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We propose a magnetic random access memory (MRAM) cell that utilizes field-induced switching and is applicable to high-speed memories. The MRAM cell, called the shape-varying MRAM cell, has three free layers, each having different shapes and functions, and achieves low write-current switching with high thermal stability and high external field robustness. We show analytically that one of the layers contributes to the low write-current switching and another contributes to the thermal stability. We also show the results of a micromagnetic simulation, in which write current of <0.5 mA, write time of <2 ns, energy barrier (ΔE/kBT)>100, and external field robustness of >32 Oe were obtained. [ABSTRACT FROM AUTHOR]

Current-driven domain wall motion from pinning sites in nanostrips with perpendicular magnetic anisotropy is studied by using micromagnetic simulations, supported by a one-dimensional model of wall dynamics. The threshold current density of perpendicular anisotropy strips is much smaller than that of in-plane anisotropy strips, and is almost independent of the pinning potential strength. This results from the narrower domain wall width, smaller hard-axis anisotropy, and the larger ratio of the depinning field and hard-axis anisotropy. In the one-dimensional model with a zero damping constant, the threshold current density is found to be about 0.72 of the intrinsic threshold current density for a perfect strip in a strong pinning regime that corresponds to strips with perpendicular magnetic anisotropy. The fact that the threshold current density from the pinning sites is smaller than the intrinsic current density is because the effective field, equivalent to the pinning potential, enhances a breakdown in the pinning site. Moreover, in the strong pinning regime, an opposite-direction depinning hardly ever occurs after current pulse is turned off. These features of strips with perpendicular magnetic anisotropy are attractive for magnetic random access memories where the domain wall should be moved stably between the pinning sites with the small current pulse. [ABSTRACT FROM AUTHOR]

We calculate writing characteristics of two kinds of low-current magnetic random access memory (MRAM) devices using a single domain model. A four-layer synthetic antiferromagnet (SAF) has four antiferromagnetically coupled ferromagnetic (FM) layers, and a two-SM-layer-FC SAF has a two-FM-layer SAF with ferromagnetically coupled (FC) two soft magnetic (SM) layers. The threshold field of these free layers can be calculated analytically. The spin flop field of four-FM-layer SAF film is independent of the strength of antiferromagnetic coupling (AFC) between inner films. The saturation field increases with increasing the AFC strength. The spin flop field of the two-SM-layer-FC SAF film decreases, and the saturation field increases with decreasing ferromagnetic coupling strength. However, activation energy analysis demonstrates that both types of films exhibit a decrease in activation energy near the spin flop field. Considering the activation energy decrease during the toggling write sequence and a sufficient writing margin, MRAM with four ferromagnetic layers decreases writing current by 40%. [ABSTRACT FROM AUTHOR]

We have developed a shape-varying magnetic tunneling junction (MTJ) (SVM) which has a high MR ratio and low write current for use in high-speed magnetic random access memory (MRAM) cells. Combining NiFe that has low anisotropy to CoFeB which has high anisotropy through the nonmagnetic layer by interlayer exchange coupling (synthetic ferromagnetic coupling free layer: SFF), the anisotropy of SFF was reduced much more than that of CoFeB, and the MR ratio was improved much more than that of NiFe. The switching magnetic field (Hsw) of SFF was reduced as the thickness of NiFe increased. The Hsw of SFF for 0.24×0.48 μm2 MTJ was 30 Oe when the thickness of CoFeB was 1.5 nm and that of NiFe was 3.0 nm. Furthermore Hsw was reduced to 18 Oe by varying the shape of the MTJ of NiFe to 0.48×0.48 μm2; the shape of the MTJ of CoFeB was not changed (0.24×0.48 μm2). Combining the SVM and a write-line-inserted structure, we obtained a write current of 0.9 mA and an MR ratio of 140%. The Hsw was 40 Oe and its thermal stability factor was 82. These properties are sufficient for operating MRAMs over 500 MHz. [ABSTRACT FROM AUTHOR]

Current driven domain wall motion in nanostrips with perpendicular magnetic anisotropy was analyzed by using micromagnetic simulation. The threshold current density of perpendicular anisotropy strips in adiabatic approximation was much smaller than that of in-plane anisotropy strips, and it reduced with thickness reduction. The differences originate from the differences in domain wall width and hard-axis anisotropy. Also, the threshold current density of perpendicular anisotropy strips required to depin from a pinning site was quite small although the threshold field of the strips was sufficiently large relative to those of in-plane anisotropy strips. [ABSTRACT FROM AUTHOR]

