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This paper proposes a brand new non-volatile XNOR logic-gate that works as both a logic-gate and a nonvolatile memory. It can unify a micro-processor and a memory and it is expected that it saves energy consumption associated with data transfer and enhances band-width of data transfer between the devices.

Voltage-control spintronics memory (VoCSM) is a spintronics-based memory that uses the voltage-control-magnetic-anisotropy (VCMA) effect as a selection method and the spin-Hall effect as a write method, and is a candidate for future nonvolatile main memory because of its advantages such as high density, low energy consumption, and robustness against read disturbance. In this paper, we demonstrate VoCSM with a self-aligned heavy-metal electrode, which is the most desirable in mass production and can expect the lowest critical current ( $I_{C})$ due to its narrow electrode structure. We obtain the value of $I_{C}$ of around $400~\mu \text{A}$ for a magnetic tunnel junction of size $35~{\mathrm{ nm}} \times 245$ nm by using a self-aligned heavy-metal electrode. The value is lowered to one-third that of the conventional VoCSM structure and is comparable with that of spin-transfer torque writing. Moreover, $I_{C}$ decreased due to the VCMA effect and reached $260~\mu \text{A}$ for $\text{V}_{\mathrm {MTJ}} = -0.8$ V. We also found that the diffusion spin current from the lateral region, which is the concern matter of this structure, did not contribute very much to the switching due to the extremely short spin diffusion length of the heavy-metal electrode. It indicates that VoCSM with a self-aligned heavy-metal electrode may lead to the production of high-density and low-energy-consumption memory.

We successfully demonstrated write tests in voltage-control spintronics memory (VoCSM) in which in-plane magnetic tunnel junctions (MTJs) with aspect ratio of about 1 (AR1) were used. Retention energy as large as that in MTJs with an aspect ratio of 2.5 was stored in AR1-MTJs. We controlled the strain introduced into the AR1-MTJs to store retention energy owing to strain-induced anisotropy. Results in the write tests show the possibility of high-density VoCSM using in-plane MTJs whose cell area is as small as 4F2.

We report a novel convolutional neural network (CNN) accelerator utilizing “voltage control spintronics memory” (VoCSM). High throughput processing is achieved by high speed in-“nonvolatile memory”-computation using high density VoCSM array. Since VoCSM has largest endurance even in short write pulse of all nonvolatile memories, write access speed can be increased. Also, density of processing element is higher than conventional SRAM based one, throughput is further increased. These technique reduces latency of CNN by 46% (ternary) and 73% (binary) compared to conventional CMOS processor with 8bit integer (INT8) for CIFAR-10 classification task. Also, energy is reduced by 72% (ternary) and 86% (binary).

Magnetic random-access memory (MRAM) using spin-transfer torque for write operations has been intensively developed as a technology for saving energy. The authors' recently presented voltage-controlled spintronic memory (VoCSM), which instead employs the spin Hall effect for writing, is here refined. High writing efficiency in VoCSM is achieved by means of an $a$-TaB/$\ensuremath{\beta}$-Ta spin Hall electrode, which features reduced write-current density, low write-error rate, strong durability, and high breakdown voltage. This improved VoCSM is seen as a path to high-density, high-speed nonvolatile memory with low power consumption.

We investigated a voltage-control spintronics memory (VoCSM) with a shunt-free design process and W spin-Hall electrode. We successfully reduced the write current to 62 μA at a 5-ns write pulse (48 μA at a 20-ns write pulse), utilizing a high spin-Hall efficiency by developing the shunt-free design process and optimized W spin-Hall electrode. Moreover, the device was very reliable, exhibiting properties such as a low write error rate ( 1 × 1012 cycles). The spin-Hall MRAM and VoCSM with the shunt-free design process and W spin-Hall electrode could lead to high-speed nonvolatile memory with low power consumption and high durability.

Voltage Control Spintronics Memory (VoCSM) is a magnetic memory combining the spin Hall effect and Voltage Controlled Magnetic Anisotropy (VCMA). It has the potential to make MRAM work faster. In this paper, we described memory design of a write window and a read window of high-speed VoCSM from experimental data. The design windows of both writing and reading are large enough for gigabit memory.

