Magnetic Tunnel Junction System Research Articles

Ultra-thin van der Waals magnetic tunnel junction based on monoatomic boron vacancy of hexagonal boron nitride.

We present a novel strategy to create a van der Waals-based magnetic tunnel junction (MTJ) comprising a three-atom layer of graphene (Gr) sandwiched with hexagonal boron nitride (hBN) layers by introducing a monoatomic boron vacancy in each hBN layer. The magnetic properties and electronic structure of the system were investigated using density functional theory (DFT) and the transmission probability of the MTJ was investigated using the Landauer-Büttiker formalism within the non-equilibrium Green function method. The Stoner gap was created between the spin-majority channel and the spin-minority channel on the local density of states of the hBN monoatomic boron-vacancy (VB) near the Fermi energy, creating a possible control of the spin valve by considering two magnetic alignment of hBN(VB) layers, anti-parallel configuration (APC) and parallel configuration (PC). The transmission probability calculation results showed a high electron transmission in the PC of the hBN(VB) layers and a low transmission when the APC was considered. A high tunneling magnetoresistance (TMR) ratio of approximately 400% was observed when comparing the APC and PC of the hBN(VB) layers in hBN(VB)/Gr/hBN(VB), giving the highest TMR ratio for the thinnest MTJ system.

Large Perpendicular Magnetic Anisotropy in Ta/CoFeB/MgO on Full-Coverage Monolayer MoS2 and First-Principles Study of Its Electronic Structure.

A perpendicularly magnetized spin injector with a high Curie temperature is a prerequisite for developing spin optoelectronic devices on two-dimensional (2D) materials working at room temperature (RT) with zero applied magnetic field. Here, we report the growth of Ta/CoFeB/MgO structures with large perpendicular magnetic anisotropy (PMA) on full-coverage monolayer (ML) molybdenum disulfide (MoS2). A large perpendicular interface anisotropy energy of 0.975 mJ/m2 has been obtained at the CoFeB/MgO interface, comparable to that observed in magnetic tunnel junction systems. It is found that the insertion of MgO between the ferromagnetic (FM) metal and the 2D material can effectively prevent the diffusion of the FM atoms into the 2D material. Moreover, the MoS2 ML favors a MgO(001) texture and plays a critical role in establishing the large PMA. First-principles calculations on a similar Fe/MgO/MoS2 structure reveal that the MgO thickness can modify the MoS2 band structure, from a direct band gap with 3ML-MgO to an indirect band gap with 7 ML-MgO. The proximity effect induced by Fe results in splitting of 10 meV in the valence band at the Γ point for the 3ML-MgO structure, while it is negligible for the 7 ML-MgO structure. These results pave the way to develop RT spin optoelectronic devices based on 2D transition-metal dichalcogenide materials.

Analytical expressions for the ferromagnetic resonance mode intensity and linewidths for a weakly coupled magnetic tunnel junction system

Ferromagnetic Resonance mode intensity, I, field linewidth, ΔH, and frequency linewidth, Δf, have been investigated for a weakly coupled magnetic tunnel junction (MTJ) system. The analysis applies for a coupled trilayer, i.e., two ferromagnetic thin films separated by a non magnetic thin film and for a MTJ system consisting of the trilayer and an antiferromagnetic layer. For this particular weakly coupled system, analytical expressions for I, ΔH, and Δf have been determined. The linewidth expressions can be separated into the parts characterizing the individual thin films and magnetic coupling dependent parts; the mode linewidths can be viewed as those of uncoupled thin films with equivalent damping parameters. The intensity and the linewidths are discussed for different applied magnetic fields, for the saturated and unsaturated systems, and for the two types of coupling. It will be shown how the mode intensity behavior can be used as a probe to distinguish the ferromagnetic from the antiferromagnetic coupling.

