Norikazu Ohshima

Crystallization of germanium–antimony–tellurium amorphous thin film sandwiched between various dielectric protective films

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...

Micromagnetic analysis of current driven domain wall motion in nanostrips with perpendicular magnetic anisotropy

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.

Control of Multiple Magnetic Domain Walls by Current in a Co/Ni Nano-Wire

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.

Domain Wall Motion Induced by Electric Current in a Perpendicularly Magnetized Co/Ni Nano-Wire

The authors show experimental results on domain wall motion induced by electric current in a Co/Ni nano-wire with perpendicular magnetic anisotropy. The motion was detected electrically by using the anomalous Hall effect. Threshold current density for the domain wall motion was found to decrease with decreasing the wire width, where the minimum threshold current density of approximately 5×1011 A/m2 was observed for the wire width of 70 nm.

A 90nm 12ns 32Mb 2T1MTJ MRAM

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.

Control of Domain Wall Position by Electrical Current in Structured Co/Ni Wire with Perpendicular Magnetic Anisotropy

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.

Current-Driven Domain Wall Motion in CoCrPt Wires with Perpendicular Magnetic Anisotropy

We report the direct observation of current-driven domain wall (DW) motion in a CoCrPt wire with perpendicular magnetic anisotropy. Magnetic force microscopy showed that a single DW introduced in the wire is displaced back and forth by positive and negative pulsed current. This is the first demonstration of the current-driven DW motion in a metallic magnetic wire with perpendicular magnetic anisotropy in the absence of a magnetic field.

Magnetic field insensitivity of magnetic domain wall velocity induced by electrical current in Co/Ni nanowire

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.

Relation between critical current of domain wall motion and wire dimension in perpendicularly magnetized Co/Ni nanowires

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.

Analysis of current-driven domain wall motion from pinning sites in nanostrips with perpendicular magnetic anisotropy

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 t...