Fe Magnetic Tunnel Junctions Research Articles

Structural and magneto-transport properties of lattice-mismatched epitaxial Fe/SrO/MgO/Fe magnetic tunnel junctions

We investigated the structural and magneto-transport properties of Fe/SrO/MgO/Fe magnetic tunnel junctions (MTJs), in which SrO has a large lattice mismatch (Δa/a ∼ 21%) with Fe. Despite such a large Δa/a, structural analyses revealed that a fully epitaxial Fe(001)/SrO(001)/MgO(001)/Fe(001) structure was successfully formed. We observed high magnetoresistance (MR) ratios up to 98% at 20 K (65% at room temperature), suggesting that coherent spin-polarized tunneling occurs through the SrO(001) tunnel barrier. We also demonstrated that the resistance-area product and bias-voltage (Vhalf), at which the MR ratio reaches half of the zero-bias value, were comparable to the values reported in epitaxial Fe/MgO/Fe MTJs. Since SrO has a smaller Δa/a with Si (∼5%) than that between MgO and Si (∼23%), the results offer that this material is a promising candidate as a high-quality tunnel barrier for Si-based spin transport devices.

Machine learning analysis of tunnel magnetoresistance of magnetic tunnel junctions with disordered MgAl2O4

Through Bayesian optimization and the least absolute shrinkage and selection operator (LASSO) technique combined with first-principles calculations, we investigated the tunnel magnetoresistance (TMR) effect of Fe/disordered-MgAl2O4(MAO)/Fe(001) magnetic tunnel junctions (MTJs) to determine structures of disordered-MAO that give large TMR ratios. The optimal structure with the largest TMR ratio was obtained by Bayesian optimization with 1728 structural candidates, where the convergence was reached within 300 structure calculations. Characterization of the obtained structures suggested that the in-plane distance between two Al atoms plays an important role in determining the TMR ratio. Since the Al-Al distance of disordered MAO significantly affects the imaginary part of complex band structures, the majority-spin conductance of the {\Delta}1 state in Fe/disordered-MAO/Fe MTJs increases with increasing in-plane Al-Al distance, leading to larger TMR ratios. Furthermore, we found that the TMR ratio tended to be large when the ratio of the number of Al, Mg, and vacancies in the [001] plane was 2:1:1, indicating that the control of Al atomic positions is essential to enhancing the TMR ratio in MTJs with disordered MAO. The present work reveals the effectiveness and advantage of material informatics combined with first-principles transport calculations in designing high-performance spintronic devices based on MTJs.

Origin of the large voltage-controlled magnetic anisotropy in a Cr/Fe/MgO junction with an ultrathin Fe layer: First-principles investigation

Voltage-controlled magnetic anisotropy (VCMA) has attracted broad interest due to its high efficiency in switching magnetization. Large VCMA was experimentally observed in Cr/Fe/MgO junction with ultrathin Fe layer [Nozaki et al., Phys. Rev. Appl. 5, 044006 (2016)], whose underlying mechanism was still not clear however. The Cr/Fe/MgO/Fe magnetic tunnel junction (MTJ) is also well known for its quantum-well (QW) states and as-induced spin-dependent resonant tunneling [Greullet et al., Phys. Rev. Lett. 99, 187202 (2007)]. Here, in order to uncover the relation between the large VCMA and the QW states, we developed a $k$-resolved VCMA calculation method combined with the second-order perturbation theory to investigate it. We find the VCMA coefficient reaches $\ensuremath{-}297$ fJ/V m matching well with the previous experiment with three monolayers (MLs) of Fe. The coefficient oscillates strongly and even changes its sign with increasing the number of Fe MLs. Comparing the $k$-resolved VCMA with the Fermi surface of the interfacial Fe atom, the screening charges theory for VCMA was verified. For $2--9$ MLs Fe, interestingly, the QW states of ${\mathrm{\ensuremath{\Delta}}}_{1}$ electron at the $\mathrm{\ensuremath{\Gamma}}$ point provide large (no) contribution to the VCMA with odd (even) MLs. Moreover, the change of the orbital-resolved Fermi surface at the interfacial Fe atom also plays an important role on VCMA oscillation, which as well as the QW states results in the largest VCMA for 3-ML Fe. Our results deepen the understanding of the large VCMA in the Cr/Fe/MgO junction, which would be helpful to design a practical MTJ with large VCMA.

