High Tunnel Magnetoresistance Research Articles

Design of Double Pinned Perpendicular Magnetic-Tunnel-Junction Spin-Valve Exhibiting Multiple Resistance States

Perpendicular spin-transfer-torque magnetic random access memory (p-STT MRAM) cells consist of a perpendicular magnetic tunnel junction and a selective device has been researched intensively because of the possibility of overcoming the scaling limitations of the current dynamic random access memory (DRAM) below the 10-nm design rule [1]. p-STT-MRAM requires high tunneling magnetoresistance (TMR) ratio greater than 150%, sufficient thermal stability (Δ=KuV/kBT) above 75 for a ten-year retention-time, and low switching current (J C0 ~ 1 MA/cm2) must be achieved for low power consumption [2-3]. However, the thermal stability is hard to maintain when scaling down in 10-nm scale of the p-STT MRAM cells for terabit integration. It requires high interfacial anisotropy K i to maintain the thermal stability requirement for 10-year retention time [4]. In this study, we designed a p-MTJ spin-valve with a double pinned structure to realize two-bit operation to secure a larger scaling limit (see Fig. 1(a)). We investigated the dependency of the number of [Co/Pt]n multilayers on the coupling strength to the pinned layers in order to control the spin-electron direction of the pinned layers. The total number of [Co/Pt]n SyAF multilayers ferro-coupled to the bottom pinned layer was reduced from 9 to 3 layers to reduce the roughness of the MgO tunneling barrier to increase the TMR ratio. Also, the number of [Co/Pt]n of the top upper SyAF multilayers was modulated so that the coercive field (H c) required to switch the spin-electron direction of the top pinned layer increased from about 0.3 kOe to 1 kOe which is sufficient to ensure the memory margin for two-bit operation, as shown in Fig. 1(b). Then, we investigated the multiple resistance states with the R-H loop of the double pinned p-MTJ spin-valve, as shown in Fig. 1(c). It showed four different resistance states depending on the spin-electron direction of the free layers and pinned layers as shown in Fig. 1(d) with a maximum TMR ratio of 166%. This implies that the double pinned p-MTJ spin-valve may be suitable for terabit integration compared to that of the single-bit operating p-STT MRAM memory cells. Acknowledgements This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No.2017R1A2A1A05001285) and Brain Korea 21 PLUS Program in 2014.

Site-Resolved Contributions to the Magnetic-Anisotropy Energy and Complex Spin Structure of Fe/MgO Sandwiches

Fe/MgO-based Magnetic Tunnel Junctions (MTJs) are among the most promising candidates for spintronic devices due to their high thermal stability and high tunneling magnetoresistance. Despite its apparent simplicity, the nature of the interactions between the Fe and MgO layers leads to complex finite size effects and temperature dependent magnetic properties which must be carefully controlled for practical applications. In this letter, we investigate the electronic, structural and magnetic properties of MgO/Fe/MgO sandwiches using first principles calculations and atomistic spin modeling based on a fully parameterized spin Hamiltonian. We find a large contribution to the effective interfacial magnetic anisotropy from the two-ion exchange energy. Minimization of the total energy using atomistic simulations shows a surprising spin spiral ground state structure at the interface owing to frustrated ferromagnetic and antiferromagnetic interactions, leading to a reduced Curie temperature and strong layer-wise temperature dependence of the magnetization. The different temperature dependences of the interface and bulk-like layers results in an unexpected non-monotonic temperature variation of the effective magnetic anisotropy energy and temperature-induced spin-reorientation transition to an in-plane magnetization at low temperatures. Our results demonstrate the intrinsic physical complexity of the pure Fe/MgO interface and the role of elevated temperatures providing new insight when interpreting experimental data of nanoscale MTJs.

