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Controlling the position of a magnetic domain wall with electric current may allow for new types of non-volatile memory and logic devices. To be practical, however, the threshold current density necessary for domain wall motion must be reduced below present values. Intrinsic pinning due to magnetic anisotropy, as recently observed in perpendicularly magnetized Co/Ni nanowires, has been shown to give rise to an intrinsic current threshold J(th)(0). Here, we show that domain wall motion can be induced at current densities 40% below J(th)(0) when an external magnetic field of the order of the domain wall pinning field is applied. We observe that the velocity of the domain wall motion is the vector sum of current- and field-induced velocities, and that the domain wall can be driven against the direction of a magnetic field as large as 2,000 Oe, even at currents below J(th)(0). We show that this counterintuitive phenomenon is triggered by Walker breakdown, and that the additive velocities provide a unique way of simultaneously determining the spin polarization of current and the Gilbert damping constant.

We study current-driven domain-wall (DW) motion and thermal stability of Co/Ni strips with DWs at the step boundaries by detecting DW resistance. The depinning current is around 0.5 mA, while the pinning field is 300 Oe, which corresponds to the thermal stability factor of 54 estimated by self-distribution of the pinning field. The depinning current decreases as the strip width decreases and does not strongly depend on the pinning field. On the other hand, the thermal stability factor increases as the pinning field increases. Therefore, an even lower current with sufficient thermal stability can be achieved by decreasing the strip width and increasing the pinning field.

The fact has been known that lime is one of the most important materials as flux in steelmaking. The last decade, the average supply of lime for flux is up to 5.5 million tons/year. Fluorspar is convenient material for making flux as dropping of melting point of slag, a number of steel making plants didn't pay attention to the standards of lime but for chemical compound and reactivity. But nowadays, because the solution of fluorine in soil is becoming serious, and the use of fluorspar has been restricted as flux. So, the characteristic of lime, such as surface area and microstructure are drawing the attention for making slag.In this paper, it is reviewed that improving of characteristics of quick lime by various calcination methods.

We have proposed detecting a domain wall behavior in a strip with perpendicular magnetic anisotropy by measuring both extraordinary Hall effect and domain wall resistance with unique placement of three via contacts. By using this detecting technique, we have confirmed that the domain wall can be initialized at the boundary of the stepped structure of a Co/Pt multilayer strip. We have observed nucleation independent of the current direction and the propagation depending on it. We could not find any domain wall motion by spin-transfer torque from the boundary until 1.8 times 1012 A/m2, probably because of the smaller polarization and/or the larger damping constant of the Co/Pt multilayer.

In this paper, we have calculated the intrinsic threshold current density of domain wall (DW) motion in nanostrips with perpendicular magnetic anisotropy (PMA) and have estimated writing properties of magnetic random access memories (MRAMs) with DW motion. Carrying out a micromagnetic simulation, we revealed that the intrinsic threshold current density decreases with decreases in the strip thickness, width, and magnetization, whereas it did not depend significantly on magnetocrystalline anisotropy and exchange stiffness. These results showed good agreement with one-dimensional (1-D) analysis. We also found that current-induced DW motion in PMA strips may have potential for use in low-write-current MRAMs. For a width of less than roughly 100 nm, comparable properties to those of existing memories can be obtained.

This paper describes a recently developed 16-Mb toggle magnetic random access memory (MRAM). It has 100-MHz burst modes that are compatible with a pseudo-SRAM even though the toggle cell requires reading and comparing sequences in write modes. To accelerate operating clock frequency, we propose a distributed-driver wide-swing current-mirror scheme, an interleaved and pipelined memory-array group activation scheme, and a noise-insulation switch scheme. These circuit schemes compensate the toggle cell timing overhead in write modes and maintain write-current precision that is essential for the wide operational margin of MRAMs. Because toggle cells are very resistant to write disturbance errors, we designed the 16-Mb MRAM to include a toggle MRAM cell. The MRAM was fabricated with 0.13-mum CMOS and 0.24-mum MRAM processes with five metal layers.

