High-density Storage Applications Research Articles

Design and Simulation Analysis of a 3TnC MLC FeRAM Using a Nondestructive Readout and Offset-Canceled Sense Amplifier for High-Density Storage Applications.

Hf0.5Zr0.5O2-based multi-level cell (MLC) ferroelectric random-access memory (FeRAM) has great potential for high-density storage applications. However, it is usually limited by the issues of a small operation margin and a large input offset. The study of circuit design and optimization for MLC FeRAM is necessary to solve these problems. In this work, we propose and simulate a configuration for a Hf0.5Zr0.5O2-based 3TnC MLC FeRAM macro circuit, which also presents a high area efficiency of 12F2 for each bit. Eight polarization states can be distinguished in a single fabricated Hf0.5Zr0.5O2-based memory device for potential MLC application, which is also simulated by a SPICE model for the subsequent circuit design. Therein, a nondestructive readout approach is adopted to expand the reading margin to 450 mV between adjacent storage levels, while a capacitorless offset-canceled sense amplifier (SA) is designed to reduce the offset voltage to 20 mV, which improves the readout reliability of multi-level states. Finally, a 4 Mb MLC FeRAM macro is simulated and verified using a GSMC 130 nm CMOS process. This study provides the foundation of circuit design for the practical fabrication of a Hf0.5Zr0.5O2-based MLC FeRAM chip in the future, which also suggests its potential for high-density storage applications.

Enhanced Resistive Switching Performance through Air-Stable Cu2AgSbI6 Thin Films for Flexible and Multilevel Storage Application.

Herein, the lead-free halide perovskite films with different Cu-to-Ag ratios (Cu3-xAgxSbI6, x = 0, 1, 2, or 3) have been prepared by a spin-coating method at low temperature. The enhanced resistive switching (RS) performance of more uniform SET/RESET voltages and the endurance up to at least 1600 cycles are found in the RS memory with a device structure of Ag/PMMA/Cu2AgSbI6/ITO. The device performance is not degraded under different bending angles and after 103 bending cycles, which is beneficial for flexible memory applications. The appropriately increased activation energy of the perovskites with the partial substitution of Ag atoms, which would lead to a more robust filament formed, is proposed to explain the enhanced RS mechanism. Importantly, the effective size and number of filaments measured by conductive AFM are introduced to confirm the multilevel storage effect of Cu2AgSbI6. The multilevel storage characteristics with four resistance levels are demonstrated by various compliance currents. Moreover, the Cu2AgSbI6 memory devices still exhibit enhanced RS properties and multilevel storage after 75 days of exposure to ambient conditions. Our study provides a strategy for improving the stability and high-density storage applications of halide perovskite RS memory devices.

Current-induced magnetization switching in Pt/Co/W and Pt/Co/W0.82Pt0.18 structures with perpendicular magnetic anisotropy

Magnetization switching caused by current-generated spin-orbit torques in heavy metal/ferromagnetic metal/heavy metal heterostructures has become a significant issue due to its excellent performance in low-power and high-density storage applications. In our work, based on Pt/Co/W and Pt/Co/W0.82Pt0.18 structures with perpendicular magnetic anisotropy (PMA), the effective spin Hall angles () of the Pt/Co/W and Pt/Co/W0.82Pt0.18 structures with different thicknesses of W and W0.82Pt0.18 alloy were determined by the harmonic measurements. We observed that the spin Hall angles were related to the thickness of the top-heavy metal layer. The magnetization switching efficiency is improved by the combined action of the bottom Pt and the top W due to the opposite signs of the spin Hall angles for the two heavy metals. When the thickness of the top-heavy metal layer reaches 3 nm, both Pt/Co/W and Pt/Co/W0.82Pt0.18 show their maximum effective spin Hall angles of 0.244 ± 0.008 and 0.223 ± 0.007, respectively. Nevertheless, the effective spin Hall angles were decreased with the thickness beyond 4 nm, which can be explained by the phase of W becomes a pure α-W from the mixed-phase of α-W and β-W. It indicates that the spin Hall angles of β-W were larger than α-W. Our results confirm an effective method to enhance the spin Hall angles and decrease the switching current density.

