Types Of Magnetic Tunnel Junctions Research Articles

Probing edge condition of nanoscale CoFeB/MgO magnetic tunnel junctions by spin-wave resonance

We investigate spin-wave resonance in nanoscale CoFeB/MgO magnetic tunnel junctions (MTJs) with a perpendicular easy axis and various free-layer sizes. Two types of MTJs are fabricated by different process conditions, and the spin-wave resonance is measured with homodyne-detected ferromagnetic resonance. We focus on the distance between resonance frequencies of the uniform and spin-wave modes as a function of the free-layer size in order to examine the effect of the edge state of MTJs. A marked difference is observed between the two types of MTJs, and the result is consistently reproduced by a model assuming free- or fixed-edge boundary conditions with or without reduced magnetic properties near the pattern edge for each MTJ. The obtained results indicate that the edge state of nanoscale MTJs is crucially affected by the process condition, and spin-wave resonance can serve as a sensitive probe for the edge condition.

Reduced Thermal Variation of Perpendicular Magnetic Anisotropy in Magnetically Stiffened Dual-W Composite Storage Layer for Spin-Transfer-Torque Magnetic Random-Access Memory

This article reports a type of magnetic tunnel junction (MTJ) with an expanded middle layer, for spin-transfer-torque magnetic random-access memory (STT-MRAM). This data-storage layer of the form Fe-Co-B/W/Co/W/Fe-Co-B sandwiches a ferromagnet with high exchange stiffness between two tungsten films, and thus is much less sensitive to temperature than that in a conventional MTJ. Such a storage layer is promising for spintronic memory that must operate across a wide range of temperatures, as in automobile applications.

Enhanced Annealing Stability of Exchange-Biased Pinned Layer in Magnetic Tunnel Junction Using Ta/Ru/Ta/Ru Underlayer

To realize highly sensitive analog sensor using magnetic tunnel junctions (MTJs), there is a demand for MgO-MTJs with a high signal-to-noise ratio. Thermal annealing is important for improving the crystalline quality of MgO barrier in order to enhance the tunneling magnetoresistance (TMR) ratio and decrease the noise level of MTJs. However, annealing at excessively high temperatures causes degradation of the exchange bias field ( $H_{\text {ex}}$ ) in the exchange-biased pinned layer (EB-PL). In this paper, we aim to develop a MgO-MTJ with high annealing stability by investigating the effect of underlayer structure on the annealing stability of EB-PL in MTJs. We fabricated two types of MTJ, one with a Ta/Ru underlayer (TR-UL) and one with a Ta/Ru/Ta/Ru underlayer (TRTR-UL). The results of evaluating the $H_{\text {ex}}$ of MTJs annealed in the temperature range of 320 °C–420 °C found that the MTJ containing a TRTR-UL (TRTR-MTJ) has higher annealing stability compared with that of the MTJ containing a TR-UL (TR-MTJ). Furthermore, the results of measuring the TMR ratio and 1/ $f$ noise level of the MTJs annealed at 380 °C showed that the TRTR-MTJ exhibits a higher TMR ratio and lower noise level than the TR-MTJ. To determine the mechanism by which the annealing stability improves, we carried out microstructural analysis of both MTJs and found that thermal Mn diffusion in the TRTR-MTJ was smaller than that in the TR-MTJ, and the crystallinity of IrMn on the TRTR-UL was found to be clearly improved compared with that on the TR-UL. As a result of $d$ -spacing analysis of each layer in the TRTR-UL, the second Ta layer deposited on the Ru layer was found to have $d$ -spacing corresponding to the (110) textured $\alpha $ -Ta structure and to be highly crystalline, whereas the first Ta layer was not found to have the components of (110) textured $\alpha $ -Ta structure. These results indicate that the Ta-on-Ru structure is responsible for the high crystallinity of the IrMn layer disposed on the TRTR-UL, resulting in the high annealing stability thanks to small Mn diffusion.

Micromagnetic Calculation for Magnetic Tunnel Junctions With Equal Parallel and Antiparallel Thermal Stability

In magnetic tunnel junctions (MTJs) with perpendicular magnetic easy axes, the retention times for parallel and antiparallel states are different because of dipolar coupling with the reference layer (RL). To make retention times symmetrical, we model three types of MTJs that have single or synthetic ferrimagnetic (SyF) RLs with stepped structures. Micromagnetic calculations are performed on the basis of the Landau–Lifshitz–Gilbert equation of motion to evaluate the stray fields from the RLs of each design. A SyF structure with a step on the bottom magnetic layer has the smallest stray field of 2 mT at lateral dimensions of 30 nm × 30 nm in the layer. Symmetric thermal stability for parallel and antiparallel states should help to realize high-density nonvolatile magnetic random-access memory and logic chips.