The requirement of low power consumption, high speed, high endurance for emerging next generation universal memory, a three terminal (separate read/write terminals) spin-orbit-torque (SOT) based non-volatile memory (SOT-MRAM) devices have been gaining much interests than two-terminal spin-transfer-torque MRAM (STT-MRAM) due to its improved magnetization switching, exploiting the spin Hall effect (SHE) mechanism [1–6]. The free layer of SOT-MRAM devices, consists of heavy metal (HM)/ferromagnet metal (FM) bilayer structure, essentially rely on two factors, first the magnitude of SOT from HM which is proportional to spin Hall angle (SHA), and the second is the effective damping constant $( \alpha _{\mathrm{eff}})$ of FM in FM/HM bilayer structure. Since the switching current in SOT-MRAM using in-plane magnetization of magnetic tunnel junction (i-MTJ) is inversely proportional to HM’s SHA and directly proportional to FM’s $\alpha_{\mathrm{eff}}$, we need to search for HM materials with high SHA and at the same time with low $\alpha _{\mathrm{eff}}$ of FM for HM/FM bilayer structure combination. Previously, we have shown the excellent magnetization switching behavior of SOT-MRAM using Ta 50 B 50 (HM), and evaluated its SHA by spin Hall magnetoresistance (SMR) measurement [7]. However, analysis of SHA based on SMR assume a simple drift-diffusion model and as for estimation of the effective damping constant, SMR cannot be applied. In this study, the spin-transfer ferromagnetic resonance (ST-FMR) method was used to evaluate both SHA of Ta 50 B 50 (HM) and $\alpha _{\mathrm{eff}}$ of Co 40 Fe 40 B 20 (FM) for Ta 50 B 50 ($t$ nm)/Co 40 Fe 40 B 20 (4 nm) bilayer structure. The ST-FMR mixed-signal can be fitted by the combination of symmetrical and asymmetrical components by $\mathrm {V_{Total}} = \mathrm {V_{Sym}} + \mathrm {V_{Asym}}$, where $\mathrm {V_{Total}}$ is total mixed signal, $\mathrm {V_{Sym}}$ is total symmetrical signal due to spin current, and $\mathrm {V_{Asym}}$ is total asymmetrical component due to charge current. The ST-FMR signal, its fitting and extracted each individual components are shown in Fig. 1 (a). The SHA evaluation is not straightforward, since other parameters/mechanisms, which are taking place simultaneously, have to be taken into account. At ferromagnetic resonance two phenomenons are taking place concurrently, the SHE from HM to FM and spin pumping (SP) from FM to HM. The SP induced spin current is converted back to charge current in HM through the inverse spin Hall effect (ISHE) which is reflected in symmetrical ST-FMR voltage signal and can be represented by $\mathrm {V_{Sym}} = \mathrm {V_{ST-FMR}} + \mathrm {V_{ISHE}}$. In order to remove the SP influence or considering contribution only from SHE i.e. $\mathrm {V_{ST-FMR}}$, the ratio of $\mathrm {V_{ST-FMR}}$ to $\mathrm {V_{Sym}}$ defined by η, is crucial and can be expressed as $\eta = \mathrm{V_{ST-FMR}}/ \mathrm{V_{Sym}} = \mathrm{V_{ST-FMR}}/ (\mathrm{V_{ST-FMR}}+ \mathrm{V_{ISHE}})$. This ratio η is very important, since both SHE and ISHE have the same angle dependence (due to similar mechanism), it is very difficult to isolate them, as suggested by other groups too [8, 9]. In addition, the influence of transparency $T$ at Ta 50 B 50 /Co 40 Fe 40 B 20 interface has to be taken into account for reliable SHA estimation [9, 10]. The transparency, which controls the spin trasnmittance at interface, is related to spin mixing conductance at the HM/FM interface. The spin mixing conductance was calculated from the enhancement of damping constant $( \Delta \alpha )$ of FM due to spin pumping from its intrinsic value $( \alpha _{0})$. The real value of SHA can be calculated by incorporating interface transparency $T$ and isolating the ISHE $( \mathrm{V_{ISHE}})$ contribution from $\mathrm{V_{Sym}}$, that is η. These contributions are incorporated by mathematically multiplying $T$ and η to conventional SHA $\Theta _{\mathrm{SH(r)}} = \eta T \Theta _{\mathrm{SH(m)}}$, where $\Theta _{\mathrm{SH(r)}}$ is the real value of SHA while $\Theta _{\mathrm{SH(m)}}$ is the observed value of SHA estimated by typical approach. In the observed SHA value, neither SHE contributions were separated nor was interface transparency considered, which leads to the overestimation of SHA. The values η, $T$ and $\eta T$ are shown as a function of Ta 50 B 50 layer thickness in Fig. 1 (b). As it is clear that the ratio η decreases systematically with Ta 50 B 50 layer thickness, however the interface transparency $T$ increases steeply and then saturates at around 2.5 nm. Their product $\eta T$ shows initial enhancement and after peak, a systematic decrease is observed. It is obvious that at lower Ta 50 B 50 thickness, interface transparency controls whereas at higher thickness the $V_{ST-FMR}$ contribution dominates. In the Fig. 1 (c), each individual symmetrical components are shown, which is required for reliable SHA estimation. The dependency of $\Theta _{\mathrm{SH(r)}}$ on Ta 50 B 50 layer thickness is illustrated in Fig. 1(d). By considering only SHE contribution and interface transparency we are able to evaluate the real value of SHA. This SHA value is nearly consistent with the derived SHA from separately fabricated SMR devices [7]. In conclusion, we have successfully estimated the SHA (-0.18) of Ta 50 B 50 by isolating SHE contribution from total ST-FMR signal combined with transparency at Co 40 Fe 40 B 20 Ta 50 B 50 interface. Additionally, we were able to achieve desired low damping constant $( \alpha _{\mathrm{eff}}) \approx 0.008$ for Co 40 Fe 40 B 20 /Ta 50 B 50 bilayer which is also crucial factor for realizing low switching current for future SOT-MRAM applications.

