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Fully oxidized poly(1-(N,N-dimethylamino)pyrrole)s (PDMAPyn+), which are synthesized through doping with anions with the same number of polymer units as the original pyrrole, have been proposed as candidates for molecular flat-band magnets from the theoretical calculations of Aoki and coworkers (Phys. Rev. B: Condens. Matter Mater. Phys., 2003, 68, 140403; Phys. Rev. B: Condens. Matter Mater. Phys., 2010, 82, 235127). Here we describe the synthesis of neutral forms of oligo(1-(N,N-dimethylamino)pyrrole)s (ODMAPys) from the dimer (2) to the dodecamer (12). The crystal structures of 2–6 (i.e., up to the hexamer) were determined by single crystal X-ray analysis. The chemical and electrochemical formation of their oxidized forms is also reported. Electrochemical measurements of the ODMAPys indicated that oligomers 4, 6, and 8 underwent 2e− oxidation, whereas 10 and 12 showed quasi-reversible 4e− oxidation. The oxidized forms maintained considerable chemical stability; 42+, 62+, 82+, 104+, and 124+ were isolated as SbCl6− salts. The neutral forms were colorless (λmax = 240–270 nm), whereas the oxidized forms exhibited strong blue color with λmax = 670–950 nm. EPR spectroscopy indicated that the dications 42+, 62+, and 82+ in the solid salts were intrinsically diamagnetic, but the tetracation of 104+ was paramagnetic at 4.4 K, which was supported by magnetic measurements. These physical properties of the ODMAPys are different from those common to polypyrroles, but consistent with their predicted unique electronic structures.

This study aims to investigate the creep motion of a magnetic domain wall (DW) induced by spin-Hall torque (SHT) in a Ta/Co–Ni/Pt hetero-structured thin wire using photoemission electron microscopy. The displacement of the DW was recorded every time after the duration of an electric current pulse. Results revealed that the existence of pinning centers and the inhomogeneity of the thermal diffusion property of the wire may result in an unstable DW motion. On the contrary, this data also suggested that the SHT induced from both the Ta and Pt layers can significantly improve the DW velocity, which was found to be three orders of magnitude larger than that previously reported for the MgO/Co–Ni/Pt system (Taniguchi et al., Appl. Phys. Exp. 7, 053005 (2014)).

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.

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.

Structural and magnetic properties of magnetic clad lines used for magnetoresistive random access memory (MRAM) devices were analyzed to optimize the clad-line fabrication process. Patterns of 520 nm lines and 280 nm spaces with a depth of 300 nm with magnetic cladding films of Ta/NiFe/Ta were fabricated to characterize the structure and magnetic properties of cladding films by transmission electron microscopy and magnetic measurements. A high susceptibility with a small hysteresis was obtained when a NiFe cladding film highly oriented to fcc (111) was formed perpendicular to the sidewall depth direction. The optimized cladding film was integrated in MRAM cells and we confirmed that the clad lines reduced the flop current of a toggle-MRAM cell by more than 50% compared with a cell having unclad lines. The flop current distribution can be reduced by applying a magnetic field of 12 kOe in the direction of word lines at room temperature. We successfully confirmed writing operation in a 4-Mbit MRAM cell with these optimized magnetic clad lines.

Embedded magnetoresistive random access memory (MRAM) with multi-level interconnects necessitates that magnetic tunnel junction (MTJ) devices have a thermal stability of 350 °C or higher during fabrication. We have improved thermal stability of MRAM devices using SiN protective film deposited by high-density plasma chemical vapor deposition (HDP-CVD) at 200 °C. The MTJ devices with HDP-CVD SiN protective film did not degrade after post-annealing at 350 °C, which suggests that the HDP-CVD process reduced oxide metal on the etched surface of the MTJ devices and that the SiN film blocked H2O diffusion from the interlayer dielectric film during post-annealing at 350 °C. We also fabricated a 1-kbit MRAM array and experimentally demonstrated thermal stability at 350 °C.