Estimation of Rectifying Performance for Terahertz Wave in Newly Designed Fe/ZnO/MgO/Fe Magnetic Tunnel Junction

We fabricated fully epitaxial Fe/ZnO/MgO/Fe magnetic tunnel junctions (MTJs) with low junction resistance-area products (several $\Omega$ $\mu$m$^2$) and conducted a theoretical estimation of square-low rectifying performance for a terahertz electromagnetic wave. Effective current responsivity up to 0.09 A/W at 1 THz was obtained under zero-bias voltage condition at room temperature. The result is approximately half the value of the best result obtained for experiments in semiconductor-based diodes, performed under similar conditions. The study strongly suggests that this MTJ system has a great potential for the rectifying element of the terahertz wave.

Theoretical study of rectifying properties in a terahertz regime with a zero-bias voltage for fully epitaxial Fe/ZnO/MgO/Fe magnetic tunnel junctions

We conducted a theoretical study of high-frequency rectifying properties with a zero-bias voltage in epitaxial Fe(001)/ZnO(001)/MgO(001)/Fe(001) magnetic tunnel junctions (MTJs) in conjunction with the expectation of the element being applied as a square-low detector. By optimizing device parameters such as insulator thickness and the junction area, we obtained current responsivity of up to 0.12 A W−1 at 1 THz (with a cut-off frequency of 1.5 THz), which is comparable to the best results obtained in experiments for semiconductor-based diodes, performed under similar conditions. The results indicate that this MTJ system has great potential for use as a high-frequency rectifying element in a terahertz regime.

Influence of HfO2 interlayers on magnetocrystalline anisotropy in Fe|MgO|Fe magnetic tunnel junction: First-principles investigation

Perpendicular magnetic anisotropy (PMA) is important for MgO based magnetic tunnel junction and magnetic random access memory to be integrated on a large scale due to high thermal stability and low critical switching current. Here, we applied the density functional theory to study the effect of HfO2 inserting layers on PMA of Fe|MgO|Fe tunnel junction. It was found that the magnetocrystalline anisotropy (MCA) of the junction for 5 layers Fe electrode was up to 1.95 mJ/m2 with one unit cell HfO2 interlayer, while it was 1.72 mJ/m2 without the interlayer. More importantly, analyzed by the layer and orbital-resolved MCA based on the second-order perturbation theory, MCA characters and the underlying mechanism of PMA become very different after inserting HfO2. The remarkable difference is the MCA contribution of the second interfacial Fe layers, which is about 0.4 mJ/m2 for Fe|MgO|Fe junctions, while it was larger than 0.7 mJ/m2 for Fe|HfO2|MgO|HfO2|Fe junctions. Furthermore, Fe-dz2 and O-pz hybridization plays a crucial role in MCA contribution from the first interfacial Fe layers since the interfacial Fe–O bond length reduces from 2.20 Å to 1.77 Å with inserting HfO2 layers. Besides, the reduction in Fe–O bond length can redistribute the orbital-resolved electrons of the second and third closest Fe layer to the interface to enhance their absolute values of MCA contributions, which results in the strong dependence of MCA on Fe thickness.

Nontrivial spatial dependence of the spin torques in L10 FePt-based tunneling junctions

We present an {\it ab-initio} study of the spin-transfer torque in a Fe/MgO/FePt/Fe magnetic tunnel junctions. We consider a FePt film with a thickness up to six unit cells, either in direct contact with the MgO spacer or with an intercalated ultra-thin Fe seed layer. We find that in the FePt layer the torque is not attenuated as strongly as in the case of pure Fe. Moreover, in FePt the torque alternates sign at the Fe and Pt atomic planes throughout the stack for all FePt thicknesses considered. Finally, when Fe is intercalated between MgO and L$1_0$-FePt, the torque is sharply attenuated and it is transferred to FePt only for a Fe seed layer that is less than two-atomic-planes thick. We attribute these features to the different spatial profiles of the exchange and correlation field and the induced non-equilibrium spin accumulation. The calculated tunnelling magneto-resistance of the Fe/MgO/FePt/Fe junctions studied is enhanced with respect to the one of Fe/MgO/Fe, while it is reduced with Fe intercalation. Our work shows that L$1_0$-FePt junctions can be promising candidates for current-operated magnetic devices and that the magnetic texture at the atomic scale has an important effect on the spin transfer torque.