Interplay of composition and anisotropy on evolution of microstructural, static and dynamic magnetic properties of CoFeB thin films on annealing

The crystallization of CoFeB electrode is essential for achieving high tunneling magnetoresistance (TMR) in magnetic tunnel junctions (MTJs) with MgO barrier. In this study, we systematically investigated the influence of annealing on the structural and magnetic properties of CoFeB (23 nm) thin films using X-ray diffraction (XRD), magnetization (M) vs. magnetic field (H) and ferromagnetic resonance (FMR) measurements. The variations in composition and uniaxial magnetic anisotropy (UMA) of as-deposited CoFeB thin films results in a different crystalline state on annealing. It was found that the partial crystallization takes place for CoFeB compositions with high Boron content and exhibiting large UMA. This initial stage of crystallization is associated with the quick release of stress and will result in low coercivity, smaller crystallite size and a decrease in uniaxial anisotropy which will eventually lead to lower magnetic damping. However, on complete crystallization for other compositions with the formation of bcc-CoFe (110) phase, both the crystallite size and coercivity increases and cubic anisotropy emerge which results in very enhanced damping. The FMR linewidth has non-linear frequency dependence which gives direct evidence that relaxation is not exclusively governed by Gilbert damping, but it is also important to consider the contribution from two-magnon scattering (TMS). The TMS dominates the damping process in crystallized films and is mainly caused by the anisotropy dispersion created due to the grain growth. Thus, the work connects the change in microstructure and anisotropy to the magnetization dynamics, in particular, the FMR linewidth.

Enhancement of tunneling magnetoresistance by inserting a diffusion barrier in L1-FePd perpendicular magnetic tunnel junctions

We studied the tunnel magnetoresistance (TMR) of L10-FePd perpendicular magnetic tunnel junctions (p-MTJs) with an FePd free layer and an inserted diffusion barrier. The diffusion barriers studied here (Ta and W) were shown to enhance the TMR ratio of the p-MTJs formed using high-temperature annealing, which are necessary for the formation of high quality L10-FePd films and MgO barriers. The L10-FePd p-MTJ stack was developed with an FePd free layer with a stack of FePd/X/Co20Fe60B20, where X is the diffusion barrier, and patterned into micron-sized MTJ pillars. The addition of the diffusion barrier was found to greatly enhance the magneto-transport behavior of the L10-FePd p-MTJ pillars such that those without a diffusion barrier exhibited negligible TMR ratios (<1.0%), whereas those with a Ta (W) diffusion barrier exhibited TMR ratios of 8.0% (7.0%) at room temperature and 35.0% (46.0%) at 10 K after post-annealing at 350 °C. These results indicate that diffusion barriers could play a crucial role in realizing high TMR ratios in bulk p-MTJs such as those based on FePd and Mn-based perpendicular magnetic anisotropy materials for spintronic applications.

Cross-point-type spin-transfer-torque magnetoresistive random access memory cell with multi-pillar vertical body channel MOSFET

In this paper, from the viewpoint of cell size and sensing margin, the impact of a novel cross-point-type one transistor and one magnetic tunnel junction (1T–1MTJ) spin-transfer-torque magnetoresistive random access memory (STT-MRAM) cell with a multi-pillar vertical body channel (BC) MOSFET is shown for high density and wide sensing margin STT-MRAM, with a 10 ns writing period and 1.2 V VDD. For that purpose, all combinations of n/p-type MOSFETs and bottom/top-pin MTJs are compared, where the diameter of MTJ (DMTJ) is scaled down from 55 to 15 nm and the tunnel magnetoresistance (TMR) ratio is increased from 100 to 200%. The results show that, benefiting from the proposed STT-MRAM cell with no back bias effect, the MTJ with a high TMR ratio (200%) can be used in the design of smaller STT-MRAM cells (over 72.6% cell size reduction), which is a difficult task for conventional planar MOSFET based design.

Magnetic tunnel junction thermocouple for thermoelectric power harvesting

The thermoelectric power generated in magnetic tunnel junctions (MTJs) is determined as a function of the tunnel barrier thickness for a matched electric circuit. This study suggests that lower resistance area product and higher tunnel magnetoresistance will maximize the thermoelectric power output of the MTJ structures. Further, the thermoelectric behavior of a series of two MTJs, a MTJ thermocouple, is investigated as a function of its magnetic configurations. In an alternating magnetic configurations the thermovoltages cancel each other, while the magnetic contribution remains. A large array of MTJ thermocouples could amplify the magnetic thermovoltage signal significantly.