The critical current density required for current-induced domain wall motion (Jc) was reduced by decreasing the magnetization (Ms) of the free layer film of a U-shaped pattern for a domain wall motion memory. The Ms was decreased by adding the nonmagnetic elements Ta and Cu to NiFe films. The Jc of the U-shaped patterns with NiFe–Ta and –Cu decreased as the proportion of Ta and Cu in the NiFe increased. The relations between Ms and Jc were examined and it was found that Jc decreased monotonically from 1.2×1012 to 0.8×1012 A/m2 as Ms decreased from 1.0 to 0.6 T regardless of what other materials were used. It is considered that the decrease in Ms caused a decrease in the hard axis anisotropy of the pattern, and the Jc decreased as predicted by a one-dimensional model. [ABSTRACT FROM AUTHOR]

We have developed a write-line-inserted magnetic tunneling junction (MTJ) (WLIM) for use in low-write-current magnetoresistance random access memory (MRAM). The write current of the WLIM was reduced to 0.98 mA and its thermal stability factor was 85 for a 0.32×0.48 μm2 MTJ. We evaluated the switching property of the WLIM in an external magnetic field (Hsw_ext) and an internal magnetic field (Hsw_int). We found that Hsw_ext was larger than Hsw_int when the aspect ratio of the MTJs was less than 1.5. Furthermore, we obtained a high write-current magnetic-field efficiency of 13.0 Oe/mA when the aspect ratio of the MTJs was low. These properties mean that the WLIM structure has advantages for use in low-write-current MRAM. [ABSTRACT FROM AUTHOR]

An edge oxidization effect on magnetization reversals is investigated for submicron-patterned magnetoresistive tunneling junctions (MTJs). By the MTJ edge oxidization which causes the MTJ edge saturation magnetization (Ms) reduction, the switching field distributions (SFDs) for 0.24×0.48 μm2 MTJs are reduced to less than 10%. The offset fields and the kinks in resistance-magnetic-field curves are reduced. Micromagnetic simulation results predict that the edge magnetization reversals are suppressed by the MTJ edge Ms reduction and the edge domain size at the remanent states becomes small. Consequently, the edge domain motion suppression by the edge oxidization is effective for decreasing the SFDs. [ABSTRACT FROM AUTHOR]

A conceptual material design for magnetic tunneling junction cap layer realizing a steep NiFe/AlOx interface is proposed. Tunnel magneto resistance stack of cap/NiFe/AlOx/CoFe/Ru/CoFe/PtMn/Ta//sub was prepared. Maximum magnetoresistance (MR) ratios of nonmagnetic-NiFeZr, Zr, Ta, Ru, and Rh caps at 0 V were 55%, 28%, 50%, 43%, and 42%, respectively. The decrease of MR ratio and the increase of resistance area product RA with Ru cap compared to Ta cap correlate with the partial oxidation of the NiFe/AlOx interface occurring in additional postannealing, which was confirmed by focused-ion-beam–transmission-electron-microscope–energy-dispersive-x-ray-fluorescence observation. Since standard electrode potential is Ta

A write-operating window with a 100% functional bit yield was successfully obtained by the control of stray fields from synthetic antiferromagnetic (SAF) pinned layers in conventional magnetic random access memories with rectangular magnetic tunneling junction bits. The stray fields were controlled by a newly developed ion-beam etching technique without causing damage and by a precise setting of the SAF pinned layer thickness, and are balanced with Néel coupling fields. As a result, it was found that symmetric switching astroid curves with no offset were obtained and switching distributions were minimized at the zero offset field. [ABSTRACT FROM AUTHOR]

We have investigated the temperature dependence of the current-induced magnetic domain wall (DW) motion in a perpendicularly magnetized Co/Ni nanowire at various temperatures and with various applied currents. The carrier spin polarization was estimated from the measured domain wall velocity. We found that it decreased more with increasing temperature from 100 K to 530 K than the saturation magnetization did. [ABSTRACT FROM AUTHOR]

Spin wave attenuation in the layered [FeNi/Pt]6/FeNi thin films was investigated by the time-domain electrical measurement. The spin-wave waveform was detected with an asymmetric coplanar strip transmission line, as an induced voltage flowing into a fast oscilloscope. We report that the amplitude of a spin-wave packet was systematically changed by controlling the thickness of a platinum layer, up to a maximum change of 50%. The virtues of spin wave, ultrafast propagation velocity and non-reciprocal emission, are preserved in this manner. This means that the Pt layer can manipulate an arbitral power-level of spin-wave input signal (reliable attenuator). [ABSTRACT FROM AUTHOR]