In recent years, writing data in magnetic random access memory (MRAM) utilizing voltage controlled magnetic anisotropy (VCMA) has attracted much attention for its potential low power consumption [1]. We proposed voltage-control spintronics memory (VoCSM) which had high-efficient and deterministic writing properties [2]. In order to realize those memories, three features of a large VCMA, a large surface anisotropy Ks, and a large tunneling magnetoresistance (TMR) should coexist. In addition, a large spin-Hall angle is a must for VoCSM. Many challenges based on MgO tunneling barrier/ferromagnetic layer (FL) such as CoFeB thin films combined with various materials as an insertion layer at the MgO/FL interface or as an underlayer of FL showed improved VCMA but were concerned to fail in the coexistence of the feature because of very thin storage-layer or degraded lattice growth between MgO and CoFeB [3]–[5]. As a result, none of them have had a practical meaning as a memory cell so far. In this study, the experiments were conducted in which the insertion position of Ir was changed in MgO/CoFeB/Ta thin films. Each of the interface layer, the interlayer and the underlayer of Ir showed an increase in VCMA, and the largest VCMA was obtained in the case of inserting the Ir interlayer into the CoFeB layer. In addition, both the resistance-area product (RA) and TMR ratio decreased greatly when using the Ir interface layer, but clearly improved by employing the Ir interlayer. The base multilayer structure for VCMA measurement was Ta (5 nm)/MgO $(\sim 3$ nm)/CoFeB (1–2 nm)/Ta (5–8 nm), which was deposited on a thermally oxidized Si substrate. The CoFeB layer was set to in-plane magnetization, and the base stack of IrMn/ CoFe/Ru/CoFeB /MgO/CoFeB/Ta with a reference layer was prepared for RA and TMR measurement by using current in-plane tunneling (CIPT). The multilayers for VCMA were patterned and etched into the device size with one side of 3 to $50 \mu \mathrm {m}$ and their hysteresis curves were measured using the magneto-optical polar Kerr effect. The effective perpendicular magnetic anisotropy field Hk $_{eff}$ of the CoFeB layer was measured while bias voltage was applied to the device, and the variation of Ks depending on the electric field E was evaluated as the VCMA coefficient. Figure 1 shows the VCMA coefficients (–dKs/dE) of the MgO/CoFeB/Ta thin films as the “Base” sample, “Interface” sample in which Ir (0.2 or 0.3 nm) is layered at the MgO/ CoFeB interface, “Interlayer” sample in which Ir (0.3 nm) is inserted in the middle of the CoFeB layer, and “Underlayer” sample in which Ir (0.5 nm) is formed between the CoFeB and the Ta layer. All coefficients of the “Interface”, “Interlayer”, and “Underlayer” samples increased more than that of the “Base” sample in terms of each average value, although each coefficient had a certain degree of dispersion. The Ks in the “Interface” sample also increased more than in the “Base” sample at each average value, however, the largest Ks (maximum of 2.2 erg/cm $^{2})$ and VCMA (maximum of 190 fJ/ Vm) were obtained in the “Interlayer” sample. The relationship between RA and TMR ratio in the MTJ samples similar to Fig. 1 with the reference layer is plotted in Fig. 2. Both RA and the TMR ratio in the “Underlayer” sample were almost the same as those in the “Base” sample, but both decreased in the “Interface” sample and further decreased by increasing the Ir layer thickness from 0.2 to 0.3 nm. In the “Interlayer” sample, the deterioration of RA was not observed, and although the TMR ratio decreased, it still showed a high value of more than 120%. By comparison at the Ir thickness of 0.3 nm, it can be seen that both RA and TMR are clearly improved by changing from the Ir interface layer to the Ir interlayer. In summary, we successfully found the practical MTJ structure as a memory cell which realized coexistence of a large VCMA, a large Ks, and a large TMR for the first time. The structure is expected to have a large spin-Hall effect as well. This work was partly supported by the ImPACT Program of the Council for Science, Technology and Innovation (Cabinet Office, Government of Japan).