Magnetization distribution and spin transport of graphene/h-BN/graphene nanoribbon-based magnetic tunnel junction

Motivated by recent electronic transport measurement of boron nitride-graphene hybrid atomic layers, we studied magnetization distribution, transmission and current-bias relation of graphene/h-BN/graphene (C/BN/C) nanoribbon-based magnetic tunnel junctions (MTJ) based on density functional theory and non-equilibrium Green's function methods. Three types of MTJs, i.e. asymmetric, symmetric (S) and symmetric (SS), and two types of lead magnetization alignment, i.e. parallel (PC) and antiparallel (APC), are considered. The results show that the magnetization distribution is closely related to the interface structure. Especially for asymmetric MTJ, the B/N atoms at the C/BN interface are spin-polarized and give finite magnetic moments. More interesting, it is found that the APC transmission of asymmetric MTJ with the thinnest barrier dominates over the PC one. By analyzing the projected density of states, one finds that the unusual higher APC transmission than PC is due to the coupling of electronic states of left ZGNR and right ZGNR. By integrating transmission, we calculate the current-bias voltage relation and find that the APC current is larger than PC current at small bias voltage and therefore reproduces a negative tunnel magnetoresistance. The results reported here will be useful and important for the design of C/BN/C-based MTJ.

High-Density and Robust STT-MRAM Array Through Device/Circuit/Architecture Interactions

In spin-transfer torque magnetic random access memory (STT-MRAM), retention-, write-, and read-failures negatively impact the memory yield and density. In this paper, we jointly consider device-circuit-architecture layers to implement high-density STT-MRAM array while meeting the target yield requirement. Different types of magnetic tunnel junctions are considered at the device level, and error correcting codes (ECCs) in conjunction with invert-coding are employed as an architectural solution. Through cross-layer interactions, we present a design methodology to optimize bit-cell area while satisfying the target yield and energy consumption under process variation. Furthermore, we explore the use of invert-coding along with ECC in order to achieve higher memory density than that obtained using ECC alone. Our proposed technique can improve memory density further by proper selection of thermal stability factor based upon two observations: 1) invert-coding can fix multiple write/read failures with small storage overhead and 2) as thermal stability factor increases, retention-failure probability exponentially decreases, and thus, simple ECC is good enough for retention failure correction.

Magnetic tunnel junctions using Co/Pt multilayered free layers with perpendicular magnetic anisotropy

Co/Pt multilayered films with perpendicular magnetic anisotropy have a large magneto-optical Kerr effect. To use the films with a submicron magneto-optical light modulator driven by spin transfer switching, we fabricated two types of magnetic tunnel junctions (MTJs) with Co/Pt multilayered films for the free layers. One is an fcc-based MTJ, another is a bcc-based MTJ with CoFeB/MgO/CoFeB junction. The fcc-based MTJ with a Ag buffer layer on the bottom electrode showed a large coercive force of the pinned layer, a large Kerr rotation angle of 0.3 degree in the free layer and a tunnel magnetoresistance (TMR) ratio of 3.8%. In the CoFeB/MgO/CoFeB junction, an X-ray diffraction pattern of an MgO layer showed a large MgO(002)-orientation. However, the TMR ratio was less than 3 %. An MTJ with a Ta buffer layer between the CoFeB layer and the Co/Pt multilayered films in the free layer was prepared. The Ta buffer was used to alleviate a lattice mismatch between bcc-CoFeB/MgO/CoFeB and fcc-Co/Pt multilayer. The peak intensity of the MgO(002)-orientation was increased up to 2 times. This result suggests that the crystalline texture of the bcc-CoFeB/MgO/CoFeB junction is strongly influenced by the fcc-Co/Pt multilayered films.

Non-equilibrium Green's function approach to the quasiparticle transport and magnetoresistance in hybrid magnetic tunnel junctions

A generalized approach to study quasiparticle transport across hybrid magnetic tunnel junctions (MTJs) is formulated using the non-equilibrium Green's function technique. This formalism allows for arbitrary thicknesses of the electrodes and the central scattering region comprising of materials with multiple electronic bands, and incorporates the many body interactions present in the electrode regions. While the method can be used to study the transport characteristics of various types of MTJs, we have used it to study the tunneling characteristics and magnetoresistance (MR) of MTJs in which s– f interaction is present at the electrode layers. It is also used to study the transport characteristics of MTJs with hybrid electrodes and double barrier. The magnetic correlation present in the electrodes is found to strongly influence the TMR. Eventhough the magnetic correlation in general suppress the TMR, the TMR is found to be enhanced strongly for certain band occupations of the electrodes. We observe a fall of TMR with increase in the number of layers in the insulating region. Band occupation of the metallic layer present at the middle of the insulating layers in the double barrier MTJ is found to be important in deciding its tunnel characteristics. Origin of the different types of behavior of TMR is analyzed in terms of the spin-dependent tunnel currents.