Voltage-controlled interlayer coupling in perpendicularly magnetized magnetic tunnel junctions

Magnetic interlayer coupling is one of the central phenomena in spintronics. It has been predicted that the sign of interlayer coupling can be manipulated by electric fields, instead of electric currents, thereby offering a promising low energy magnetization switching mechanism. Here we present the experimental demonstration of voltage-controlled interlayer coupling in a new perpendicular magnetic tunnel junction system with a GdOx tunnel barrier, where a large perpendicular magnetic anisotropy and a sizable tunnelling magnetoresistance have been achieved at room temperature. Owing to the interfacial nature of the magnetism, the ability to move oxygen vacancies within the barrier, and a large proximity-induced magnetization of GdOx, both the magnitude and the sign of the interlayer coupling in these junctions can be directly controlled by voltage. These results pave a new path towards achieving energy-efficient magnetization switching by controlling interlayer coupling.

Resonant and Switching Fields for a Weakly Coupled Magnetic Tunnel Junction System

The ferromagnetic resonance (FMR) and the magnetization switching fields have been analytically determined for weakly coupled Magnetic Tunnel Junction or spin valve systems. Such systems consist of an antiferromagnetic (AF) and two ferromagnetic (F) thin films separated by a nonmagnetic one. Both bilinear [Formula: see text] and biquadratic [Formula: see text] coupling parameters are used to describe the interlayer exchange coupling, while the exchange anisotropy field [Formula: see text] characterizes the interfacial ([Formula: see text]/AF) interaction. It will be shown how [Formula: see text], [Formula: see text], [Formula: see text] and the in-plane anisotropy fields can be derived for the experimental measure of the FMR field and the different magnetization switching fields.

Competing Anisotropy-Tunneling Correlation of the CoFeB/MgO Perpendicular Magnetic Tunnel Junction: An Electronic Approach.

We intensively investigate the physical principles regulating the tunneling magneto-resistance (TMR) and perpendicular magnetic anisotropy (PMA) of the CoFeB/MgO magnetic tunnel junction (MTJ) by means of angle-resolved x-ray magnetic spectroscopy. The angle-resolved capability was easily achieved, and it provided greater sensitivity to symmetry-related d-band occupation compared to traditional x-ray spectroscopy. This added degree of freedom successfully solved the unclear mechanism of this MTJ system renowned for controllable PMA and excellent TMR. As a surprising discovery, these two physical characteristics interact in a competing manner because of opposite band-filling preference in space-correlated symmetry of the 3d-orbital. An overlooked but harmful superparamagnetic phase resulting from magnetic inhomogeneity was also observed. This important finding reveals that simultaneously achieving fast switching and a high tunneling efficiency at an ultimate level is improbable for this MTJ system owing to its fundamental limit in physics. We suggest that the development of independent TMR and PMA mechanisms is critical towards a complementary relationship between the two physical characteristics, as well as the realization of superior performance, of this perpendicular MTJ. Furthermore, this study provides an easy approach to evaluate the futurity of any emerging spintronic candidates by electronically examining the relationship between their magnetic anisotropy and transport.

Comprehensive and Macrospin-Based Magnetic Tunnel Junction Spin Torque Oscillator Model- Part II: Verilog-A Model Implementation

The rapid development of the magnetic tunnel junction (MTJ) spin torque oscillator (STO) technology demands an analytical model to enable building MTJ STO-based circuits and systems so as to evaluate and utilize MTJ STOs in various applications. In Part I of this paper, an analytical model based on the macrospin approximation, has been introduced and verified by comparing it with the measurements of three different MTJ STOs. In Part II, the full Verilog-A implementation of the proposed model is presented. To achieve a reliable model, an approach to reproduce the phase noise generated by the MTJ STO has been proposed and successfully employed. The implemented model yields a time domain signal, which retains the characteristics of operating frequency, linewidth, oscillation amplitude and DC operating point, with respect to the magnetic field and applied DC current. The Verilog-A implementation is verified against the analytical model, providing equivalent device characteristics for the full range of biasing conditions. Furthermore, a system that includes an MTJ STO and CMOS RF circuits is simulated to validate the proposed model for system- and circuit-level designs. The simulation results demonstrate that the proposed model opens the possibility to explore STO technology in a wide range of applications.