We have investigated the distribution of the flop field, which represents writing field in a toggle magnetic random access memory (MRAM). We analyzed the factors of the distribution by dividing them into the 45deg and f 35deg directions in (Hx, Hy) coordinates. We found that the distribution in the 135deg direction is mainly caused by stress-induced anisotropy and can be effectively suppressed by adopting materials that maintain low magnetostriction even after the fabrication process. On the other hand, we found that the distribution in the 45deg direction depends on the texture and atomic structure of the ferromagnetic layers, and that low distribution can be attained when the ferromagnetic layers are constructed from small crystalline grains or amorphous materials. We demonstrated a toggle MRAM with a distribution of the flop field only half that of the previously reported magnetic tunnel junction stack structure.

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.

This paper describes newly developed magnetic random access memory (MRAM) cell technology suitable for high-speed memory macros embedded in next-generation system LSIs: a two-transistor one-magnetic tunneling junction (2T1MTJ) cell structure, a write-line-inserted MTJ, and a 5T2MTJ cell structure. The 2T1MTJ cell structure makes it possible to significantly improve the write margin and accelerate the operating speed to 200 MHz. Its high compatibility with SRAM specifications and its wide write margin were confirmed by measuring 2T1MTJ MRAM test chips. Although the cell structure requires a small-writing-current MTJ, the current can be reduced to 1mA using the newly developed write-line-inserted MTJ. Further development to reduce the current down to 0.5 mA is required to obtain a cell area of 1.9 mum2, which is smaller than the SRAM cell area, in the 0.13-mum CMOS process. The 5T2MTJ cell structure also enables random-access operation over 500 MHz because the sensing signal is amplified in each cell. Random access time of less than 2 ns can be achieved with SPICE simulation when the magnetic resistance is 5 kOmega and the magnetoresistive (MR) ratio is more than 70%

The authors have investigated wire width dependence of threshold current density for current-induced magnetic domain wall (DW) motion in perpendicularly magnetized Co/Ni nanowires. Threshold current density decreased with reducing the wire width, and the lowest threshold value of 1.6×1011 A/m2 was observed at the narrowest width of 36 nm. The micromagnetic simulation shows that the observed width dependence of threshold current density can be understood by the adiabatic spin transfer model.

The read/write characteristics for perpendicular magnetic recording media of focused-ion-beam (FIB)-etched recording heads were investigated. It was found that the trailing edge of an FIB-etched head produces a higher gradient in the magnetic field perpendicular to the medium than a head which has not been etched. The signal-to-noise ratio of the medium increased with the FIB-etched write gap. A high-Bs and thin pole increased the magnetic field's gradient in the perpendicular direction, resulting in excellent read/write characteristics.

A magnetic recording head with a Co-Ni-Fe/Ni-Fe composite write pole was designed. The saturation induction Bs of Co-Ni-Fe is 2 T. In a computer simulation of a 3D static magnetic field, a recording head with Co-Ni-Fe films on both sides of a write gap generated a longitudinal write field of 9.9 kOe at a magnetic spacing of 40 nm from the write gap. The write characteristics of the recording head with the dual high-Bs structure designed above was tested for media with high coercivities of 4.5 to 7 kOe. The overwrite performance is over 30 dB for a medium with high coercivity of 7 kOe. Straight patterns of magnetic transitions written on the medium with a coercivity of 7 kOe were observed by using a magnetic force microscope (MFM). On the other hand, bends were found in the track-edges of magnetic transitions written on a medium with a coercivity of 4.5 kOe. This indicates that the recording head generated too large a write field for the medium with a coercivity of 4.5 kOe.

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 effects of recording heads on off-track noise characteristics were examined. It was found that an FeTaN head with a narrow gap and high Bs was very effective for decreasing both on-track noise and off-track noise, in com- parison with a NiFe head with a narrow gap or a merged- type NiFe head with a wide gap. It was also confirmed that, according to readback waveform analysis and MFM images observation, the fluctuations of the magnetic transition in track-edge regions for magnetic patterns recorded by the FeTaN head were much smaller than for those recorded by the merged-type NiFe head with a wide gap. It will become increasingly important to improve recording heads as their track width becomes narrower.

Narrow track technologies for shielded MR heads are discussed in terms of off-track performance and increased head output. Permanent magnet bias heads are increasingly superior to exchange coupling bias heads as tracks become narrower, because of the excellent off-track performance of the permanent magnet type. Increasing the sense current is one way to increase head output. However, the current increase raises the temperature of the MR element and degrades performance. An analysis of the thermal behavior of the shielded MR head shows that increasing the thermal conduction of the shield and gap layers, which are the main sources of heat radiation, and improving the crystalline properties of the Ni-Fe film effectively suppress the rise in temperature of the MR element.

