is observed that the current and STT components show oscillatory behavior due to quantum well states in the middle NM layer and resonant tunneling effect. We also study effect of difference in the thickness of the MgO insulators. Bias dependence demonstrate the magnitude enhancement of the current and in-plane STT in new asymmetric DBMTJs (A-DBMTJs) compared with symmetric DBMTJs (S-DBMTJs), however, perpendicular STT decreases in the A-DBMTJs. Results also show different behavior compared with conventional asymmetric MTJs and spin valves (SVs). Therefore, one can design new memory devices by means of suitable insulator and FM electrodes with proper thicknesses.

Dynamic and Magnetotransport Properties of Perpendicularly Magnetized CoFeB Magnetic Tunnel Junctions

We present the study of the dynamic properties of CoFeB (CFB) based magnetic tunnel junctions (MTJ) by Brillouin light scattering spectroscopy and micro-strip ferromagnetic resonance technique as well as magnetoresistance measurements. Two different MTJs: single and double barrier systems have been investigated. The dynamic studies revealed mostly similar perpendicular anisotropy field for both CoFeB layers for the single barrier MTJ with the thickest CFB layers, while two sensibly different perpendicular anisotropy fields were revealed in the case of the double barrier MTJ with the thickest CFB soft and hard layers. Modeling the spin wave frequencies behavior allows us the analysis of the experimental data and the derivation of the pertinent magnetic parameters. Our results suggest an interfacial origin of the anisotropy. Therefore, by changing the thickness of the perpendicularly magnetized CFB layers, it is possible to obtain a free and pinned layer usually ensured by the use of synthetic antiferromagnetic material. The tunnel magnetoresistance has been measured in elliptical elements of double barrier MTJ of different sizes. Strong size-dependent switching fields and antiparallel state being more unstable in the smallest device are observed. Patterning effect, lowering the anisotropies around the edge areas and increasing the dipolar coupling that favors the parallel state in the case of stacked layers, are probably the origin.