Negative differential resistance in magnetic tunnel junction systems

We study the electrophysical properties of the Fe/MgO/Fe magnetic tunnel junctions (MTJ) with impurities. Sample structures are fabricated on top of fine‐crystalline glass–ceramic substrates by e‐beam evaporation in a relatively low vacuum (∼10−4 Torr). The influence of the first magnetic layer fabrication conditions on the degradation of the MTJ is explained by the interlayer diffusion. Various models of electrophysical processes in MTJ on polycrystalline substrates are discussed. The current–voltage (I–V) characteristics of the fabricated structures are found to exhibit a region with negative differential resistance, similar to the one in tunneling diodes. We explain this phenomenon by the formation of excitons in the MgO layer modified by the conductive impurity atoms and their diffusion. The obtained results will be useful in the development of MRAM devices containing MTJs and tunneling diodes.

Effects of In-Plane Magnetization Orientation on Magnetic and Electronic Properties in a Bcc Co (001)/Rock Salt MgO (001)/Bcc Co (001) Magnetic Tunnel Junction System: <I>Ab Initio</I> Calculations

Ab initio calculations were performed on a fully epitaxial bcc Co (001)/rock salt MgO (001)/bcc Co (001) magnetic tunnel junction system for two cases where the magnetization is parallel to bcc Co [100] and to bcc Co [110]. Structural optimization reveals that the two cases are equivalent systems and that the Co electrodes contract in the z-direction whereas the MgO insulating barrier expands. The magnetic moments of each monolayer vary slightly in each case; furthermore, only the magnetic moment at the surface of the Co atom shows any enhancement (12%). The layer decomposed density of states profiles reveals that the bonding character of the junction interface is derived mainly from the 2p-3d hybridization of the MgO and Co interfacial atoms.

Influence of interface state in Fe/MgO/Fe magnetic tunnel junction system: C modified interfaces—a first principle study

The Fe/MgO/Fe magnetic tunnel junction with C modified interfaces has been studied based on the first principle density function theory method under a finite bias voltage for thin (five layers) and thick (ten layers) MgO barriers. Positive and negative tunneling magnetic resistance (TMR) ratios are obtained as a function of the interface structure. We found that the tunneling conductance is highly nonlinear for asymmetric systems with C at one side of the barrier, and even a sign reversal of the TMR as a function of the bias was found to be in agreement with experiments.

Coupling Asymmetry Effect on the Coherent Spin Current Through a Ferromagnetic Lead/Quantum Dot/Ferromagnetic Lead System

The spin dependent transport through asymmetric double barrier magnetic tunnel junction system is theoretically studied, based on the non-equilibrium Green's function obtained via the equation of motion method. In the system, a single energy level is considered in the semiconductor quantum dot, which is asymmetrically coupled to the left and the right leads. It is found that due to the asymmetric coupling, the threshold voltages for charge current I - V characteristics are different for positive and negative applied bias, so are the spin current I - V characteristics. Besides, it is also found that the direction of current flow also modifies the electron occupancy within the dot.

The effect of image force and temperature on electrical characteristics of layer-type MTJs

Tunnel resistivity R (= V/J) as a function of voltage has been calculated for magnetic tunnel junction systems, such as ferromagnet-insulator-ferromagnet three-layer structures. Our early study [1] has been carried out for determining the barrier characteristics in the MTJs based on Ni80Fe20/Al2O3/Co systems, by fitting the experimental I-V characteristics to Simmons' and Brinkmann's models without image force. In this paper, the image force is taken into account for approaching closely to a real barrier potential. The effect of dielectric constant, and temperature on the tunnel characteristics has also been investigated.