High performance perpendicular magnetic tunnel junction with Co/Ir interfacial anisotropy for embedded and standalone STT-MRAM applications

High volume spin transfer torque magnetoresistance random access memory (STT-MRAM) for standalone and embedded applications requires a thin perpendicular magnetic tunnel junction (pMTJ) stack (∼10 nm) with a tunnel magnetoresistance (TMR) ratio over 200% after high temperature back-end-of-line (BEOL) processing up to 400 °C. A thin reference layer with low magnetic moment and strong perpendicular magnetic anisotropy (PMA) is key to reduce the total thickness of the full pMTJ stack. We demonstrated strong interfacial PMA and a perpendicular Ruderman-Kittel-Kasuya-Yosida exchange interaction in the Co/Ir system. Owing to the additional high PMA at the Ir/Co interface in combination with a conventional CoFeB/MgO interface in the Ir/Co/Mo/CoFeB/MgO reference layer, the full film pMTJ showed a TMR ratio over 210% after annealing at 400 °C for 150 min. The high TMR ratio can be attributed to the thin stack design by combining a thin reference layer with the efficient compensation by a thin pinned layer. The annealing stability may be explained by the absence of solid solution in the Co-Ir system and the low oxygen affinity of Mo in the reference layer and the free layer. High device performance with a TMR ratio over 210% was also confirmed after subjecting the patterned devices to BEOL processing temperatures of up to 400 °C. This proposed pMTJ design is suitable for both standalone and embedded STT-MRAM applications.

Perpendicular magnetic tunnel junctions with Mn-modified ultrathin MnGa layer

Perpendicular magnetic tunnel junctions (p-MTJs) with a MgO barrier and a 1-nm-thick MnGa electrode were investigated by inserting several monolayers (MLs) of Mn. The tunnel magnetoresistance (TMR) ratio systematically increased when increasing the Mn layer thickness with a maximum of 18 (38.4)% at 300 (5) K for a Mn layer thickness of 0.6–0.8 nm. This ratio is five times higher compared to that without the Mn layer. The perpendicular magnetic anisotropy (PMA) field and the PMA constant of the ultrathin MnGa layer also increased up to 62–90 kOe and 6.2–11.3 Merg/cm3, respectively, with an increase in the Mn interlayer thickness, even for the ultrathin regime of the MnGa layer. For p-MTJs showing a high TMR and PMA, electron microscopy indicated the presence of 3–4 MLs of Mn at the MnGa/MgO interface; thus, the Mn modification enhanced the TMR as well as improved the PMA. This may be a promising finding to develop a Mn-based free layer for spin-transfer-torque devices for high-recording-density magnetoresistive random access memory and a sub-THz oscillator/detector.

Double MgO-based Perpendicular Magnetic-Tunnel-Junction Spin-valve Structure with a Top Co2Fe6B2 Free Layer using a Single SyAF [Co/Pt]n Layer

A new perpendicular spin-transfer-torque magnetic-tunnel-junction (p-MTJ) spin-valve was developed to achieve a high tunneling magnetoresistance (TMR) ratio. It had a double MgO-based spin-valve structure with a top Co2Fe6B2 free layer and incorporated a single SyAF [Co(0.4 nm)/Pt(0.3 nm)]3 layer and a new buffer layer of Co(0.6)/Pt(0.3)/Co(0.4). It had a TMR ratio of 180% and anisotropy exchange field (Hex) of 3.44 kOe after ex-situ annealing of 350 °C for 30 min under a vacuum below 10−6 torr and a perpendicular magnetic field of 3 tesla, thereby ensuring a memory margin and avoiding read disturbance failures. Its high level of performance was due to the face-center-cubic crystallinity of the MgO tunneling barrier being significantly improved by decreasing its surface roughness (i.e., peak-to-valley length of 1.4 nm).

