Fe Tunnel Junctions Research Articles

Integration of graphene and two-dimensional ferroelectrics: properties and related functional devices.

Ferroelectric (FE) thin films have been investigated for many years due to their broad applications in electronic devices. It was recently demonstrated that FE functionality persists in ultrathin films, possibly even in monolayers of two-dimensional (2D) FEs. However, the feasibility of 2D-based FE functional devices remains an open challenge. Here, we employ density-functional-theory calculations to propose and document the possible integration of graphene with 2D FE materials on metal substrates in the form of functional FE devices. We show that monolayers of proposed M2O3 (M = Al, Y) in the quintuple layer (QL) In2Se3 structure are stable 2D FE materials and that QL-M2O3 is a functional tunnel barrier in a graphene/QL-M2O3/Ru heterostructure. The QL-M2O3 barrier width can be modulated by its polarization direction, whereby the heterostructure can function as a prototype ferroelectric tunnel junction. Moreover, alternating the polarization of QL-M2O3 modulates the doping type of graphene, enabling the fabrication of graphene p-n junctions. By design, the proposed heterostructures can in principle be fabricated by intercalation, which is known to produce high-quality, large-scale 2D-based heterostructures.

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.

Tunneling Magnetoresistance and Spin-Dependent Diode Performance in Fully Epitaxial Magnetic Tunnel Junctions With a Rocksalt ZnO/MgO Bilayer Tunnel Barrier

Metal/insulator/metal (MIM) tunnel diodes are promising for high-frequency rectifier systems, such as for energy harvesting in the infrared-to-terahertz range, where typical semiconductor devices cannot operate. For practical applications, though, better rectification is needed. The authors fabricate fully epitaxial Fe/ZnO/MgO/Fe tunnel junctions exhibiting 96% magnetoresistance at room temperature, as well as greatly enhanced rectification performance, due to magnon excitations at the Fe/barrier interfaces. This is an exciting result for energy harvesting at longer wavelengths.

Thermally induced dynamics in ultrathin magnetic tunnel junctions

We consider the magnetization dynamics induced by thermally induced spin transfer torques in thin Fe|MgO|Fe tunnel junctions. The magnetization dynamics is described by the Landau-Lifshitz-Gilbert equation, including the thermal torques as computed from first principles. We show that the angular skewness of the torques importantly modifies the dynamics, e.g., leading to different instability criteria.

Thermal electric effects in Fe|GaAs|Fe tunnel junctions

We study the spin polarized thermoelectric effects on Fe|GaAs|Fe tunnel junction using a generalized Landauer-Büttiker formalism, where the energy flow is described on the same footing as the electric current. The Seebeck coefficient of tunnel junction will change sign as the GaAs thickness increases. We demonstrate the thermally induced STT on Fe|GaAs|Fe tunnel junction is robust against the interfacial defects and is non-negligible.

Effect of structural relaxation and oxidation conditions on interlayer exchange coupling in Fe|MgO|Fe tunnel junctions

The effect of structural relaxation and interfacial oxidation is demonstrated from first principles to have a crucial impact on interlayer exchange coupling (IEC) in crystalline Fe|MgO|Fe magnetic tunnel junctions (MTJs). It is shown that the IEC becomes antiferromagnetic for the relaxed structure in contrast to ferromagnetic for unrelaxed Fe|MgO|Fe MTJs. Furthermore, the antiferromagnetic IEC is strongly enhanced in the presence of oxygen vacancies while it is decreased by overoxidation and may even become ferromagnetic for sufficiently high oxygen concentration at the Fe∣MgO interface. These results were supported using a tight-binding model and provide an explanation for recent experimental studies.

Electrical switching in Fe/V/MgO/Fe tunnel junctions

Bipolar hysteretic resistance switching in epitaxial Fe/V/MgO/Fe magnetic tunnel junctions is observed in highly reproducible $I(V)$ curves and found to be modified by the frequency of the bias voltage sweep. Observation of slow relaxation of the resistance state values is reported. A model is proposed that takes into account the incidence of time-dependent electric-field-induced migration of atomic species on the effective barrier thickness. This model provides a good qualitative agreement with experimental data.

A two-band model of spin-polarized transport in Fe/Cr/MgO/Fe magnetic tunnel junctions

Theoretical studies of spin dependent transport in Fe/Cr/MgO/Fe tunnel junctions with noncollinear alignment of magnetizations of metallic layers comprising these magnetic tunnel junctions are presented. Calculations are performed with use of nonequilibrium Green function technique in the framework of the Keldysh formalism. Electronic band structure of ferromagnetic electrodes is modeled within a two-band model with majority and minority states being s-like and d-like electrons, respectively. Furthermore, interfacial s-d hybridization is taken into account and calculated using perturbation corrections for the wave and Green functions. It is shown that in the presence of Cr layer at the Fe/MgO interface, the contribution from s-d hybridization to the total current is much stronger in the antiparallel magnetizations configuration compared to the parallel one leading to decrease in tunnel magnetoresistance values in agreement with earlier reports.