We have developed a top free type magnetic tunnel junction (MTJ) patterning technique that involves tunnel barrier etching with enough time margins for chemical assisted ion etching (CAIE). The approximately 1 nm thick tunnel barrier enables stopping the etching process in a shorter time margin. We have found that no aluminum-oxide barrier shorting by re-deposition occurred in chlorine based CAIE, when the etching depth into anti-ferromagnetic IrMn layer was less than 5 nm. The magnetoresistance (MR) ratio of the whole MTJ patterns reached 35% on a 6-inch wafer. The time margin of the etching was 120 s, which is long enough for an magnetic random access memory (MRAM) process.

Controlling the displacement of a magnetic domain wall is potentially useful for information processing in magnetic non-volatile memories and logic devices. A magnetic domain wall can be moved by applying an external magnetic field and/or electric current, and its velocity depends on their magnitudes. Here we show that the applying an electric field can change the velocity of a magnetic domain wall significantly. A field-effect device, consisting of a top-gate electrode, a dielectric insulator layer, and a wire-shaped ferromagnetic Co/Pt thin layer with perpendicular anisotropy, was used to observe it in a finite magnetic field. We found that the application of the electric fields in the range of ± 2-3 MV cm(-1) can change the magnetic domain wall velocity in its creep regime (10(6)-10(3) m s(-1)) by more than an order of magnitude. This significant change is due to electrical modulation of the energy barrier for the magnetic domain wall motion.

Electrical control of magnetic properties is crucial for device applications in the field of spintronics. Although the magnetic coercivity or anisotropy has been successfully controlled electrically in metals as well as in semiconductors, the electrical control of Curie temperature has been realized only in semiconductors at low temperature. Here, we demonstrate the room-temperature electrical control of the ferromagnetic phase transition in cobalt, one of the most representative transition-metal ferromagnets. Solid-state field effect devices consisting of a ultrathin cobalt film covered by a dielectric layer and a gate electrode were fabricated. We prove that the Curie temperature of cobalt can be changed by up to 12 K by applying a gate electric field of about ±2 MV cm(-1). The two-dimensionality of the cobalt film may be relevant to our observations. The demonstrated electric field effect in the ferromagnetic metal at room temperature is a significant step towards realizing future low-power magnetic applications.

Laminated Fe-Ta-N heads for Hi-Vision digital VTRs have been developed. The Fe-Ta-N film is suitable for use as a head core material in combination with high-coercivity media, because of its high saturation magnetization, soft magnetic properties, high Vickers hardness and corrosion resistance. The head showed a smaller pulse width, higher reproduced output and C/N, and superior recording/playback characteristics in the high frequency range above 20 MHz, compared with Sendust heads in the same design. Laminated Fe-Ta-N heads are expected to be suitable for high data transfer rate digital recording, such as Hi-Vision digital VTRs.

The spin transfer torque is essential for electrical magnetization switching. When a magnetic domain wall is driven by an electric current through an adiabatic spin torque, the theory predicts a threshold current even for a perfect wire without any extrinsic pinning. The experimental confirmation of this 'intrinsic pinning', however, has long been missing. Here, we give evidence that this intrinsic pinning determines the threshold, and thus that the adiabatic spin torque dominates the domain wall motion in a perpendicularly magnetized Co/Ni nanowire. The intrinsic nature manifests itself both in the field-independent threshold current and in the presence of its minimum on tuning the wire width. The demonstrated domain wall motion purely due to the adiabatic spin torque will serve to achieve robust operation and low energy consumption in spintronic devices.

It has become clear that the internal stress (?) of thermally treated sendust films changes depending on the treatment temperature, the thermal expansion coefficient (?) of the substrate, and the film oxygen content. The measured value for low-oxygen-content sendust film is 169×10?7. Values of calculated using the value of for Sendust film show good agreement with measured values.

We have developed a Co-Ni-Fe write head with a short yoke length for high-speed recording. By reducing the yoke length to 9.5 /spl mu/m, the eddy currents induced in a yoke with a relatively low resistivity (0.2 /spl mu//spl Omega/m) were reduced. The head of this short yoke had good write performance for a medium with a coercivity of 400 kA/m (5000 Oe) at frequencies up to 250 MHz (the overwrite less than -30 dB, and nonlinear transition shift less than 7%).