Revealing the interfaces of the hybrid system MgO/Co/GaAs(0 0 1): a structural and chemical investigation with XPS and XPD

Bcc metals and MgO are used in technological research for building magnetic tunnel junctions (MTJs), because they yield a high tunnel magnetoresistance. Thin insulating barriers are of great importance in realizing MTJs. Combined with electrons spin-injected into GaAs, tunneled electrons can be detected and manipulated. We report on a synchrotron radiation based x-ray photoelectron spectroscopy and x-ray photoelectron diffraction study on the system MgO/Co(bcc)/GaAs(0 0 1) for ultra-low Co and MgO coverages (, ). As a result, we obtain a Co3Ga alloy at the Co/GaAs interface in the rare D03 structure. This structure is only 6.07 Å thick, and serves as a template for the metastable Co(bcc) structure. Co(bcc) itself grows heavily distorted in the (0 0 1) direction for the first two unit cells, due to the D03 template. The MgO/Co interface reveals a weak bonding between MgO and Co(bcc) without Co oxidation, since no compound formation was observed. Additionally, MgO grows in an amorphous phase for a thickness of . At , it crystallizes in a compressed unit cell where every second layer is shifted toward the (0 0 1) direction compared to the bulk halite structure.

Resonant Spin-Transfer-Torque Nano-Oscillators

Spin transfer torque nano-oscillators are potential candidates for replacing the traditional inductor based voltage controlled oscillators in modern communication devices. Typical oscillator designs are based on trilayer magnetic tunnel junctions which are disadvantaged by low power outputs and poor conversion efficiencies. In this letter, we theoretically propose to use resonant spin filtering in pentalayer magnetic tunnel junctions as a possible route to alleviate these issues and present device designs geared toward a high microwave output power and an efficient conversion of the d.c. input power. We attribute these robust qualities to the resulting non-trivial spin current profiles and the ultra high tunnel magnetoresistance, both arising from resonant spin filtering. The device designs are based on the nonequilibrium Green's function spin transport formalism self-consistently coupled with the stochastic Landau-Lifshitz-Gilbert-Slonczewski's equation and the Poisson's equation. We demonstrate that the proposed structures facilitate oscillator designs featuring a large enhancement in microwave power of around $775\%$ and an efficiency enhancement of over $1300\%$ in comparison with typical trilayer designs. We also rationalize the optimum operating regions via an analysis of the dynamic and static device resistances. This work sets stage for pentalyer spin transfer torque nano-oscillator device designs that extenuate most of the issues faced by the typical trilayer designs.

Robust and Cascadable Nonvolatile Magnetoelectric Majority Logic

Nonvolatile logic computations are of particular interest due to their zero standby leakage power consumption. In this paper, we explore a nonvolatile majority logic using voltage-driven magnetoelectric (ME) switching of ferromagnets. Specifically, we employ ME magnetic tunnel junctions (ME-MTJs) connected in parallel to construct a majority gate. We also present the cascadability of the proposed majority gate using CMOS inverters. Furthermore, through a mixed-mode simulation framework consisting of magnetization dynamics and electron transport model, we analyze the robustness of the proposed majority gate under process variations. Our analysis shows that due to parallel connection of the ME-MTJs, high tunnel magnetoresistance (TMR) ratios are required for proper functioning of the proposed majority gate, in presence of variations. In order to relax the constraints on the TMR requirements, we present an alternate implementation of the majority logic that does not require a parallel connection of ME-MTJs, thereby increasing the robustness against process variations. Energy and delay metrics are presented for a full adder implementation using the proposed majority gates.

Strong anti-strain capacity of CoFeB/MgO interface on electronic structure and state coupling**Project supported by the National Key Research and Development Program of China (Grant No. 2016YFB0400801), the National Natural Science Foundation of China (Grant Nos. 61774128, 61674124, 11604275, 11304257, and 61227009), the Natural Science Foundation of Fujian Province of China (Grant Nos. 2017J01012, 2014J01026, 2016J01037, and 2015J01028), and the Fundamental

Electronic structure and spin-related state coupling at ferromagnetic material (FM)/MgO (FM = Fe, CoFe, CoFeB) interfaces under biaxial strain are evaluated using the first-principles calculations. The CoFeB/MgO interface, which is superior to the Fe/MgO and CoFe/MgO interfaces, can markedly maintain stable and effective coupling channels for majority-spin state under large biaxial strain. Bonding interactions between Fe, Co, and B atoms and the electron transfer between Bloch states are responsible for the redistribution of the majority-spin state, directly influencing the coupling effect for the strained interfaces. Layer-projected wave function of the majority-spin state suggests slower decay rate and more stable transport property in the CoFeB/MgO interface, which is expected to maintain a higher tunneling magnetoresistance (TMR) value under large biaxial strain. This work reveals the internal mechanism for the state coupling at strained FM/MgO interfaces. This study may provide some references to the design and manufacturing of magnetic tunnel junctions with high tunneling magnetoresistance effect.