Temperature and bias voltage dependence of tunnel magnetoresistance effect in magnetic tunnel junctions with Co2Fe(Al,Si) electrodes fabricated by MBE

The temperature and bias voltage dependence of tunnel magnetoresistance ratio of two types of magnetic tunnel junctions were investigated. These two types of junctions have almost the same ordered structured Co2FeAl0.5Si0.5 Heusler bottom electrodes and different barrier layers of Al oxide or MgO, and top electrodes. A similar temperature dependence of tunnel magnetoresistance ratio measured at low bias voltage was found. A larger asymmetric bias voltage dependence of the tunnel magnetoresistance was obtained for the junction with Al oxide barrier compared with the junction with MgO barrier, and negative tunnel magnetoresistance ratio was also observed for both junctions.

New Type of Magnetic Tunnel Junction Based on Spin Filtering through a Reduced Symmetry Oxide: FeCo|Mg3B2O6|FeCo

Magnetic tunnel junctions with high-tunneling magnetoresistance values such as Fe|MgO|Fe capitalize on spin filtering in the oxide region based on the band symmetry of incident electrons. However, these structures rely on magnetic leads and oxide regions of the same cubic symmetry class. A new magnetic tunnel junction (FeCo|Mg(3)B(2)O(6)|FeCo) is presented that uses a reduced symmetry oxide region (orthorhombic) to provide spin filtering between the two cubic magnetic leads. Complex band structure analysis of Mg(3)B(2)O(6) based on density functional calculations shows that significant spin filtering could occur in this system. This new type of magnetic tunnel junction may have been fabricated already and can explain recent experimental studies of rf-sputtered FeCoB|MgO|FeCoB junctions where there is significant B diffusion into the MgO region.

Structural and tunneling properties of magnetic tunnel junctions with Al–O and MgO barrier

Two types of magnetic tunnel junctions (MTJs) with the configuration: substrate Si(1 0 0)/SiO 2 47 nm/buffer/IrMn 12 nm/CoFe 2.5 nm/Al–O 1.5 nm/NiFe 3 nm/Ta 5 nm and Si(1 0 0)/SiO 2 47 nm/buffer/IrMn 10 nm/CoFeB 3 nm/MgO 2 nm/CoFeB 4 nm/Ta 5 nm were prepared by the sputtering technique with two different buffers: A-Cu 25 nm and B-Ta 5 nm/Cu 25 nm. The B buffer caused a high texture of MTJs whereas in the case of the A buffer junctions texture was weak. Crystallites in the textured layers grew in a columnar like shape that induced interfacial roughness. High textured buffer B caused high interfacial roughness that reduced the resistance-area (RA) product due to a barrier thickness fluctuation. RA also changed substantially depending on the type of a barrier. The highest RA product ∼15 MΩ μm 2 was achieved for a low textured junction with Al–O barrier whereas in the high textured MgO sample RA product was ∼100 kΩ μm 2. Tunnel magnetoresistance (TMR) measured at room temperature was about 45% for the samples with Al–O barrier, whereas for the samples with MgO barrier TMR was about three times higher and achieved 140%.

Bias dependence of tunnel magnetoresistance in spin filtering tunnel junctions: Experiment and theory

A spin filter is a type of magnetic tunnel junction in which only one of the electrodes is magnetic and the insulating barrier is ferro- or ferrimagnetic. We report on spin-dependent transport measurements and their theoretical analysis in epitaxial spin filters integrating a tunnel barrier of the high-Curie-temperature ferrimagnetic spinel $\mathrm{Ni}{\mathrm{Fe}}_{2}{\mathrm{O}}_{4}$, with half-metallic ${\mathrm{La}}_{2∕3}{\mathrm{Sr}}_{1∕3}\mathrm{Mn}{\mathrm{O}}_{3}$ and Au electrodes. A positive tunnel magnetoresonance of up to $\ensuremath{\sim}50%$ is obtained at low temperature, which we find decreases with bias voltage. In view of these experimental results, we propose a theoretical treatment of the transport properties of spin filters with epitaxial magnetic barriers, based on an elliptical variation of the decay rates within the spin-dependent gaps in analogy with what was calculated for nonmagnetic barrier materials such as MgO or $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$. Whereas the spin filtering efficiency for zero bias is of one sign, we show that this can easily change with bias; the degree of change hinges on the energy variation of the majority and minority spin decay rates of the transmission across the barrier. We point out some shortcomings of approaches based on models in which the transmission is related to spin-dependent barrier heights, and some implications for future experimental and theoretical research on spin filters.

