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.
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.
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.
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.Introduction An MRAM is a strong candidate for a high-speed non-volatile RAM due to its high speed random access, unlimited endurance, and compatibility with a scaled-down CMOS processes [1][2][3] . A 2T1MTJ cell was proposed for a high-speed MRAM macro in SoC operating at over 200 MHz [4] . Toggle MRAM makes it possible to enlarge the operating margins [3] , however, such field writing MRAMs have difficulties in scalability of the writing currents. Developing and adopting new magnetic technologies will be required for size reduction [5][6] . We propose a new MRAM cell for scalability of the writing current. The proposed cell has 2T1MTJ structure and it is suitable for a high-speed MRAM [4] .DW-seesaw Cell The proposed cell includes an MTJ with a U-shaped free layer ( Fig. 1a-b). The magnetization directions of the two end parts are fixed, and the two corners are DW trapping sites. A DW moves in the central part, and the DW positions are corresponded to the stored data. A tunnel barrier and a pinned magnetic layer are formed on the central part, and the DW position is detected by measuring the MTJ resistance. In writing, the fixed magnetization region acts as the source of the spin-polarized electrons. The electrons flow from one fixed magnetization region to the other through the DW moving region. The DW should move in line with the electron flow direction [7] . Its behavior is like a seesaw, and we call the cell a DW-seesaw. While a Race-track-memory deals with a number of DWs [8] , a single DW is treated in the DW-seesaw cell. Each fixed magnetization region is connected to a transistor (Fig. 1c). The circuitry of the cell (Fig. 1d) is the same as the 2T1MTJ cell in [4] and it suitable for high speed operation. This memory cell has some advantages. As the cell size is reduced, the writing current and time decrease. The current paths of the writing and reading operations are separated, and the writing current goes through only the free layer. Consequently, the tunnel barrier will be free from the voltage stress and the stored data will be free from the reading disturbance. This is a completely different point to a usual spin injection MRAM. Magnetic ConfigurationThe magnetic configurations of a 300-nm-wide, 10-nm-thick NiFe element were calculated using OOMMF [9] , a micro magnetic simulator. Figs. 2a and b depict the results for a 0-state and a 1-state, respectively. The magnetic configuration was directly observed by magnetic force microscopy (MFM) (Fig. 2c). The bright spots in the right corners indicated DWs. Moreover, we measured dc resistance of the left (R left ) and right (R right ) legs...
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