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.

Reactive and anisotropic etching of magnetic tunnel junction (MTJ) stacked films has been achieved using pulse-time-modulated (TM) plasma. While corrosion and delamination of MTJs are observed in continuous wave discharge plasma, a chlorine pulse-time-modulated plasma achieved a high MTJ etching rate without corrosion or delamination. The authors think that the negative ions enhance the chemical reactions on the surface of magnetic films. The magnetic characteristics are also significantly improved by using TM plasma because of reduced residues and improved tapered profiles. Accordingly, TM plasma etching is a promising candidate for high-rate and damage-free MTJ etching for magnetoresistive random access memory devices.

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.

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.

We examined the magnetic and writing properties of clad writing lines for magnetic random access memory (MRAM) devices. After fabricating them, we applied a magnetic field of 1.2 T to them at room temperature. We call this operation initialization. Initialization reduced the flop current deviation of the single magnetic tunnel junction toggle cells. We fabricated a 4-Mbit toggle memory array with the clad lines, using initialization. It exhibited a large operating margin and a reduced switching current.

Crystallization processes and structures were analyzed, by using modified dynamical reflectance spectroscopy, x-ray diffraction measurement, and transmission electron microscopy for a germanium–antimony–tellurium (Ge–Sb–Te) ternary amorphous film on different underlayer materials: a silicon nitride (Si3N4) film and a zinc sulfide–20 mol % silicon dioxide (ZnS–SiO2) film. The crystallization temperature was almost the same (about 430 K) for each sample. Above the crystallization temperature, the Ge–Sb–Te film on the Si3N4 underlayer film exhibited a face-centered-cubic (fcc) structure and grains grew gradually with transformation to a hexagonal structure, whereas the film on the ZnS–SiO2 underlayer film exhibited mixed phases of the fcc and hexagonal structure and abrupt grain growth at 520 K. These crystallization differences are thought to be caused by the interfacial atomic arrangement of amorphous Ge–Sb–Te. This arrangement depends on the chemical affinity at the interface.

Crystallization processes were studied for germanium–antimony–tellurium (Ge–Sb–Te) ternary amorphous thin film as a single layer or sandwiched between various dielectric films, such as silicon dioxide (SiO2), siricon nitride (Si3N4), tantalum oxide (Ta2O5), zinc sulfide (ZnS), and ZnS–20 mol % SiO2. The processes were analyzed quantitatively, based on transmittance changes in Ge–Sb–Te films heated either exothermally or isothermally. Both Kissinger equation and Johnson–Mehl–Avrami kinetic analysis were adopted to estimate activation energy and the reaction order of the processes. Ge–Sb–Te single‐layer amorphous film crystallized in two stages, nucleation and crystal growth. These two processes can be distinguished by exothermal crystallization patterns. By sandwiching this film into dielectric films, crystallization activation energy increases and the nucleation processes are affected. The Si3N4 and Ta2O5 dielectric films accelerate the nucleation, while the SiO2 films inhibit it, and the ZnS and ZnS–20 m...

The domain wall (DW) velocity above the Walker field drops abruptly with increasing magnetic field, because of the so-called Walker breakdown, where the DW moves with a precessional mode. On applying the higher field, the DW velocity again starts to increase gradually. We report the DW propagation around this local minimum regime in detail, investigated through the time-resolved electrical detection technique, with a magnetic tunnel junction. Just above the Walker field, we succeeded in detecting the precessional motion of the DW in a real-time regime, while a different mode appeared around the local minimum of the DW velocity.

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.

A variety of dibenzosiloles and -germoles as well as dithiophenosilole (VII) are prepared, better tolerating the functional groups as the methods applied so far.

A variety of dibenzosiloles and -germoles as well as dithiophenosilole (VII) are prepared, better tolerating the functional groups as the methods applied so far.

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 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.