Control of epitaxy-induced magnetocrystalline anisotropy in a molecular beam epitaxy–grown Co∕MgO∕Fe∕MgO(100) pseudo-spin-valve

We show that by obtaining a crystalline structure in a Co∕MgO∕Fe tunnel junction, we are able to control the magnetic anisotropies in the spin valve and therefore, induce independent switching of the two magnetic layers. In situ and ex situ structural characterization confirms that the multilayer is fully epitaxial, with smooth interfaces throughout. In confirmation of the high quality of the insulating barrier, we also present a layer-selective measurement of the magnetization of the top electrode using current-in-plane transport measurements.

Bias dependent inversion of tunneling magnetoresistance in Fe∕GaAs∕Fe tunnel junctions

The authors investigated spin dependent transport through Fe∕GaAs∕Fe tunnel junctions. The tunneling magnetoresistance (TMR) effect was probed for different types of Fe∕GaAs interfaces. For interfaces cleaned by hydrogen plasma the TMR effect is increased and is observable at room temperature. If an epitaxial Fe∕GaAs(001) interface is involved, the tunnel junction exhibits a bias dependent inversion of the TMR effect. This is a first experimental signature for band structure effects at an Fe∕GaAs interface and is relevant for spin injection experiments.

Interlayer exchange coupling in Fe∕MgO∕Fe magnetic tunnel junctions

Interlayer exchange coupling (IEC) in fully epitaxial Fe∕MgO∕Fe(001) tunnel junctions with wedge-shaped MgO layers is measured at room temperature from the unidirectional shift of the Kerr hysteresis loop. It is found that the IEC is antiferromagnetic for small MgO thickness but changes sign at 0.8nm. Ab initio calculations of IEC show that this behavior can be explained by the presence of O vacancies in the MgO barrier which makes IEC antiferromagnetic for thin barriers. With increasing MgO thickness the resonance contribution to IEC from localized defect states is reduced resulting in the ferromagnetic coupling typical for perfect MgO barriers.

Conductance calculations for quantum wires and interfaces: Mode matching and Green’s functions

Landauer's formula relates the conductance of a quantum wire or interface to transmission probabilities. Total transmission probabilities are frequently calculated using Green function techniques and an expression first derived by Caroli. Alternatively, partial transmission probabilities can be calculated from the scattering wave functions that are obtained by matching the wave functions in the scattering region to the Bloch modes of ideal bulk leads. An elegant technique for doing this, formulated originally by Ando, is here generalized to any Hamiltonian that can be represented in tight-binding form. A more compact expression for the transmission matrix elements is derived and it is shown how all the Green function results can be derived from the mode matching technique. We illustrate this for a simple model which can be studied analytically, and for an Fe|vacuum|Fe tunnel junction which we study using first-principles calculations.

High-field magnetoresistance of Fe∕GaAs∕Fe tunnel junctions

We investigate transport through 6–10nm thin epitaxial GaAs(001) barriers sandwiched between polycrystalline iron films. Apart from a pronounced tunneling magnetoresistance effect at low magnetic fields, we observe a distinct negative magnetoresistance (MR) at low and a positive MR at higher temperatures. We show that the negative MR contribution is only observed for the ferromagnetic iron contacts but is absent if iron is replaced by copper or gold electrodes. Possible explanations of the negative MR involve suppression of spin-flip scattering or Zeeman splitting of the tunneling barrier, but neither of these explanations is fully consistent with the data.

Magnetotransport properties of Co–Fe∕Al–N∕Co–Fe tunnel junctions with large tunnel magnetoresistance ratio

The magnetotransport properties of tunnel junctions with the structure of Ta 5nm∕Cu20nm∕Ta 5nm∕Ni76Fe242nm∕Cu 5nm∕Mn75Ir2510nm∕Co71Fe29 4nm∕Al–N∕Co71Fe29 4nm∕Ni76Fe2420nm∕Ta5nm were investigated. When the Al thickness, nitridation time, and annealing temperature were 1nm(0.8nm), 50s(35s), and 280°C(300°C), tunnel magnetoresistance (TMR) ratio and resistance-area product were 49%(34%) and 3×104Ωμm2(1.5×104Ωμm2), respectively. In order to clarify the annealing temperature dependence of TMR ratio, the local transport properties were studied using conducting atomic force microscope for Ta5nm∕Cu20nm∕Ta5nm∕Ni76Fe242nm∕Cu5nm∕Mn75Ir25 10nm∕Co71Fe29 4nm∕Al(0.8nm)–N junction fabricated with the nitridation time of 35s and Ar+N2 plasma as a function of annealing temperature. The large TMR at 300°C, where the TMR ratio of the corresponding magnetic tunnel junction had the maximum value of 34%, could be well elucidated by the enhancement of the average barrier height ϕave, and the reduction of its fluctuation. After annealing at 340°C, the leakage current was observed and the TMR ratio decreased.