MRAM has been developed since 1980s until now with several ups and downs. The ultimate purpose is to realize non-volatile working memories to save energy consumption of conventional volatile working memories such as SRAM and DRAM. However all of non-volatile memories including MRAM have been facing a dilemma of non-volatility and high energy consumption in their active mode because non-volatility has led to large writing-energy consumption, $E_{w}$. As a result, they have been used as data storages and none of them overcame the historical dilemma for busy applications. This is the one of the reasons why MRAM has not had big markets so far. Recently, the possibilities of overcoming the dilemma were demonstrated by both STT-MRAM and VoCSM [1], [2]. STT has better maturity but less room for further improvement. On the other hands, VoCSM has poor maturity but better potentials in terms of higher writing efficiency and better endurance [3]. In this talk, STT technologies and VoCSM technologies is reviewed with respect to saving energy consumption and remaining issues for VoCSM will be discussed at the conference.

In order to reduce energy consumption of memory hierarchy, several non-volatile memories have been developed. However all of the candidates consume more energy in active modes than save in stand-by modes for busy mobile applications. Therefore, usage of non-volatile memories has been limited to storage of data. This is a historical dilemma that all the candidates have not solved. In order to solve the historical dilemma, we proposed voltage-control spintronics memories (VoCSM) and proved the fundamental writing property and its high efficiency. Also we proposed and succeeded in demonstrating unique writing schemes of the high-speed architecture and the high-density architecture.

Our new proposal of voltage-control spintronics memory (VoCSM) in which spin-orbit torque in conjunction with the voltage-control-magnetic-anisotropy effect works as the writing principle showed small switching current of $37~\mu \text{A}$ for about 350 $K_{B}T$ switching energy. This indicates VoCSM’s writing efficiency is so high that VoCSM would be applicable for deep learning memories requiring ultra-low power consumption.

Patterning damage at the sidewall in a magnetic tunnel junction (MTJ) was observed precisely using a rectangular MTJ where deterioration in crystallinity is easier to identify than in the case of a dot-shaped conventional MTJ. A 200–500 nm-square rectangular MTJ was patterned by a 200 eV ion beam (IB). Cross-sectional transmission electron microscopy was used for damage observation. A bright-field image showed that crystallinity deteriorated to a depth of $\sim 1.3$ nm in the MgO-barrier layer. A Fourier transform mapping image and a dark-field transmission electron microscopy image indicated the existence of an amorphous region at the patterning edge in the MgO layer. IB etching is one of the strong candidates for magnetic random access memory (MRAM) fabrication. However, a typical IB etching energy, e.g., 200 eV, introduces a damage depth of several monolayers at the patterned surface. Since nearly damage-free-patterned surface would be needed for high-density MRAM with nanoscale MTJs of $\sim 10$ nm in diameter, IB etching with much lower energy would be necessary for fabrication.