The temperature and time dependencies of magnetic anisotropies for NiFe layers pinned by two typical antiferromagnetic films, NiO and FeMn, were studied to understand the stability of unidirectional anisotropy. Torque curves were measured isothermally after application of a magnetic field from a direction perpendicular to the unidirectional anisotropy for certain time periods. Applying the field caused the torque curve to shift toward the field direction and its amplitude to decrease. The amount of shift and the normalized decrease in the torque amplitude were proportional to the logarithmic time of the applied perpendicular field. The changes observed in the NiO/NiFe film are twice as large as those in the NiFe/FeMn film at the same holding time and temperature. The external magnetic field caused rotation in the direction of the unidirectional anisotropy and a reduction in the exchange coupling strength. These changes may be explained by the thermal fluctuation aftereffect model. The unidirectional anisotropy in the NiO/NiFe film was less stable compared with that in the NiFe/FeMn film.

We fabricated toggle magnetic random access memories with clad writing lines. First, we evaluated the structures and magnetic properties of sputter-deposited cladding layers. The substrate bias during the deposition affected not only the sidewall coverage, but also the crystallinity and magnetic properties of the cladding. The optimized clad lines reduced the writing current to as low as 50% of that of unclad lines. Moreover, the writing current deviation divided by the average current of magnetic tunnel junction cells with clad lines was as low as that with unclad lines. Using the optimized clad lines, we constructed memory arrays with a large operating margin and reduced switching current

The behavior of a three-terminal domain wall (DW) device with a perpendicularly magnetized CoFeB free layer and underlying hard magnets was investigated. In a Ta/CoFeB/MgO free layer without hard magnets, a current-induced DW motion in the direction of electron flow was observed. In a device having a hard magnet under each end of the free layer, we found that a DW nucleated by injecting current played an important role in the switching of magnetization. We concluded that the switching of magnetization in our device is due to the displacement in the direction of electron flow of the DW created by current. After deriving the principle of operation through experiments, we describe a way to reduce the current required for writing by increasing the thickness of the hard magnets.

Thickness dependence of current-induced domain wall (DW) motion in a perpendicularly magnetized [Co/Ni]N multilayered wire containing Ta/Pt capping and Pt/Ta seed layers has been studied. The thickness of the magnetic layer was controlled by the stacking number, N. The threshold current density for driving DW had a local minimum at N = 3 and the velocity of DW motion decreased with N. Estimation of carrier spin polarization from measurements of DW velocity revealed that a thinner Co/Ni stack adjacent to the Pt layers reduced the carrier spin polarization and the strength of adiabatic spin transfer torque.

A single vortex-core switching induced by current injection into a ferromagnetic disk is detected in real time using a three-terminal device with the tunneling magnetoresistance junction. The real time observation reveals that the electrical core switching is completed within a few 100 ps. It is also found that the core switching takes place at the specific positions inside the disk.

The effect of inserting a Pt layer into a Co/Ni multilayered film with a perpendicular magnetization in terms of current-induced magnetic domain wall (DW) motion has been investigated. We found that the current density required to obtain DW motion was approximately 1.9 times greater in Co/Ni/Pt wire than in Co/Ni wire, in spite of the Co/Ni and Co/Ni/Pt multilayered films having almost identical magnetic properties. Estimation of spin polarization from measurements of DW velocity indicated that the decreased spin polarization caused by insertion of the Pt layers accounts for the increased current density required for DW motion in Co/Ni/Pt wire.

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 single vortex-core switching in a ferromagnetic disk is detected in real time by using a three-terminal device with the tunneling magnetoresistance junction. We show that the device works as a vortex core memory cell, where reading and writing can be done in an all-electrical way: binary data corresponding to the core direction can be read out electrically as the amplitude of the output, while the data can be written electrically by applying a pulsed current.