Decorated Tetrathiafulvalene‐Based Ligands: Powerful Chemical Tools for the Design of Single‐Molecule Magnets

This Minireview covers the design and characterization of coordination lanthanide complexes involving TTF‐based ligands. The specific design of TTF‐based ligands allowed the isolation of complexes with magnetic properties such as Single‐Molecule Magnets (SMMs) behavior and the studies of magnetic modulations due to supramolecular interaction, molecular engineering, magnetic dilution as well as isotopic enrichment. A careful design leads to TTF‐based ligands displaying several coordination sites in order to rationally elaborate polynuclear systems with multi‐SMM behavior or to auto‐assembly SMMs. Their redox activity allowed the investigation of coordination lanthanide complexes in several oxidation states and their consequences on optical and magnetic properties. The complete experimental and theoretical studies of such systems contributed to the understanding of the magnetic properties of lanthanide ions for futures applications in high density storage and quantum computing.

High-Quality Hexagonal Nonlayered CdS Nanoplatelets for Low-Threshold Whispering-Gallery-Mode Lasing.

Low threshold micro/nanolasers have attracted extensive attention for wide applications in high-density storage and optical communication. However, constrained by quantum efficiency and crystalline quality, conventional semiconductor small-sized lasers are still subjected to a high lasing threshold. In this work, a low-threshold planar laser based on high-quality single-crystalline hexagonal CdS nanoplatelets (NPLs) using a self-limited epitaxial growth method is demonstrated. The as-grown CdS NPLs show multiple whispering-gallery-mode lasing at room temperature with a threshold of ≈0.6 µJ cm-2 , which is the lowest value among reported CdS-based lasers. Through power-dependent lasing studies at 77 K, the lasing action is demonstrated to originate from a exciton-exciton scattering process. Furthermore, the edge length- and thickness-dependent lasing threshold studies reveal that the threshold is inversely proportional to the second power of lateral edge length while partially affected by vertical thickness, and the lasing modes can be sustained in NPLs as thin as 60 nm. The lowest threshold emerges with the thickness of ≈110 nm due to stronger energy confinement in the vertical Fabry-Pérot cavity. The results not only open up a new avenue to fabricate nonlayered material-based coherent light sources, but also advocate the promise of nonlayered semiconductor materials for the development of novel optoelectronic devices.

An overview of resistive random access memory based high-density crossbar array

In the global era of knowledge economy and electronic information, semiconductor memories play a crucial role in the storage of the massive information. In order to achieve a higher integration density, the microelectronics process node is pushed forward to the next generation according to the Moores Law. However, the current mainstream nonvolatile memory technology based on charge storage, such as flash memory, is rapidly running into its physical limit due to the tradeoffs between the high speed, long time retention and low power operation. Therefore, several new types of nonvolatile memories based on other storage concepts have been intensively investigated to replace Flash. Resistive random access memory (RRAM) device, which is based on resistance change modulated by electrical stimulus, has been considered as one of the most promising candidate for next-generation nonvolatile memory due to its potential advantages for simple structure, fast switching speed, excellent scalability, three-dimensional (3D) stackable integration, and good compatibility with the current complementary metal oxide semiconductor (CMOS) technology. However, a crossbar array consisting of only RRAM cell suffers unavoidable cross-talk interference due to leakage current paths through neighboring unselected cells with low resistances, leading to a misreading problem, the biggest hindrance for the high-density memory application. It can be effectively tackled by the addition of necessary nonlinearity to the RRAM by integrating a highly non- linear and bidirectional selector device. In this paper, we give an overview on the familiar architectures to diminish the sneak current in crossbar array, including 1D1R (one diode one resistor), 1S1R (one selector one resistor), 1CRS1R (one CRS device one resistor, complementary resistive switch (CRS) devices forming by two back-to-back connected memory cells) and 1R (one resistor) with self-rectifying effect. In the meantime, we discuss the research trends and the challenges of high-density storage based on RRAM passive cross-array. Finally, future research direction and prospects, as well as main challenges awaiting RRAM high-density memory are given. The application of diodes with large forward current density, high rectification ratio, low preparation temperature and easy 3D integration are the key to achieve high density in 1D1R structure integration. It is of great significance to find materials with better performance and applicability, and to carry out theoretical research on the integrated applications of 1S1R and 1CRS1R structures. Since the 1R with self-rectifying effect is simpler and more cost-effective than other integrated structures, further research to clarify intrinsic physical mechanism of 1R, and develop high-performance self-rectifying RRAM devices with large rectification ratio, high current density, uniformity, stability, and reliability are the significant processes to the achievement of RRAM high-density storage applications.

