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 (H x , H y ) 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.
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.
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.
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
We have developed a new magnetic random access memory with current-induced domain wall (DW) motion (DW-motion MRAM). We confirmed its potential of 0.1-mA and 2-ns writing with sufficient thermal ...stability. The obtained properties indicate that this MRAM can replace conventional high-speed embedded memories.
We studied a scaling property of a three-terminal domain wall (DW)-motion device, which is one of the promising candidates for future low-power nonvolatile memory and logic-in-memory architecture. ...Using several assumptions, we derived the scaling factor of the switching current, switching time, resistance of the write-current path, and data storage stability. We also quantitatively evaluated the variation of these parameters with the device size. It was found that the switching current and time decrease almost linearly with the device size, while the variation of the resistance of the write-current path is negligible. The switching current and time for 32-nm-wide device are less than 100 μA and 2 ns, respectively. A required critical field which assures a sufficient thermal stability of stored data was calculated for each generation. Furthermore, future issues and intrinsic limiter for the size reduction were discussed.
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.
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 mu m, the eddy currents induced in a yoke with a relatively low ...resistivity (0.2 mu 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%).
The thermal asperity of the tunneling magnetoresistive (TMR) head was studied using Zip-type flexible media, and compared with that of the anisotropic magnetoresistive (AMR) head. The examined head ...had a shielded structure with the TMR element close to the air bearing surface. Nevertheless, it generated relatively small thermal asperity even when the mechanical spacing between the head and medium was less than 20 nm, at which large and frequent thermal asperity was observed when the AMR head was used. Such relatively small thermal asperity of the TMR head Is attributed mainly to the small low-temperature coefficient of the electrical resistivity of the TMR element.
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