Magnetic tunnel junctions

Fueled by the ever-increasing demand for larger hard disk drive storage capacities, extensive research over the past decade has resulted in the development of AlO x - and TiO x -based magnetic tunnel junctions that exhibit a large magnetoresistive effect at room temperature. As their commercialization in various applications begins, a new type of magnetic tunnel junction with a crystalline MgO tunnel barrier has emerged that shows a much larger room-temperature magnetoresistive effect. We present a brief overview of the development of magnetic tunnel junctions, introducing the underlying physics. We also discuss two important commercial applications: read sensors in hard disk drives and memory elements in magnetoresistive random access memory. An emphasis is placed on the material aspects of magnetic tunnel junctions.

Temperature and voltage dependence of magnetic barrier junctions with a nonmagnetic spacer

The temperature and voltage dependence of spin transport is theoretically investigated in a new type of magnetic tunnel junction, which consists of two ferromagnetic outer electrodes separated by a ferromagnetic barrier and a nonmagnetic (NM) metallic spacer. The effect of spin fluctuation in magnetic barrier, which plays an important role at finite temperature, is included by taking the mean-field approximation. It is found that, the tunnel magnetoresistance (TMR) and the electron-spin polarization depend strongly on the temperature and the applied voltage. The TMR and spin polarization at different temperatures show an oscillatory behavior as a function of the NM spacer thickness. Also, the amplitude of these oscillations is regularly reduced when the temperature increases. The maximum TMR value, varies approximately from 270 $\%$ in reverse bias (at T = 0 K) to 25 $\%$ in forward bias (at $T\geq T_C$ ).

High thermal stability of magnetic tunnel junctions with oxide diffusion barrier layers

We developed two types of magnetic tunnel junctions (MTJs) that showed high thermal stability. One is a PtMn exchange-biased spin-valve MTJ with a CoFe/Al-oxide (AlOx)/NiFe free layer and a CoFeTaOx/CoFe pinned layer, and the other is a pseudo-spin-valve (PSV) MTJ with a CoFe/AlOx/NiFe soft layer, where AlOx and CoFeTaOx act as barriers for Ni and Mn diffusion toward the tunnel barrier, respectively. After 390 °C-1H annealing, the PSV MTJs maintained 28% and the SV MTJs 39% of tunnel magnetoresistance. Transmission electron microscopy observation of the SV MTJs after 380 °C-1H annealing revealed that the migrated Mn atoms were trapped at the CoFeTaOx layer.

Tunnel magnetoresistance in double spin filter junctions

We consider a new type of magnetic tunnel junction, which consists of twoferromagnetic tunnel barriers acting as spin filters (SFs), separated by anonmagnetic metal (NM) layer. Using the transfer matrix method and thefree-electron approximation, the dependence of the tunnel magnetoresistance(TMR) on the thickness of the central NM layer, bias voltage and temperature inthe double SF junction are studied theoretically. It is shown that the TMR andelectron-spin polarization in this structure can reach very large values undersuitable conditions. The highest value of the TMR can reach 99%. By anappropriate choice of the thickness of the central NM layer, the degree of spinpolarization in this structure will be higher than that of the single SF junctions.These results may be useful in designing future spin-polarized tunnelling devices.

Tunnel current, conductance and magnetoresistance in double-barrier magnetic tunnel junctions

In this paper, we design a new type of magnetic tunnel junction (MTJ), which consists of two ferromagnetic layers separated by two adjacent insulating barriers which have different barrier heights and different dielectric constants. Based on the nearly-free-electron approximation, the tunnel current, conductance and magnetoresistance in the double-barrier MTJ are discussed, respectively, by the treatment of the transfer matrix method. Our numerical results show that there exists an asymmetrical tunneling phenomenon in the double-barrier MTJ under the forward and reverse biases, which is modulated by the configuration of magnetizations of two ferromagnetic layers. We predict that our results will stimulate experimental efforts on fabricating the double-barrier MTJ.