Evidence of Magnetic Inversion in Single Ni Nanoparticles.

Superparamagnetism is an unwanted property of small magnetic particles where the magnetization of the particle flips randomly in time, due to thermal noise. There has been an increased attention in the properties of superparamagnetic particles recently, because of their potential applications in high density storage and medicine. In electron transport through single nanometer scale magnetic particles, the current can also cause the magnetization to flip randomly in time, even at low temperature. Here we show experimental evidence that when the current is then reduced towards zero in the applied magnetic field, the magnetization can reliably freeze about a higher anisotropy-energy minimum, where it tends to be inverted with respect to the magnetic field direction. Specifically, we use spin-unpolarized tunneling spectroscopy of discrete levels in single Ni particles 2–4 nm in diameter at mK-temperature, and find that the the magnetic excitation energy at the onset of current decreases when the magnetic field increases, reaching near degeneracy at nonzero magnetic field. We discuss the potential for spintronic applications such as current induced magnetization switching without any spin-polarized leads.

Magneto-optical Kerr effect in ZnTMO2 (TM=Cr, Mn, Fe, Co and Ni)

First principles generalized gradient full potential density-functional calculations were performed to predict the optical and magneto-optical (MO) properties of the chalcopyrite compounds ZnTMO2, TM=Cr, Mn, Fe, Co and Ni. Detailed investigation of the electronic band structure and density of states is reported. The optical properties in the 0–8eV energy range are analyzed in terms of band structure transitions. As for the magneto–optical properties, our results show that the studied compounds have peaks in the Kerr rotation ranging from infrared to ultraviolet radiation, with ZnFeO2 having the highest Kerr rotation angle of 10° and −8.46° at 0.35eV and 4.59eV, respectively. The peaks in the Kerr spectra were assigned to the optical and magneto–optic contributions. Our calculated function of merit for these compounds indicates that these compounds might be useful for technological application in high density storage.

First-principles study on surface reconstruction and magnetic phase stability of an FePt3 film on a Pt(110) substrate

The FePt3 alloy is one of the most investigated materials for high density storage applications due to its rich variety of magnetic structures which transform sensitively depending on change in its local structure. Here, we present the ab-initio total energy and electronic structure calculations within the framework of density functional theory for an FePt3 film of 0.5 nm in thickness on a Pt (110) surface. The results show that a missing-row surface reconstruction along the \([1\overline 1 0]\) direction is energetically more stable over the unreconstructed clean surface, which is attributed to the energy gain by the spill out of p-electron charge to the large facet area from Pt atoms at the second and third atomic layers. The missing-row reconstruction is found to enhance the stability of the ferromagnetic phase over the antiferromagnetic bulk ground-state phase and to induce possible concurrence of a meta-stable atomic structure with an in-plane anti-phase boundary along the orientation of missing-row in addition to the conventional L12 surface, implying the observation of various magnetic phases.

Coexistence of diode-like volatile and multilevel nonvolatile resistive switching in a ZrO2/TiO2 stack structure

Diode-like volatile resistive switching as well as nonvolatile resistive switching behaviors in a Cu/ZrO2/TiO2/Ti stack are investigated. Depending on the current compliance during the electroforming process, either volatile resistive switching or nonvolatile resistive switching is observed. With a lower current compliance (<10 μA), the Cu/ZrO2/TiO2/Ti device exhibits diode-like volatile resistive switching with a rectifying ratio over 106. The permanent transition from volatile to nonvolatile resistive switching can be obtained by applying a higher current compliance of 100 μA. Furthermore, by using different reset voltages, the Cu/ZrO2/TiO2/Ti device exhibits multilevel memory characteristics with high uniformity. The coexistence of nonvolatile multilevel memory and diode-like volatile resistive switching behaviors in the same Cu/ZrO2/TiO2/Ti device opens areas of applications in high-density storage, logic circuits, neural networks, and passive crossbar memory selectors.