60% magnetoresistance at room temperature in Co–Fe/Al–O/Co–Fe tunnel junctions oxidized with Kr–O2 plasma

The influence of the mixed inert gas species for plasma oxidization process of a metallic Al layer on the tunnel magnetoresistance (TMR) was investigated for a magnetic tunnel junction (MTJ), Ta 50 Å/Cu 200 Å/Ta 200 Å/Ni–Fe 50 Å/Cu 50 Å/Mn75Ir25 100 Å/Co70Fe30 25 Å/Al–O/Co70Fe30 25 Å/Ni–Fe 100 Å/Cu 200 Å/Ta 50 Å. Using Kr–O2 plasma, a 58.8% of TMR ratio was obtained at room temperature after annealing the junction at 300 °C, while the achieved TMR ratio of the MTJ fabricated with usual Ar–O2 plasma remained 48.6%. A faster oxidization rate of the Al layer by using Kr–O2 plasma is a possible cause to prevent the over oxidization of the Al layer, which depolarizes the surface of the underlaid ferromagnetic electrode, and to realize a large magnetoresistance.

Transport properties of epitaxial Fe–GaN–Fe tunnel junctions on GaAs

Using an ultra-high vacuum deposition system equipped with an electron cyclotron resonance microwave plasma nitrogen source, epitaxial Fe–GaN–Fe layers were grown on GaAs for the evaluation of GaN as a spin-dependent tunneling barrier. The tunnel junction conductance was strongly temperature dependent. Above 10 K thermionic emission over the barrier was the dominant transport mechanism, while at lower temperature direct tunneling prevailed. At 4 K, the I– V characteristics were non-linear as expected for metal–insulator–metal systems. To date present, spin-dependent tunneling was not observed.

TRANSPORT PROPERTIES ON La0.7Sr0.3MnO3/Al2O3/Fe TUNNEL JUNCTIONS

Manganese oxide La\-\{0.7\}Sr\-\{0.3\}MnO\-3 produced on (001) LaAlO\-3 substrate by means of sol gel spin coating method was used as the base electrode of LaSrMnO/Al\-2O\-3/Fe tunnel junctions. The I V characteristic in the high bias region of this system was shown to be similar to that of the conventional tunnel junctions. Anomalous temperature dependence of tunneling resistance was observed to be a positive temperature coefficient of resistance. This phenomenon was attributed to the high voltage applied and was simply elucidated from the density states vs. energy diagram.

Ferromagnetic tunnel junctions with plasma-oxidized Al barriers and their annealing effects

Cross-geometrical Ni–Fe/Co/Al–AlO/Co/Ni–Fe/Fe–Mn/Ni–Fe tunnel junctions were fabricated by magnetron sputtering. To form the insulating layer, an Al layer was reverse sputtered in an atmosphere of either oxygen or oxygen–argon mixture at low power after deposition. The oxidization time necessary to form an AlO barrier was much shorter than that by natural oxidization, lasting for only a half to a few minutes. By adding argon to oxygen, the oxidization was slowed down and high MR ratios were obtained for a wide range of time. A magnetoresistance (MR) ratio of 16% was observed in the as-deposited junction when the barrier was oxidized in oxygen plasma for 35 s. In addition, the MR ratio increased to 24% by annealing at 300 °C. In as-deposited junctions, the tunnel resistances were increased by increasing the plasma oxidization time, but the MR ratios gradually decreased. The estimated tunnel barrier width increased and the barrier height decreased with the plasma oxidization time. After annealing, the MR ratio increased only for those junctions oxidized for short times. This suggests that the remaining Al between the AlO and the Co surface plays an important role in the effects of annealing.

Evidence for spin fluctuations in vanadium from a tunneling study of Fermi-liquid effects.

The Fermi-liquid parameter ${G}_{0}$ is found to be close to zero in vanadium, providing new experimental evidence for the importance of spin fluctuations in this material. Al/${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$/V and V/${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$/Fe tunnel junctions were used to observe the renormalization caused by Fermi-liquid interactions of the spin splitting of the vanadium's superconducting density of states in a magnetic field. This was done by fitting the theory of Rainer to the measured dynamic tunneling conductance. Excellent agreement was found with this theory, and the results were consistent with critical field measurements made on the same vanadium films. The spin-orbit scattering rate ${b}_{\mathrm{so}}$ was determined to be 0.07\ifmmode\pm\else\textpm\fi{}0.03.