Low power consumption of high-speed memories such as cache memories will be realized by use of magnetic random access memory (MRAM). We have proposed the voltage-control spintronics memory (VoCSM) as a writing method. The VoCSM has a large operation margin because the current path in writing and the voltage path in reading are separated. The fabrication process we have employed is the two-step self-alignment (TSSA) process that is advantageous for reducing write current and thus addresses one of the MRAM issues. The electrical properties of the VoCSM indicate that the write error rate (WER) steeply decreases depending on write current. This shows the VoCSM is available for low write current. For highspeed operation, differential operation is demonstrated. In this operation, two magnetic tunnel junctions (MTJs) are simultaneously written at write current, and then either of them is in the high-resistance state and the other is in the low-resistance state. Furthermore, we propose and demonstrate that the write current during differential operation is reduced by the U-shape, since the write current beneath two MTJs absolutely flows to the opposite directions in this shape. The VoCSM is suitable for application to high-speed memories.

We propose a new spintronics-based memory architecture with 2 MTJs and 4 transistors as a unit cell for high-speed application. The architecture employs spin-Hall effect as a writing principle and voltage-control-magnetic-anisotropy (VCMA) effect as a write speed acceleration. We successfully demonstrated the unique complementary flash-writing scheme and proved a potential of ultrahigh speed writing with the prototype unit-cell and the test-element.

Magnetization switching was observed in real time in voltage-control spintronics memory (VoCSM), which utilizes a hybrid writing method that combines spin–orbit torque and voltage-controlled magnetic anisotropy (VCMA). VCMA lowers the switching energy barrier between parallel and antiparallel states, leading to a reduction in the write current and acceleration of magnetization switching. Real-time observation revealed that this acceleration of magnetization switching was accomplished by reducing the incubation and switching times. Using this writing method, high write durability was realized over 1013 write cycles using 2-ns write pulses. These results indicate that VoCSM realizes high speed and highly reliable switching with low power consumption and has the potential to shape next-generation computing systems.Magnetization switching was observed in real time in voltage-control spintronics memory (VoCSM), which utilizes a hybrid writing method that combines spin–orbit torque and voltage-controlled magnetic anisotropy (VCMA). VCMA lowers the switching energy barrier between parallel and antiparallel states, leading to a reduction in the write current and acceleration of magnetization switching. Real-time observation revealed that this acceleration of magnetization switching was accomplished by reducing the incubation and switching times. Using this writing method, high write durability was realized over 1013 write cycles using 2-ns write pulses. These results indicate that VoCSM realizes high speed and highly reliable switching with low power consumption and has the potential to shape next-generation computing systems.

New equations for hydraulic efficiency conversion from a model to a prototype centrifugal pump including mixed and axial flow types have been developed and proposed in this paper. In order to establish a set of conversion equations applicable for all type numbers of pumps, the following factors related to the conversion equations were examined. 1) The ratio of scalable loss to total hydraulic loss was examined by using CFD (Computational Fluid Dynamics) analysis. The ratio is related with the effect of actual complex velocity distributions in the flow passages in the impeller and diffuser/volute casing. 2) The conversion equation was constructed by two terms dealing with flow passages in two major hydrodynamic components, impeller and diffuse/volute separately, where contributions of each component to the efficiency step-up was expressed explicitly. 3) The friction coefficient ratio between a model and prototype pump was expressed in a simplified equation, applicable for both hydraulically-smooth and transitional surfaces. This expression was found to be useful to determine the relationship between equivalent machined surface roughness and uniform sand roughness, as the friction coefficient diagram expressed for uniform sand roughness is used for conversion of hydraulic losses between the model and prototype pumps. 4) The difference between the friction coefficients for decelerating flow and for a flat plate with uniform flow was examined by CFD analysis, and it was found that the friction coefficient for a flat plate can be used for the conversion without causing any substantial deviation.