The dependence of TMR on the barrier thickness, bias voltage and asymmetry in Fe/ZnO/Fe MTJs: A DFT study

Density functional theory (DFT) and non-equilibrium Green's function (NEGF) methods are used to simulate spin-polarized transport properties in Fe(001)/ZnO(001)/Fe(001) magnetic tunnel junctions (MTJs) with an even number of ZnO monolayers (ML). The barrier thickness conductance, tunneling magneto-resistance (TMR) ratio, and tunneling current are investigated for parallel and antiparallel alignment of the magnetization of electrodes. The results indicate that in zero and non-zero bias voltages the slower decay of conductance in the parallel alignment configuration, compared to antiparallel alignment, leads to increased TMR ratio for thicker barriers. Additionally, it is shown that, across all thicknesses of the ZnO barrier, the TMR ratio is highly sensitive to the applied bias voltage and decreases with increasing voltage. Moreover, analysis of the 4-ML MTJ reveals that, for the same density of states (DOS) in the symmetric structure, increasing the DOS at the Fermi level of antiparallel alignment configuration in the asymmetric case results in a negative TMR ratio. The results suggest that a ZnO rock-salt type potential barrier is as suitable as the traditional MgO barrier for spintronic purposes in MTJs.

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.

Effect of MgO Underlying Layer on the Growth of GaOx Tunnel Barrier in Epitaxial Fe/GaOx/(MgO)/Fe Magnetic Tunnel Junction Structure.

We investigated the effect of a thin MgO underlying layer (~3 monoatomic layers) on the growth of GaOx tunnel barrier in Fe/GaOx/(MgO)/Fe(001) magnetic tunnel junctions. To obtain a single-crystalline barrier, an in situ annealing was conducted with the temperature being raised up to 500 °C under an O2 atmosphere. This annealing was performed after the deposition of the GaOx on the Fe(001) bottom electrode with or without the MgO(001) underlying layer. Reflection high-energy electron diffraction patterns after the annealing indicated the formation of a single-crystalline layer regardless of with or without the MgO layer. Ex situ structural studies such as transmission electron microscopy revealed that the GaOx grown on the MgO underlying layer has a cubic MgAl2O4-type spinel structure with a (001) orientation. When without MgO layer, however, a Ga-Fe-O ternary compound having the same crystal structure and orientation as the crystalline GaOx was observed. The results indicate that the MgO underlying layer effectively prevents the Fe bottom electrode from oxidation during the annealing process. Tunneling magneto-resistance effect was observed only for the sample with the MgO underlying layer, suggesting that Ga-Fe-O layer is not an effective tunnel-barrier.

Investigation on the formation process of single-crystalline GaOx barrier in Fe/GaOx/MgO/Fe magnetic tunnel junctions

We have grown Fe(0 0 1)/GaOx(0 0 1)/MgO(0 0 1)/Fe(0 0 1) magnetic tunnel junctions (MTJs) with or without in situ annealing after the deposition of GaOx layer and performed structural characterizations by focusing on the formation process of the single-crystalline GaOx. It was found that, even without the in situ annealing, the as-grown GaOx grown on the MgO was mostly single-crystalline except near the surface region (amorphous). The crystallization temperature of the amorphous region was reduced from 500 °C down to 250 °C by depositing the Fe upper electrode (poly-crystalline). It was clarified that the crystallization of the amorphous region near the Fe/GaOx interface caused the realignments of the crystal grains in the poly-crystalline Fe upper electrode, and, as a result, the fully epitaxial Fe/GaOx/MgO/Fe structure is eventually formed. All the MTJs showed high tunneling magnetoresistance ratios (about 100%) at room temperature, which was almost independent of the formation temperature of the single-crystalline GaOx.

Negative tunneling magnetoresistance of Fe/MgO/NiO/Fe magnetic tunnel junction: Role of spin mixing and interface state

Motivated by a recent tunneling magnetoresistance (TMR) measurement in which the negative TMR is observed in MgO/NiO-based magnetic tunnel junctions (MTJs), we have performed systematic calculations of transmission, current, and TMR of Fe/MgO/NiO/Fe MTJ with different thicknesses of NiO and MgO layers based on noncollinear density functional theory and non-equilibrium Green's function theory. The calculations show that, as the thickness of NiO and MgO layers is small, the negative TMR can be obtained which is attributed to the spin mixing effect and interface state. However, in the thick MTJ, the spin-flipping scattering becomes weaker, and thus, the MTJs recover positive TMR. Based on our theoretical results, we believe that the interface state at Fe/NiO interface and the spin mixing effect induced by noncollinear interfacial magnetization will play important role in determining transmission and current of Fe/MgO/NiO/Fe MTJ. The results reported here will be important in understanding the electron tunneling in MTJ with the barrier made by transition metal oxide.