The electric field effect on magnetization properties and Curie temperature of Co ultra-thin films has been investigated. An electric field is applied to a Co film by using an electric double layer (EDL) formed in a polymer film containing an ionic liquid. The change in the Curie temperature is ∼100 K by applying the gate voltage of ±2 V, suggesting that the observed large modifications of magnetization properties are attributed to the significant change in the Curie temperature, which is induced by a large amount of carrier density control due to the formation of the EDL. [ABSTRACT FROM AUTHOR]

We have investigated the velocity of magnetic domain wall (DW) motion induced by electric currents in a Co/Ni nanowire with a perpendicular magnetic anisotropy. The DW velocity increased as current density increased and the maximum velocity of 60 m/s was observed. Furthermore, the DW velocity was found to be almost independent of external perpendicular magnetic fields in the range of -50 to +50 Oe. The mechanism of the observed field insensitivity of the current induced DW motion is also discussed. [ABSTRACT FROM AUTHOR]

The current-induced effective field in perpendicularly magnetized Ta/CoFeB/MgO wire was investigated. A threshold field decrease of 6.4 kOe/mA was observed by measuring the threshold field of Hall resistance versus the magnetic field curve with various bias currents. The decrease was probably caused by the in-plane effective field, mainly due to the Rashba effect. The effective field of the Ta/CoFeB/MgO wire was smaller and opposite in direction compared to that of Pt/Co/AlOx previously reported. [ABSTRACT FROM AUTHOR]

Current-induced domain wall motion in perpendicularly magnetized CoFeB nanowires with a stack structure of Ta(1.0 nm)/CoFeB(1.2 nm)/MgO(2.0 nm)/Ta(1.0 nm) was investigated. Domain wall motion driven by adiabatic spin-transfer torque was observed at a current of about 74 μA, corresponding to a current density of 6.2×107 A/cm2. The obtained results were compared with those of a micromagnetic simulation and the spin polarization of the CoFeB was estimated to be 0.72. [ABSTRACT FROM AUTHOR]

We investigated the relation between critical current of domain wall motion and wire dimension by using perpendicularly magnetized Co/Ni nanowires with different widths and thicknesses. The critical current, Ic, became less than 0.2 mA when w<100 nm, suggesting that magnetic random access memory with domain wall motion can replace conventional embedded memories. In addition, in agreement with theory, the critical current density, jc, decreased as wire width decreased and became much less than 5×107 A/cm2 when w<100 nm. We also performed a micromagnetic simulation and obtained good agreement between the experiment and simulation, although a few discrepancies were found. [ABSTRACT FROM AUTHOR]

We have developed a shape varying magnetic tunneling junction (SVM) with a Fe/NiFeB free layer for use in magnetic logic circuits. Inserting the thin Fe film between an NiFeB free layer and MgO tunneling barrier improved the magnetoresistance (MR) ratio: it increased up to 130%, as the thickness of the Fe film increased. In addition, the switching current distribution of the SVM was reduced to 8%. By using NiFeB as a free layer, the roughness under the MgO was reduced and the crystallization of the MgO was enhanced. This led to both the high MR ratio and the low switching current distribution. Our developed Fe (0.4 nm)/NiFeB free layer satisfies the requirement of the MTJ's characteristics that the magnetic logic circuits operate with a high bit yield. [ABSTRACT FROM AUTHOR]

A wire with perpendicular magnetic anisotropy (PA) is suitable for domain-wall-motion (DWM) cells because its critical current is small. However, it is not easy to design a cell consisting of a high magnetoresistance (MR) ratio magnetic tunneling junction (MTJ) and a DWM using PA material alone. We propose a combination of a PA-DWM element and in-plane (IP) MTJ for detection. The structure can be designed in a way that reduces the write current by the use of a perpendicular layer, yet maintains a high MR ratio by the independent use of in-plane material stacks. We fabricated a cell and ran tests to determine its read and write properties. A critical write current of 700 μA and a MR ratio of 50% were achieved. These properties are almost the same as when a DWM wire and in-plane MTJ are fabricated separately, which means it is possible to design the two elements independently. [ABSTRACT FROM AUTHOR]

We developed an accurate and precise system for controlling dew points by using an active pressure control system in a primary standard humidity generator. By using this system, an adjustment capability to a target value of 0.0008 °C was attained for dew-point generation, compared with the previous value of approximately 0.01 °C. In a two-pressure humidity generator, the drift in the dew point with time owing to a rapid drop in the water level in the presaturator and to fluctuations in atmospheric pressure was greatly suppressed. The standard deviation of the dew point obtained with our dew-point control system was 10 times smaller than the drift observed in the absence of the system. This dew-point control system makes it possible to calibrate humidity sensors at target values with greater efficiency.