Epitaxial Fe/magnesium gallium spinel oxide (MgGa2O4)/Fe(001) magnetic tunnel junctions (MTJs) were fabricated by magnetron sputtering. 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 barrier height of the MgGa2O4 barrier is much lower than that in 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., Comment: 4 figures, 1 table

Catalytic aqueous living radical polymerization was achieved through a ligand design for a ruthenium-based catalyst. A phenolic phosphine ligand [PPh2(pPhOH)] was combined with a pentamethylcyclopentadienyl (Cp*)-based tetrameric ruthenium precursor, and the resulting complex showed a high catalytic activity for aqueous living radical polymerizations of hydrophilic methacrylates (for example, poly(ethylene glycol) methacrylate and 2-hydroxyethyl methacrylate) in conjunction with a chlorine initiator [H–(MMA)2–Cl]. The catalytic system allowed very fast living polymerizations, block copolymerizations and syntheses of high-molecular-weight polymers (DPn~1000) with narrow-molecular-weight distributions. Importantly, the activity was high enough to control the polymerization using a catalytic amount of the complex, even though the polymerizations were performed at low temperature (40 °C). Such advanced catalysis was achieved by not only simple hydrophilicity of the ligand but also by a water-assisted dynamic transformation from the original coordinatively saturated form [Cp*RuCl(PR3)2; 18e; PR3=phosphine] into an unsaturated and active form [Cp*RuCl(PR3); 16e]. Water molecule(s) may also coordinate for further stabilization as demonstrated by 31P NMR analyses.

We studied the spin-transfer switching probability (Psw) in giant magnetoresistance (GMR) device with perpendicular magnetizations using short nanosecond and sub-nanosecond current pulses. A switching time of 510 picoseconds was achieved with the application of 7.5 mA, which is 4.3 times larger than the critical current at 0 K, without the application of an assisting magnetic field. Experiments with longer pulses revealed an exponential decay of the nonswitching probability (1-Psw) as a function of pulse width. Extrapolation of the results predicts an error rate of 10-19 for a pulse width of about 4.8 ns. To understand the observed pulse width dependence of Psw, we developed a formula using a macro spin model for the perpendicular magnetization system which includes the influence of thermal fluctuations in the initial magnetization direction of the free layer. The formula easily reproduces the qualitative nature of the observed Psw distributions in all time ranges.

With judiciously selected ligands (phosphines) and cocatalysts (amines), a series of highly active and functionality-tolerant pentamethylcyclopentadienyl (Cp*) ruthenium catalysts [Cp*Ru(Cl)L1L2; L1 and L2: ligands] have been developed for living radical homo- and copolymerizations universally accessible to a variety of functional methacrylates in ethanol and related alcoholic and polar media. In particular, the ligand/cocatalyst combination of tri-m-tolylphosphine [P(mTol)3; mTol = m-MeC6H5] and a hydrophilic amine, 2-dimethylamino-1-ethanol [Me2N(CH2)2OH; 2-DMAE], led to a very active and robust catalyst that induced fast polymerizations and fine molecular-weight control (Mw/Mn < 1.2) in ethanol for not only homopolymerizations but also random or block copolymerizations with pendent-functional methacrylates carrying poly(ethylene glycol) (−PEG), dimethylamino [−N(CH3)2], and hydroxyl (−OH) groups. The accessible solvents included a wide variety of alcohols (methanol, ethanol, etc.), environmentally beni...

We fabricated magnetic tunnel junctions with a 3d-transition material(X)-doped MgO (Mg-X-O) barrier, and evaluated the effect of the doping on magnetoresistance (MR) and microstructure. Among the variations of X (X = Fe, Co, Ni, Cr, Mn, Ti, V, and Zn), X = Fe and Mn showed a high MR ratio of more than 100%, even at a low resistance-area product of 3 Ωμm2. The microstructure analysis revealed that (001) textured orientation formed for X = Fe and Mn despite substantial doping (about 10 at%). The elemental mappings indicated that Fe atoms in the Mg-Fe-O barrier were segregated at the interfaces, while Mn atoms were evenly involved in the Mg-Mn-O barrier. This suggests that MgO has high adaptability for Fe and Mn dopants in terms of high MR ratio.

We experimentally demonstrate a giant voltage-controlled magnetic anisotropy (VCMA) coefficient in a crystallographically strained CoFe layer (~15 monolayers in thickness) in a MgO/CoFe/Ir system. We observed a strong applied voltage dependence of saturation field and an asymmetric concave behavior with giant VCMA coefficients of −758 and 1043 fJ V−1 m−1. The result of structural analysis reveals epitaxial growth in MgO/CoFe/Ir layers and the orientation relationship MgO(001)[110] ∥ CoFe(001)[100] ∥ Ir(001)[110]. The CoFe layer has a bcc structure and a tetragonal distortion due to the lattice mismatch; therefore, the CoFe layer has a large perpendicular magnetic anisotropy.