Current-induced magnetic domain wall (DW) displacement in a Co/Ni nano-wire with perpendicular magnetic anisotropy was investigated in real space by photoemission electron microscopy (PEEM) for the first time. DW velocity determined from the PEEM observation was 40 m s(-1) for the current density of 2.5 × 10(12) A m(-2), which was consistent with the result obtained by the electrical measurement used in our previous reports.

Magnetic domain wall (DW) motion induced by currents in a perpendicularly magnetized Co/Ni nanowire has been investigated both at room temperature and low temperature. The threshold current density was almost independent of temperature, although the external magnetic field to depin a DW was doubled at low temperature. The result is consistent with the theory based on the adiabatic spin transfer model.

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.

The relevance between electrical properties of CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs) and the crystalline orientation of MgO layer was investigated within a large-scale substrate. The tunnel magnetoresistance characteristics and the resistance-area product in CoFeB/MgO/CoFeB MTJs are degraded by deterioration of the crystalline orientation of MgO layer, which depends on sputtering conditions. That deterioration originates from damage caused by high-energy oxygen negative-ion irradiation during RF-magnetron sputtering of the MgO layer.

This paper describes experimental results obtained from measuring the dependence of device temperature on the current for domain wall motion in a Co/Ni wire having perpendicular magnetic anisotropy. Devices with different insulating layer thicknesses were prepared in order to control the device temperature. A stable domain wall motion was observed up to the temperature at which perpendicular magnetic anisotropy vanishes. Moreover, the current required for domain wall motion was independent of the device temperature.

The authors have investigated magnetic domain wall motion induced by electric currents in ferromagnetic nano-wires made of Co/Ni multilayers. The thicknesses of Co and Ni layers were changed, whereas the numbers of layer stacks of Co and Ni were the same in all samples. The sample with thinner total Co/Ni thickness showed the lower threshold current density for the domain wall motion as an overall trend, which is qualitatively in agreement with the expectation by the theory based on the adiabatic spin-transfer model. The lowest threshold current density was 2.9×1011 A/m2 obtained in the sample with the total Co/Ni thickness of 3.4 nm and the wire width of 110 nm.

The relation between critical current, critical field, and thermal stability of domain wall (DW) trapped at step pinning sites was studied using Co/Ni nanowires with perpendicular magnetic anisotropy (PMA). A sharp step structure was fabricated, which increased the critical field of DW motion, while the critical current was independent of the critical field. Also, the derived thermal stability (ΔE/kBT) for the step samples was much more than 60. These results indicate that the DW motion in PMA nanowires has potential for memory devices with both a small driving current and large thermal stability.

The point contact Andreev reflection technique was employed to estimate the spin polarization of currents in Co/Pt multilayered thin film and Co–Pt alloy film with perpendicular anisotropy. The spin polarization of currents in the Co–Pt alloy film was estimated to be 0.48, while that in Co/Pt multilayered film was 0.56. These results suggest that the spin-polarization of currents in multilayered film is enhanced compared to that in alloy film.

All-electrical control and local detection of multiple magnetic domain walls in perpendicularly magnetized Co/Ni nano-wires were demonstrated. A series of domain walls was reproducibly shifted in the same direction by the current, keeping the distance between the walls almost the same. Furthermore, the walls can be shifted back and forth depending on the direction of the pulsed currents.

Overwrite characteristics have been studied for Ge-Sb-Te phase change media under high linear velocity conditions, ranging from 11.3 m/s to 22.6 m/s, with mark-edge-recording (MER). The Ge 1 Sb 4 Te 7 recording layer composition was chosen for the rapid cooling structure to obtain a sufficient erase ratio under high linear velocity conditions. The optical optimization for the disk structure and the narrow-grooved substrate have been applied to improve erase characteristics with MER. The optical phase-difference-reproduction (PDR) has been studied to realize a high carrier to noise ratio (C/N). The pulse-width-reduction (PWR) recording compensation has been developed for high recording density. A 44.8 dB C/N was obtained for 0.68 micrometers minimum recording mark length. A -26.8 dB erase ratio was obtained at 22.6 m/s linear velocity.