High-density Magnetoresistive Random Access Memory Research Articles

Electron Correlation Enhances Orbital Polarization at a Ferromagnetic Metal/Insulator Interface: Depth-Resolved X-ray Magnetic Circular Dichroism and First-Principles Study

The Fe(CoB)/MgO interface is vital to spintronics as it exhibits the tunneling magnetoresistance (TMR) effect and interfacial perpendicular magnetic anisotropy (PMA) simultaneously. To further enhance TMR and PMA for the development of high-density magnetoresistive random access memory, it is essential to clarify the behavior of Fe atoms interfaced with MgO. This study reveals that the spin and orbital magnetic moments of Fe are enhanced at the Fe/MgO interface. The enhancement in the orbital magnetic moment is much more significant than that predicted by the standard density functional theory. Theoretical calculations based on the orbital polarization correction reproduce this enhancement, the origin of which is attributed to the electron–electron correlation resulting from electron localization at the Fe/MgO interface. The present findings highlight the importance of electron–electron correlation at ferromagnet/oxide interfaces, which has often been disregarded in spintronics.

High-density magnetoresistive random access memory operating at ultralow voltage at room temperature

The main bottlenecks limiting the practical applications of current magnetoresistive random access memory (MRAM) technology are its low storage density and high writing energy consumption. Although a number of proposals have been reported for voltage-controlled memory device in recent years, none of them simultaneously satisfy the important device attributes: high storage capacity, low power consumption and room temperature operation. Here we present, using phase-field simulations, a simple and new pathway towards high-performance MRAMs that display significant improvements over existing MRAM technologies or proposed concepts. The proposed nanoscale MRAM device simultaneously exhibits ultrahigh storage capacity of up to 88 Gb inch−2, ultralow power dissipation as low as 0.16 fJ per bit and room temperature high-speed operation below 10 ns.

Trimmed-diamond shaped toggle magnetoresistive random access memory cells

We have performed micromagnetic simulations for the design of toggle magnetoresistive random access memory (MRAM) cells to make the operating field as low as possible while keeping a reasonable margin and thermal stability. The memory cells are composed of weakly coupled synthetic antiferromagnets. The cells are diamond-shaped to suppress the formation of edge domains, which increase the operating field. The adverse effect of the diamond shape making the remanent state too stable is prevented by trimming the sharp points. The optimization of the trimming allows us to reduce the operating field and offer a pathway for realizing high-density toggle MRAM.

Toggle switching of weakly coupled synthetic antiferromagnet for high-density magnetoresistive random access memory

The toggle-switching behavior for circular and elliptic cylinder shaped memory cells of weakly coupled synthetic antiferromagnet with a diameter and thickness (ferromagnetic layer) ranging from 200 to 400 and from 2.5to5.0nm, respectively, has been studied by micromagnetic simulation. The critical fields for a circular cylinder are much larger than those predicted by a single domain model. This is due to the formation of edge domains resulting in a so-called S state. The elliptic cylinder with an aspect ratio of >1.2 is found to be necessary to prevent the increase of the start field by the edge domains.

A Novel Magnetic Tunneling Junction Shaped Cell with Large Write Operation Margin for High-Density Magnetoresistive Random Access Memory

Magnetoresistive random access memory (MRAM) is one of several candidates for low-power and high-density system-on-chip (SoC) memory. In this paper, we propose a novel magnetic tunneling junction (MTJ) shape, called the “sandglass” (or “hourglass”), that has both an excellent asteroid curve for preventing write disturbance and a minimum memory of 8F2, where F is the feature size. A unique magnetic switching mechanism that works under decreasing magnetic switching field was compared with that of conventional MTJs simulated using the Landau–Lifshitz–Gilbert (LLG) simulator. We fabricated both a sandglass MTJ of 260×420 nm2 and a conventional elliptic MTJ of 200×400 nm2. The excellent asteroid curve of the sandglass MTJ is confirmed which exhibits both a larger write operation margin and a 50% lower switching field than those of the conventional elliptic MTJ. The sandglass MTJ cell is a promising candidate for realizing high-density MRAM.

Switching and reliability issues of magnetic tunnel junctions for high-density memory device

Abstract Magnetoresistive random access memory (MRAM) is one of the promising universal memories, which combines the high speed of static RAM, high-density of dynamic RAM, and non-volatility of FLASH memory. However, MRAM faces several challenges prior to the appearance at the commercial market on a large scale. The most important challenge among them will be an issue, related to the magnetic switching. The conventional MRAM structures and writing methods cause several issues to be overcome for high packing density without cross-talk. Furthermore, small ferromagnetic elements will require high magnetic field, which will increase the power consumption of the devices. This article will review new writing schemes, such as thermal-assisted switching and spin-transfer switching methods. In addition, reliability characteristics and thermal stability of magnetic tunnel junctions will be presented, because ensuring the reliability of magnetic tunnel junctions emerges as an another challenging problem for the successful application of the new writing schemes to the high-density MRAM devices in the next generation.