High Tunnel Magnetoresistance in Mo/CoFe/MgO Magnetic Tunnel Junction: A First-Principles Study

The tunnel magnetoresistance (TMR) ratio in a magnetic tunnel junction (MTJ) is influenced by heavy metal capping layer due to the interfacial effect. We report a systematic first-principles study on MTJ based on CoFe/MgO with capping layer, demonstrate that TMR ratios are sensitive to capping layer material, and show that TMR in Mo-capped MTJ is three times as high as that in Ta-capped MTJ. Besides, TMR in Mo-capped MTJ remains high at finite voltage bias. By analyzing the transmission spectrum and density of scattering states, we found that coherent transmission of $\Delta _{1}$ state dominates the majority-spin conductance in Mo-capped MTJ, while the resonant tunneling contributes significantly in Ta-capped MTJ. The evolution of TMR for varying MgO and CoFe thickness in Mo-capped MTJ is presented. TMR oscillates as a function of CoFe thickness because of the confined wave function in ferromagnetic layer, while TMR rises with MgO thickness increasing due to the enhanced filtering effect of MgO. This work clarifies the physical mechanism on high TMR in Mo-capped MTJ, which is promising to benefit the design of spintronics device.

Effect of Finite Tunneling Magnetoresistance for the Switching Dynamics in the Spin Transfer Torque Magnetic Tunneling Junctions

We investigate the effect of tunneling magnetoresistance (TMR) on the spin transfer-torque (STT) switching behaviors in magnetic tunneling junctions. In most of the micromagnetic simulations for STT switching, a uniform current density has been assumed, which is not realistic in the high TMR devices. The local STT is proportional to the local current density, and the local current density will be determined by the local resistivity. Since higher than 150% of TMR values is required in the real STT-magnetoresistive random access memory devices, the local resistance is dramatically changed as a function of the relative spin orientation between the fixed and free layers under the constant voltage operation mode. By employing non-uniform current density in STT switching simulations using the “embedded object-oriented micromagnetic framework” scheme, we found that the details of switching behaviors such as switching time and critical current density are significantly influenced by the TMR values.

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.

(Invited) Interface Control of Advanced Electronic Devices - High-k/Metal Gate System and Magnetic Tunneling Junction