MgGa2O4 spinel barrier for magnetic tunnel junctions: Coherent tunneling and low barrier height

Epitaxial Fe/magnesium gallium spinel oxide (MgGa2O4)/Fe(001) magnetic tunnel junctions (MTJs) were fabricated by magnetron sputtering. A tunnel magnetoresistance (TMR) ratio up to 121% at room temperature (196% at 4 K) was observed, suggesting a TMR enhancement by the coherent tunneling effect in the MgGa2O4 barrier. The MgGa2O4 layer had a spinel structure and it showed good lattice matching with the Fe layers owing to slight tetragonal lattice distortion of MgGa2O4. Barrier thickness dependence of the tunneling resistance and current-voltage characteristics revealed that the height of the MgGa2O4 barrier is much lower than that of an MgAl2O4 barrier. This study demonstrates the potential of Ga-based spinel oxides for MTJ barriers having a large TMR ratio at a low resistance area product.

High Magnetoresistance in Fully Epitaxial Magnetic Tunnel Junctions with a SemiconductingGaOxTunnel Barrier

The spin field-effect transistor (spin-FET) is the ultimate spintronic device, and the ultimate issue for spin-FET technology is realizing a high magnetoresistance (MR) ratio in a ferromagnet/semiconductor/ferromagnet sandwich. The authors succeed in fabricating fully epitaxial Fe/GaO${}_{x}$/Fe magnetic tunnel junctions with MR ratios up to 92% at room temperature---a milestone in the development of spin-FETs in a vertical, rather than planar, configuration.

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.

Large voltage-induced magnetic anisotropy field change in ferrimagnetic FeGd

The voltage-induced magnetic anisotropy change of an ultrathin ferrimagnetic FeGd alloy has been characterized using V|Fe90Gd10|MgO|Fe magnetic tunnel junctions. The bias-voltage dependence of the magnetoresistance measurement reveals the voltage-induced anisotropy change in the V|Fe90Gd10|MgO junction. The magnetic anisotropy field change (ΔHk) in Fe90Gd10 is about twice that in Fe owing to the small net magnetization of the former. Therefore, employing ferrimagnetic materials is a promising option for voltage-controlled frequency-tunable spintronic devices.

First-principles study of spin transport in Fe–SiCNT–Fe magnetic tunnel junction

We report first-principles calculations of spin-dependent quantum transport in Fe–SiCNT–Fe magnetic tunnel junction (MTJ). Perfect spin filtration effect and substantial tunnel magnetoresistance are obtained, which suggests SiCNTs as a suitable candidate over CNTs for implementing 1D MTJs. The calculated tunnel magnetoresistance is several hundred percent at zero bias voltage, it reduces to nearly zero after the bias voltage of about 1 V. When the orientation of magnetic configurations of both electrodes is parallel, the zero bias spin injection factor is staggering 99% and remains reasonably high in the range of 60%–75% after the bias voltage of 0.6 V.

Taming the resistive switching in Fe/MgO/V/Fe magnetic tunnel junctions: An ab initio study

A possible mechanism for the resistive switching observed experimentally in Fe/MgO/V/Fe junctions is presented. Ab initio total energy calculations within the local density approximation and pseudopotential theory shows that by moving the oxygen ions across the MgO/V interface one obtains a metastable state. It is argued that this state can be reached by applying an electric field across the interface. In addition, the ground state and the metastable state show different electric conductances. The latter results are discussed in terms of the changes of the density of states at the Fermi level and the charge transfer at the interface due to the oxygen ion motion.

Electric and thermo spin transfer torques in Fe/Vacuum/Fe tunnel junction

We present first-principle calculations of electric and thermo spin transfer torques (STT) in Fe/Vacuum(Vac)/Fe magnetic tunnel junctions (MTJs). Our quantitative studies demonstrate rich bias dependence of STT and tunnel magneto resistance (TMR) behaviors with respect to the interface roughness. Thermoelectric effects in Fe/Vac/Fe MTJs is remarkable. We observe larger ZT of 6.2 in 8 ML clean Vacuum barrier, where the heavily restrained thermal conductance should be responsible for. Thermo-STT in Fe/Vac/Fe MTJs show same order as that in Fe/MgO/Fe MTJs with similar barrier thickness.

Magnetic anisotropy modified by electric field in V/Fe/MgO(001)/Fe epitaxial magnetic tunnel junction

Single-crystalline V/Fe(0.7 nm)/MgO(1.2nm)/Fe(20 nm) magnetic tunnel junctions are studied to quantify the influence of an electric field on the Fe/MgO interface magnetic anisotropy. The thinnest Fe soft layer has a perpendicular magnetic anisotropy (PMA), whereas the thickest Fe layer acts as sensor for magnetic anisotropy changes. When electrons are added at the PMA Fe/MgO interface (negative voltage), no anisotropy changes are observed. For positive voltage, the anisotropy constant decreases with increasing bias voltage. A huge 1150 fJ V−1 m−1 anisotropy variation with field is observed and the magnetization is found to turn from out-of-plane to in-plane of the sample with the applied voltage.