Since MRAM cells have unlimited write endurance, they can be used as substitutes for DRAMs or SRAMs. MRAMs in electronic appliances enhance their convenience and energy efficiency because data in MRAMs are nonvolatile and retained even in the power-off state. Therefore, 2 to 16Mb standalone MRAMs have been developed [1–4]. However, in terms of their random-access times, they are not enough fast (25ns) [1] as substitutes for all kinds of stand-alone DRAMs or SRAMs. To attain a standalone MRAM with both a fast random-access time and a large capacity, we adopt a cell structure with 2 transistors and 1 magnetic tunneling junction (2T1MTJ), which we previously published for a 1Mb embedded MRAM macro [5]. We need to develop circuit schemes to achieve a larger memory capacity and a higher cell-occupation ratio with small access-time degradation. We describe the circuit schemes of a 32Mb MRAM, which enable 63% cell occupation ratio and 12ns access time.

We have developed a focused ion beam trimmed write head using Al/sub 2/O/sub 3/ refilling, in which an Al/sub 2/O/sub 3/ film was deposited and a two-step lapping process was performed after FIB trimming at the air bearing surface. This process provided highly defined pole edges without any rounded corners, well-filled hollows, and a damage-free read element. Good environment-proof characteristics were also obtained, because a diamond-like carbon film was deposited as a protective layer on the element after FIB trimming. The fabricated head showed good overwrite characteristics in half-micron width writing.

We report the direct observation of the current-driven domain wall (DW) motion by magnetic force microscopy in a structured Co/Ni wire with perpendicular magnetic anisotropy. The wire has notches to define the DW position. It is demonstrated that single current pulses can precisely control the DW position from notch to notch with high DW velocity of 40 m/s., 12 pages, 3 figures

We propose a new MRAM cell that stores data in the form of the domain wall (DW) position. The DW is moved by the spin-polarized current that flows in the free layer. The cell was fabricated and the writing characteristics were investigated. A writing current of the cell was scalable, and the current density was reduced by using a new material. The cell is suitable for a high-speed MRAM that will compete with an eSRAM.

NiMn antiferromagnets with a high blocking temperature are attractive for application to the pinning layer in spin-valve heads, because a highly stable exchange coupling field is expected through their use. We have fabricated NiMn spin-valve heads having a CoZrTa bottom shield and a CoZrTa/NiFe write element, using an anisotropy control process which provides a desirable magnetization configuration in each magnetic layer. These heads showed highly stable, symmetrical, and Barkhausen noise free readback waveforms with output voltage of over 800 /spl mu/V/sub pp///spl mu/m.

A synthetic antiferromagnet (SAF) consisting of two ferromagnetic layers antiferromagnetically coupled is a key component for a magnetoresistive element. By using an SAF as a free layer of conventional MRAMs, the switching characteristics and scalability can be improved. Toggle MRAMs must use SAF free layers. When the SAFs that use a spacer layer of Ru are grown on the AlO-barrier layer of magnetic tunnel junctions (MTJs), the antiferromagnetic coupling (AFC) significantly deteriorates. Until now, a thin Ru layer( 300degC) annealing. This is a serious problem for the MgO-barrier MTJs that need high-temperature annealing. To solve these issues, we developed SAFs with an ultrathin seed layer and we used them in toggle MRAMs.

Toggling MRAM with four antiferromagnetically (AF) coupled ferromagnetic (FM) layers is proposed. The AF coupling strength between the inner FM layers should be larger than between the others. The four-FM-layer toggle cell shows not only good thermal stability but also a large write margin even with a small spin flop field. The advanced toggle cell has the potential to decrease write current by half compared with the basic two-FM-layer cell and can improve MRAM scalability.

The stress configuration of SAL (soft adjacent layer)-biased magnetoresistive (MR) elements has been analyzed, in order to study the stress-induced anisotropy effect on the MR transfer characteristics. The stress analysis, on the basis of stress measurement results in the single-layer sheet films that formed the MR elements, shows that anisotropic tensile stress of around 100 to 300 MPa is induced in the element height direction in the SAL film with an open-pattern edge structure. Furthermore, we calculated the MR transfer curves using a micromagnetic model for samples with different saturation magnetostriction constant (/spl lambda//sub s/) value for SAL films. Assuming anisotropic tensile stress of 300 MPa in the element height direction throughout the entire track region, a desirable /spl lambda//sub s/ value for SAL films is in the range from +1/spl times/10/sup -6/ to -2/spl times/10/sup -6/, because the stress-induced anisotropy in the SAL does not seriously affect the /spl mu/R transfer curves.