With the scaling of electronic devices, interface control of the constituent materials has become more and more important. Several key issues and related topics are taken up and reviewed with respect to logic and memory devices. The key issues are interface control in high-k/metal gate system for logic application and magnetic tunneling junction for memory application. High-k/Metal Gate System [1] In accordance with the requirement of CMOS scaling, replacement of thermally grown SiO2 gate dielectric with other dielectric materials with higher dielectric constant (k) has become imperative to reduce the equivalent oxide thickness (EOT) to achieve higher drivability of FET. Key factors on Hf-based high-k/metal gate (HK/MG) CMOS technology based on conventional gate first process for low operational power application are overviewed. Work-function (WF) tuning derived from ionic property of high-k material, process cost, EOT scaling with low gate leakage current are issues to be overcome. Among them, the key issue is how to control the atomic distribution in the Hf-based HK/MG system, which is absolutely requisite for WF tuning and EOT scaling. Interpretation of effective work-function (WF): For conventional gate first process, due to the interfacial reaction, the Fermi level is pinned and the WF increases with decreasing the EOT. In addition to this, due to the fixed charge in the high-k dielectric, the WF is also modulated, hence the effective WF (eWF) changes. The effect of the fixed charge is rather complicated due to an oxygen vacancy behavior in HK. Consequently, the eWF results in the “roll-off” behavior caused by the (+) charge induced by the oxygen vacancy. When the oxygen increases, i.e., (-) charge increases in the HK, the obtained WF tends to roll-up. In contrast, it tends to roll-off for more (+) charge induced by oxygen vacancy, Vo 2+in the HK. Formation process of Hf-based high-k (HK): Generally, HK is deposited by metal-organic chemical vapor deposition (MOCVD) or atomic layer deposition (ALD), however, HK dielectric formation by oxidizing a preliminarily sputtered metallic-Hf is reasonable approach for fabricating the HK gate stack in a very pure form with less water content across the wafer. And in a very important thing, by controlling the oxidation time, this process can provide precise thickness control of the interfacial layer (IL) including “zero IL” with excellent uniformity. Magnetic Tunneling Junction A magnetic tunnel junction (MTJ) for spin-transfer-torque (STT) switching is a promising building block of non-volatile working memory embedded in very large scale integrated circuits (VLSIs) because of its potential to reduce a power consumption by replacing the conventional volatile cache memories. Among many materials, a CoFeB/MgO/CoFeB stack structure has attracted much attention as a candidate of MTJ embedded in VLSIs because of its high tunnel magnetoresistance (TMR) ratio caused by a perpendicular magnetic easy axis. And what is better, this MTJ has a potential for high integration into VLSIs down to a junction size of 1X nm regime. To improve the MTJ performance, crystal structure of the CoFeB/MgO interface is a crucial issue as well as B behavior during the process treatment. Redox reaction: As an example of the interfacial reaction, a redox reaction was observed at the surface of Ta/CoFeB/MgO/CoFeB MTJ stack after annealing up to 400℃ for 1 h in vacuum, by angle-resolved X-ray photoelectron spectroscopy (XPS). For 1nm-thick tantalum capping layer of the as-deposited stack sample, both the tantalum layer and the surface of the CoFeB layer underneath the tantalum layer were naturally oxidized. By comparison of the Co 2p and Fe 2p spectra with respect to as-deposited and annealed samples, reduction of the naturally oxidized cobalt and iron atoms was found to occur at the surface of the CoFeB layer. This reduction reaction was more significant at higher annealing temperature. Oxidized cobalt and iron were found to be reduced by boron atoms that diffused toward the surface of the top CoFeB layer. By means of naturally oxidized surface of the CoFeB layer formed on SiO2, the redox reaction behavior after annealing was confirmed by angle-resolved XPS analysis[2]. This redox reaction is thermodynamically reasonable according to the Ellingham diagram. (This work is supported by Industrial Affiliation on the STT-MRAM program in CIES Consortium and ACCEL project under JST.) [1] M. Niwa (2013), Chapter 4, High Permittivity Gate Dielectric Materials, Edited by S. Karr, Springer Series in Advanced Microelectronics 43 , pp. 191-234. [2] S. Sato et al., Appl. Phys. Lett. 196, 142407 (2015).

Double MgO Based Perpendicular Magnetic-Tunnel-Junction Spin-Valve without Upper Synthetic-Anti-Ferromagnetic Multi-Layers