Perpendicular L10-FePt/MgO/Fe/L10 -FePt magnetic tunnel junction (MTJ) films with the (001) texture were successfully developed to obtain a large tunnel magnetoresistance (TMR) above 100 % at room temperature. The TMR ratio in the L10-FePt/MgO/Fe/L10-FePt MTJ was strongly dependent on the Fe interfacial layer thickness. The lattice mismatch between the MgO(001) barrier layer and the L10 -FePt(001) layer is too large for the MgO barrier layer to grow epitaxially on the L10-FePt(001) layer. The insertion of the Fe interfacial layer improves the quality of the MgO(001) barrier layer and achieves an epitaxy in the L10-FePt/MgO/Fe/L10-FePt stack. As a result, the optimization of the Fe interfacial layer thickness is a key to obtain the large TMR ratio in the MgO-based MTJ with the L10-FePt electrodes.

To enlarge the read margin of magnetic random access memory (MRAM), we developed a SiO2/Si3N4 hard mask process for magnetic tunnel junction (MTJ) stack patterning. This process can protect MTJ materials from oxidation during the resist removal process and reduces the distribution of MTJ resistance for 0.32-µm-wide bits on an 8-in.-diameter wafer more than the conventional process does. We also developed process integration for 4-Mbit toggle MRAMs: the read margin for 4-Mbit reached nearly 18σ.

For each case of current and voltage detection in series connections, and current and voltage in parallel connections in the multi-layered structure of a magnetoresistive random access memory (MRAM) cell having two serially connected magnetic tunnel junction (MTJ) layers, first, we demonstrate the existence of an optimum value for the weighting coefficient k of the resistance, the voltage dependence, and the dependence on the magnetic resistance (MR) of the MTJ. Therefore, to optimize the signal margin in multiple layers, the ability to detect series connections with high sensitivity up to an MR ratio of 1 is demonstrated quantitatively. © 2007 Wiley Periodicals, Inc. Electron Comm Jpn Pt 2, 90(3): 42–48, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecjb.20265

Critical switching current, I sw , of STT (Spin Transfer Torque)-MRAM has been reduced by several orders with perpendicular MTJ and the state-of-the-art write charge, Q w , becomes the order of 100–150fC. With the small Q w , MRAM starts to save energy consumption by 70–80% compared with a conventional memory system. Analysis of the write pulse-width dependence of I w revealed a further potential of perpendicular MTJ to reduce I w and Q w . STT-MRAM is thought to achieve a further reduction of energy consumption.

Although mobile devices such as smartphones are convenient in many respects, short battery lifetime remains an issue. Moreover, the energy consumption of the mobile processor is increasing as its performance improves. Accordingly, there are high expectations that the energy consumption of mobile devices will be reduced in order to prolong battery lifetime.

We examined the effects of the capping layers of Ta, Al-oxide (AlO), and Ru on the magnetic properties of ultrathin (1–20 nm) NiFe free layers and investigated thermal degeneration of the switching field (Hsw) in magnetic tunnel junctions (MTJs) using these caps in the cell structure of magnetoresistive random access memory (MRAM). The magnetization reversal of the free layer was found to be sensitive to the capping layer that affected various magnetic anisotropies. For postannealing temperatures of up to 350 °C, the Hsw of a NiFe(2 nm)/AlO/Ta free layer was thermally stable, whereas that of conventional NiFe(3 nm)/Ta seriously deteriorated. This is because the AlO capping layer completely prevented the thermal interdiffusion between the NiFe and Ta layers even after annealing at 400 °C. Moreover, a free layer with a small magnetostriction reduced the influence of the stress-relaxation on the free layer caused by postannealing, which also resulted in a thermally stable Hsw in MTJs.

A hybrid writing scheme that combines the spin Hall effect and voltage-controlled magnetic-anisotropy effect is investigated in Ta/CoFeB/MgO/CoFeB/Ru/CoFe/IrMn junctions. The write current and control voltage are applied to Ta and CoFeB/MgO/CoFeB junctions, respectively. The critical current density required for switching the magnetization in CoFeB was modulated 3.6-fold by changing the control voltage from −1.0 V to +1.0 V. This modulation of the write current density is explained by the change in the surface anisotropy of the free layer from 1.7 mJ/m2 to 1.6 mJ/m2, which is caused by the electric field applied to the junction. The read disturb rate and write error rate, which are important performance parameters for memory applications, are drastically improved, and no error was detected in 5 × 108 cycles by controlling read and write sequences.