Switching characteristics of submicrometer magnetic tunnel junction devices with perpendicular anisotropy

As the pattern size of magnetic tunnel junctions (MTJs) becomes smaller, the vortex of magnetization and fluctuations of switching fields caused by the shape MTJ cells will cause serious writing problems. However, a MTJ structure with perpendicular anisotropy (pMTJ) has shown low saturation magnetization and shape independence during the writing process. In this study, we considered Co∕Pd multilayers that allow better tailor-design of magnetization and anisotropy. A series of calculations based on the Landau–Lifschitz–Gilbert equation were carried out on pMTJs to investigate the effect of size variations and material properties on their transfer behaviors. In a same submicrometer cell size, pMTJ shows better properties for high-density magnetoresistive random access memory (MRAM) against synthetic antiferromagnet MTJs. As the number of bilayer Co∕Pd,n, becomes larger, the coercivity and squareness were enhanced because of shape anisotropy and large effective Ku. Even with a small field, 10 Oe, along the hard axis, the drastic decrease, nearly about 80% of switching field, occurs. As a micromagnetic result, a pMTJ with a cell size of 200 nm and n=3 shows an abrupt increase in coercivity and better squareness.

230% room-temperature magnetoresistance in CoFeB∕MgO∕CoFeB magnetic tunnel junctions

Magnetoresistance (MR) ratio up to 230% at room temperature (294% at 20 K) has been observed in spin-valve-type magnetic tunnel junctions (MTJs) using MgO tunnel barrier layer fabricated on thermally oxidized Si substrates. We found that such a high MR ratio can be obtained when the MgO barrier layer was sandwiched with amorphous CoFeB ferromagnetic electrodes. Microstructure analysis revealed that the MgO layer with (001) fiber texture was realized when the MgO layer was grown on amorphous CoFeB rather than on polycrystalline CoFe. Since there have been no theoretical studies on the MTJs with a crystalline tunnel barrier and amorphous electrodes, the detailed mechanism of the huge tunneling MR effect observed in this study is not clear at the present stage. Nevertheless, the present work is of paramount importance in realizing high-density magnetoresistive random access memory and read head for ultra high-density hard-disk drives into practical use.

Current aspects and future perspectives of high-density MRAM

Magnetoresistive random access memory (MRAM) is regarded as one of the leading candidates for universal memory [1] that will be commercialized in the foreseeable future. The figure of merits for MRAM technology includes non-volatility, high speed, high density, radiation hardness, and unlimited endurance. Very promising R & D results have been announced wordlwide. For a high density MRAM as a stand-alone memory, uniform resistance and switching control for sub-micron and deep sub-micron devices must be guaranteed. To achieve this goal, several material and device issues related with MRAM core cell should be resolved. Resistance (R) as well as magnetoresistance (MR) are limited by the uniformity of barrier thickness induced by bottom electrode. Switching issues appear controllable with a choice of appropriate shape and fine patterning process.

A novel process for highly manufacturable MRAM

We have designed new process architecture for high-density magneto-resistive random access memory (MRAM). One of the major issues in fabricating high-density MRAM is how to prevent the electrical short through the tunnel barrier without any kind of degradation of the characteristics of small magnetic tunnel junction (MTJ) devices. We have developed a novel MTJ patterning technique that involves removing the residual free layer by wet-chemical treatments after partial RIE process. Additional degradation of MTJ devices in post-patterning should be minimized.

Magnetic switching in 100 nm patterned pseudo spin valves

Progress in developing operative high-density magnetoresistive random access memory (MRAM) devices relies critically on tailoring the magnetic switching occurring in arrays of small patterned pseudo spin valve (PSV) elements. Co/Cu/NiFe PSV films, produced by sputtering in the presence of a magnetic field, have an in-plane anisotropy and switching fields of typically 10 Oe for the soft NiFe and 40 Oe for the hard Co. These films were patterned into arrays of elliptical and circular elements with dimensions of 80 nm to 10 /spl mu/m. The layered structure in these large-area arrays of compositionally modulated PSV elements is preserved through the patterning processes. Hysteresis measurements of the dot arrays show that the switching field of the hard layer increases significantly with decreasing element size, reaching 600 Oe for the smallest elements. Additionally, in patterned elements the soft layer switches prior to field reversal due to magnetostatic coupling between the layers, leading to antiparallel alignment at remanence.