Recently, the semiconductor memory device has faced the difficulties of scaling down, low power consumption and high retention time. In particular, dynamic random access memory (DRAM) has been struggling with scaling limit because of reducing capacitor area. In addition, alternative memories have been studied intensively to replace DRAM. Among them, perpendicular spin transfer torque magnetic random access memory (STT-MRAM) is considered as a strong candidate because of the small scaling down, low power consumption, high read and write speed and non-volatility. STT-MRAM has the structure of 1 transistor and 1 resistance (1T-1R) called magnetic tunnel junction (MTJ). Resistance difference between parallel and antiparallel states in MTJ indicates memory sensing margin. The large difference of tunneling magneto-resistance (TMR) is required for a good memory operation. To obtain the high TMR ratio, not only perfect switching of the free layer but also hard pinning of the pinned layer is required. It is known that pinning the pinned layer is due to the exchange coupling induced by a synthetic anti-ferro-magnetic (SyAF) layer based on Ruderman Kittel Kasuya Yosida (RKKY) interaction. In our study, therefore, we investigated how to optimize the SyAF layer structure for the proper exchange coupling. MgO tunnel barrier should be b.c.c (100) crystallinity because it acts as an electrons filter which determined whether spin direction between free and pinned layers is aligned parallel or not. In particular, we propose double MgO based perpendicular magnetic-tunnel-junction spin-valve without upper synthetic-anti-ferromagnetic Multi-layers in order to obtain high TMR ratio. In addition, we investigated the different characteristics of the p-MTJ spin-valve with and without upper SyAF multi-layers. For the experiment, two structures, double MgO based p-MTJ spin-valve with and without upper SyAF multi-layers, were fabricated on a 12-inch-diameter wafer deposited with SiO2/W/TiN films in a 12-inch multi-chamber sputtering system under a high vacuum of less than 1ⅹ10-8 torr (without breaking the vacuum; see Figs. 1a and b). The thicknesses of the tungsten (W) were varied ranging from 0.18 to 0.48 nm. All samples were subject to ex-situ annealing at 350oC for 30 min under a perpendicular magnetic field of 3 Tesla. The dependence of the TMR ratio on the thickness of W bridge layer was estimated by using current-in-plane tunneling (CIPT) at room temperature as shown in Fig.1c. It shows that a TMR ratio of p-MTJ spin-valve with upper SyAF multi-layers slightly increases at the bridge layer thickness, t w up to around 0.3 nm. Then it abruptly decreases when the thickness was increased from 0.36 to 0.5 nm. On the other hand, the TMR ratio of p-MTJ spin-valve without upper SyAF multi-layer increases at the t w up to around 0.24 m. Then, the TMR ratio abruptly decreases when the thickness was increased from 0.36 to 0.4 nm. And maximum TMR ratio of p-MTJ spin-valve with upper SyAF multi-layers is 156 % while, that of p-MTJ spin-valve without upper SyAF multi-layers is 180 %. Obviously, the TMR ratio of p-MTJ spin-valve without upper SyAF multi-layers is higher 24 % than p-MTJ spin-valve wih upper SyAF multi-layers. In our presentation, we report the different characteristics of the p-MTJ spin-valve with and without upper SyAF multi-layers by examining the static magnetization behavior, MgO crystallinity, and I-V characteristic curve. In addition, we present how these properties influence the TMR ratio and mechanism of p-MTJ spin-valve. * This work was supported by a Basic Science Research Program grant from the National Research Foundation of Korea (NRF) funded by the Korean government (MSIP) (No. 2014R1A2A1A01006474) and the Brain Korea 21 PLUS Program in 2014. Figure 1

Tunnel magnetoresistance of homocatenated silicon and germanium clusters

The spin-polarized transport characteristics of homocatenated clusters of group 14 elements have been studied using the non-equilibrium Green's function formalism. Compared with the carbon cluster, silicon and germanium clusters with extended σ-conjugation had a high tunnel magnetoresistance (TMR) ratio at a low bias voltage. This phenomenon was caused by the low energy level of the LUMO from σ-conjugation extended throughout the whole molecule and by the spin polarization of the sulfur located between the σ-conjugated system and the ferromagnetic electrode.

Bias dependence of spin transfer torque in Co2MnSi Heusler alloy based magnetic tunnel junctions

Heusler compounds are of interest as electrode materials for use in magnetic tunnel junctions (MTJs) due to their half metallic character, which leads to 100% spin polarization and high tunneling magnetoresistance. Most work to date has focused on the improvements to tunneling magnetoresistance that can stem from the use of Heusler electrodes, while there is much less work investigating the influence of Heusler electrodes on the spin transfer torque properties of MTJs. Here, we investigate the bias dependence of the anti-damping like and field-like spin transfer torque components in both symmetric (Co2MnSi/MgO/Co2MnSi) and asymmetric (Co2MnSi/MgO/CoFe) structure Heusler based MTJs using spin transfer torque ferromagnetic resonance. We find that while the damping like torque is linear with respect to bias for both MTJ structures, the asymmetric MTJ structure has an additional linear component to the ordinarily quadratic field like torque bias dependence and that these results can be accounted for by a free electron tunneling model. Furthermore, our results suggest that the low damping and low saturation magnetization properties of Heusler alloys are more likely to lead significant improvements to spin torque switching efficiency rather than their half metallic character.