Ab initio calculations of structural and magnetic properties of Fe(0 0 1)/MgO(0 0 1)/Fe(0 0 1) magnetic tunnel junction with interfacial Mg vacancy

The structural and magnetic properties of the Fe(0 0 1)/MgO(0 0 1)/Fe(0 0 1) magnetic tunnel junction (MTJ) with Mg vacancy at the interface were investigated using density functional theory. For a defect-free MTJ, the interfacial Fe–O distance was 2.06 Å and 13.8% of thickness reduction was found in electrode layers. For the Mg deficient MTJ system, the interfacial Mg atoms were positioned 0.19 Å below the O-interface atoms, resulting in the Fe–Mg distance of 1.74 Å. Each atomic slab exhibited large fluctuations of spin polarization values since complex slab separation occurred in both barrier and electrode layers. Specifically, the magnetic moment of 0.20 μB was induced for O atoms at the interface slab. Large variations in the spin polarization and magnetic moment values for Fe atoms were found in the MTJ system with Mg vacancy.

Spin tunnelling phenomena in single-crystal magnetic tunnel junction systems

A brief theoretical review points out the specific aspects of electronic transport insingle-crystal magnetic tunnel junctions employing bcc(100) Fe electrodes and a MgO(100)insulating barrier. The theoretical predictions are compared to the experimental reality inboth equilibrium and out-of-equilibrium regimes. For extremely small MgO thickness, weillustrate that the equilibrium tunnel transport in Fe/MgO/Fe systems leads toantiferromagnetic interactions. Artificial antiferromagnetic systems based on coupling byspin polarized tunnelling have been elaborated and studied. In the out-of-equilibriumregime and for large MgO barrier thickness, the tunnel transport validates specificspin filtering effects in terms of symmetry of the electronic Bloch function andsymmetry-dependent wavefunction attenuation in the single-crystal barrier. Within thisframework, we explain the experimental giant tunnel magnetoresistive effects at roomtemperature, up to 180%, measured in our simple or double barrier tunnel junctionsystems. Moreover, we illustrate that the magneto-transport properties of the junctionsmay be skilfully engineered by adjusting the interfacial chemical and electronic structure.

Study of annealing and exchange bias effects in PtMn based magnetic tunnel junction system

The crystallization structure and thickness of PtMn layer in a magnetic tunnel junction system are important factors to improve its exchange bias effect. This study shows that the PtMn layer could be changed from a FCC (111) structure to a FCT (111) structure after annealing above 270 °C. The minimum thickness of PtMn layer is found to be 10 nm for exchange coupling effect to be occurred in our MTJ system. The magnetic exchange effect between PtMn and SAF layers is near 4,300 Oe. Annealing temperatures can be higher than 400 °C for samples without patterning; however, temperature at 275 °C is too high for samples after patterning. This may be due to the breakdown of edges of the patterned samples as well as the complicated environments around the patterned samples. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Evolution of magnetic coupling in ferromagnetic tunnel junctions by annealing

In spin valve and magnetic tunnel junction systems, the soft sensing layer should be sensitive to small fields of a few hundred A/m and should not be influenced by the adjacent magnetic hard layer. In polycrystalline films, however, congruent interface roughness invokes a ferromagnetic coupling, which can be compensated by an antiferromagnetic magnetostatic dipole coupling in patterned elements. In this paper, we demonstrate the evolution of the magnetic interlayer interaction by annealing in tunnel junctions with Al2O3 barrier. Whereas the minor loop shifting field of the soft magnetic NiFe sense layer is constant or varies only slightly from 2 kA/m to 1.15 kA/m for 1–1.5 nm thick barriers, an eightfold increase of the coercivity is found after annealing above 350 °C. Microstructural investigations give evidence that small ferromagnetic clusters arise in the hard layer of the junctions (Cu/CoFe/Ru/CoFe) by diffusion of Cu, Co, and Fe. The density and the stray field of these clusters are determined by magnetic force measurements and can be correlated to the increased switching field strength.