Amorphous carbon and carbon nitride (CNx) films were deposited by using the ECR plasma sputtering method. The films were deposited on a silicon substrate at room temperature by the sputtering of a carbon target by argon and argon/nitrogen ion plasma. The properties of films were investigated by XPS, Raman, SEM, and tribological measurements. The maximum of nitrogen content was N/C=0.6 in the film deposited in a nitrogen atmosphere. The friction coefficient of deposited films increased with increasing of nitrogen content. The CSS test of ultra-thin films on an Al/TiC slider with a Si intermediate layer (2 nm) indicates that a 5-nm thickness film of carbon nitride has the most significant toughness for a magnetic head.

An Fe-based perpendicular alloy with small damping constant was applied to an MTJ storage layer and small switching current of 9 μA was obtained for a write current width of 5 ms. The efficiency of spin transfer torque writing was proved to be higher than those for in-plane MTJs. The estimated Ic for the MTJ with 50 nsec pulse width is lower than 20 μA and smaller than the drive currents of CMOS transistor at Gbits density.

In order to estimate the losses and temperature rise at the end of electrical machines, it is important to clarify electromagnetic behavior in the core end region. In this paper, the method of evaluating the leakage flux of the stator end region of large turbogenerators, combining three-dimensional magnetic field analysis and phasor diagram analysis of the end leakage flux is described. Moreover, the result of applying this method to a 60 Hz-1000 MW turbogenerator is also described.

Insertion of an ultrathin (les6 Aring) Ta-seed layer into the bottom NiFe layer of a synthetic antiferromagnet (SAF) grown on the AlO and MgO barriers of magnetic tunnel junctions (MTJs) significantly increased the crystal orientation of the SAF. This led to improvements of the strength and thermal robustness of the antiferromagnetic coupling (AFC) in the SAF. The magnetic properties of the films and MTJs using the SAFs were investigated. A low anisotropy field with a large AFC of the Ta-seed SAF reduced the writing field of toggle MRAMs

The effect of the reduction of the sidewall redeposition layer of magnetic materials is investigated for submicron-patterned magnetic random access memory (MRAM) cells. The sidewall redeposition layer is made at the first etch step of a magnetic tunnel junction (MTJ) with ion beam etching (IBE) in the case that the sidewall angle of a hard mask is steep. By controlling the etched profile at the time of the first IBE step, formation of the redeposition layer is prevented. Functional test results of 1-Kb MRAM arrays show that the sidewall redeposition layer enlarges fluctuation of switching current, and reduces the write operation region. The effect of the sidewall redeposition on the switching characteristics of MRAM arrays is explained qualitatively by micromagnetic simulation solving the Landau-Lifshitz-Gilbert (LLG) equation

Technologies for realizing high density MRAM were developed. First, new circuitry to lower the resistance of programming wires was developed. Second, both MTJ plane shape and cross-sectional structure were optimized to lower the programming current. Based on these two technologies, 16 Mb MRAM was designed, fabricated with 130 nm CMOS process and 240 nm back end MTJ process. As a result, a 1.8 V power supply MRAM with 42.3% array efficiency was successfully demonstrated

A new protruding yoke wire was proposed and investigated for reduction of programming current of magnetic random access memory (MRAM). By optimization of the overall design of the yoke wires and magnetic tunneling junctions (MTJs) considering the MTJ switching mechanism, the programming current was successfully reduced for not only the conventional MTJ but also our previously proposed uniquely shaped MTJ, which kept the good asteroid curves, owing to the protruding yoke wire.

The switching characteristic of the MTJ (magnetic tunnel junction) submicron pattern is investigated. The mechanism of the step in the M-H loop analyzed. In the M-H loop of the MTJ, which has a C-type domain, a step is created by pinning of the 360-degree domain wall, which is caused by the stray field originated in the pinned layer. The position of the steps in the M-H loops is dependent on the direction of the stray field from the pinned layer. The model explains the steps in the M-H loops obtained from the experiment.