The effect of remanent magnetic flux density (B/sub r/) and magnetic layer thickness (/spl delta/) on offtrack noise characteristics in Co-Cr-Ta alloy thin films is investigated. It has been found that decreasing the value of B/sub r/ is a more effective method of improving offtrack noise characteristics than decreasing the value of /spl delta/. Offtrack noise is shown to be due to the fluctuations in the amplitude of readback waveforms, fluctuations which are caused by fluctuations in magnetic transition amplitude. It was observed, using a magnetic force microscope, that lower B/sub r/ media had sharp boundaries at the track edge regions and the magnetization was more uniform, compared with those of higher B/sub r/ media.

Laminated hard disk heads with FeTaN films, which have high saturation magnetic flux density of 16 kG, have been developed for high density magnetic recording. Here, read/write characteristics for FeTaN heads are evaluated and compared with the value for Sendust laminated heads. The FeTaN heads have better overwrite characteristics by 10 dB than Sendust heads. Even for 2400 Oe high coercivity media, the overwrite value is -32 dB. Output voltage for the FeTaN head is at least 1.2 times higher than that for a Sendust head. Popcorn noise and wiggle noise are low enough for practical use. FeTaN heads are shown to have a great potential to achieve high areal density recording. >

FeN phase generation has been observed by x‐ray analysis in FeTaN films, which normally show α‐Fe, TaN phase, and no FeN phases. With a rise in substrate temperature Ts above 200 °C, FeN phases are generated in nitrogen reactive sputtering process with columnar structure formation, and remain after annealing. With FeN phase generation, coercivity Hc and magnetostriction λ values increase markedly. Moreover, internal stress σ for the films increases drastically. The FeTaN films, sputtered on low temperature substrate, generate no FeN phases, and show low Hc, about 0.1 Oe, and low λ. Consequently, FeN phase generation with columnar structure formation has negative effects for FeTaN films to be used as magnetic head core materials. For preparation of FeTaN films, the substrate temperature has to be controlled to a sufficiently low value for suppression of FeN phase generation.

High-performance laminated sputtered Sendust (LSS) video heads with high abrasion resistance have been realized by using a newly developed CaO-TiO/sub 2/-NiO ceramic substrate. For acceptable head abrasion for practical use of 15 mu m, head life is more than 1000 h at more than 20 m/s relative tape-head speed. Low magnetostriction Sendust films provide high reproduce voltage for the head. The LSS head, with CaO-TiO/sub 2/-NiO substrate, has sufficient performance for use as high-definition television (HDTV) video tape recorder (VTR) heads. >

High‐nitrogen‐concentration FeTaN films have been newly developed as a high performance magnetic head core material. 10.5–14.5 (at. %) N, 8–13 Ta, and balance Fe composition films, prepared by a nitrogen reactive sputtering method followed by appropriate annealing (500–550 °C), have high saturation flux density (14–17 kG) and excellent soft magnetic properties: such as low coercivity (∼0.1 Oe), high relative permeability (3000 at 20 MHz), and small saturation magnetostriction ( < 0.5 × 10−6). Fine crystal structure is one of the most essential origins for the soft magnetism of FeTaN films. The FeTaN films are expected to realize recording density increases and rubbing noise reduction with high durability and high reliability due to their high Vickers hardness (1000) and their higher corrosion resistance than Sendust film.

In this study, we have investigated perpendicular magnetic anisotropy in a MgO/Co/Pt structure under an electric field. The change in the interface anisotropy energy in a MgO/Co stack upon the application of a gate voltage was determined by using a transport measurement technique. The result indicates that a decrease in the electron number at the surface of the Co layer leads to the enhancement of the perpendicular anisotropy energy at a temperature sufficiently lower than the Curie temperature.

In this study, we have investigated magnetic domain wall motion under gate electric fields in a perpendicularly magnetized Co/Pt wire. The motion under a transparent gate electrode was directly monitored using a magneto-optical Kerr effect microscope. The domain wall velocity was determined as a function of temperature and gate voltage. It was found that this velocity could be modulated by up to two orders of magnitude by changing the gate voltage.

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.

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.

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.

Spin wave attenuation in the layered left perpendicularFeNi/Ptright perpendicular(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).

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.