Functional helical silica nanofibers with coaxial mixed mesostructures for the fabrication of PtCo nanowires that display unique geometry-dependent magnetism

Helical mesoporous silicas are notable from a self-assembly and applications points of view. We report a novel confinement-free synthesis of chloropropyl-functionalized helical mesoporous silica nanofibers (CP-HMSNFs) possessing straight channels at the fiber center surrounded by concentric short-pitch helical channels. The chloropropyl groups are found mainly distributed at the central cylindrical portion of the fibers, allowing the selective inclusion of guest species to fabricate novel nanocomposite fibers. Moreover, the chloropropyl-functionalized nanofibers were applied as a hard template to fabricate helical platinum–cobalt (PtCo) alloy nanowires with small and narrowly distributed radii of gyration. The helical metal nanowires exhibited distinct ferromagnetic properties as compared with their straight counterpart. Platinum–cobalt nanowires can be constructed into twisted, magnetically active helical shapes using a novel silica template. Mesoporous silica is a sieve-like material, ideal for directing the growth of metal nanowires. Previously, researchers have developed helical mesoporous silicas to grow spiral metal nanowires. Now, Chia-Min Yang from National Tsing Hua University in Taiwan and co-workers have made these templates more versatile. By introducing silanes bearing chloropropyl groups into the synthesis mixture, the materials self-assembled into nanofibres having a straight central core surrounded by concentric short-pitch helical channels. Because the hydrophobic chloropropyl groups localized near the nanofibre core, the team used them to selectively deposit various materials into the helical templates — iron oxide, pure platinum metal and ferromagnetic platinum–cobalt nanocomposites with possible high-density storage applications. A novel confinement-free synthesis of CP-HMNSFs possessing straight channels at fiber center surrounded by concentric short-pitch helical channels is discovered. The chloropropyl groups are found mainly located at central cylindrical part of the nanofibers. Helical PtCo nanowires with small and narrowly distributed radii of gyration are also fabricated, showing distinct ferromagnetic properties as compared with the straight counterparts.

Chirality control of nonplanar lead phthalocyanine (PbPc) and its potential application in high-density storage: a theoretical investigation.

On single-crystal surfaces, achiral molecules may become chiral owing to confinement in two dimensions (2D). Metal phthalocyanines (MPcs) on Cu(001) and Ag(100) surfaces have exhibited a chiral electronic state. However, the chirality is not always desirable since crystal defects (grain boundaries) inevitably occur between two different chiral domains during the self-assembly of single layers. In this theoretical study, we propose to utilize metal(001) substrates with different electron configurations to mediate the azimuthal orientations of nonplanar PbPc. The results show that PbPc is chiral on Cu(001) with a partially filled s orbital (3d(10)4s(1)) but achiral on Pd(001) with a completely filled d orbital (4d(10)). The mechanism that PbPc prefers achiral azimuthal orientation rather than chiral orientation on Pd(001) is clarified. In addition, we predict that PbPc can form a (3 × 4) surface reconstruction. While it is used for data storage, the capacity is almost three orders of magnitude higher than the present storage materials.

Fabrication and characterization of a piezoelectric micromirror using for optical data tracking of high-density storage

A micromirror actuated by three piezoelectric microcantilevers is presented for optical data tracking of high-density storage application. The microcantilevers are actuated by 2.5-μm-thick lead zirconate titanate (PZT) films which are deposited on the silicon-based substrate by a compatible sol–gel route. The X-ray diffraction result shows that the PZT film is perovskite structure and has a typical good ferroelectric loop. The quasi-static displacement of the mirror plate increases linearly with increasing the driving voltage and the tracking resolution on disk is as high as 8 nm/V. The micromirror also provides a high bandwidth of about 21 kHz, which is high enough to support the optical data tracking of future high-density storage.

Extraordinary hall effect in Pt- or Ni-based multilayer stacks with strong perpendicular magnetic anisotropy

We determine in high anisotropy Pt/Co- and Ni/Co-based multilayer materials, which are candidates for high density storage applications, the contributions to the extraordinary Hall effect. In order to gain more insight into the physics underlying these processes, the dependence of the extraordinary Hall resistivity on the longitudinal resistivity is used to identify the skew scattering and the combined intrinsic and side jump effects when analyzing their variation with temperature. Furthermore, the influence of an additional amorphous CoFeB layer is shown, revealing a possibility to enhance the extraordinary Hall effect using this material.

Carbon-doped Ge2Sb2Te5 phase change material: A candidate for high-density phase change memory application

Carbon-doped Ge2Sb2Te5 material is proposed for high-density phase-change memories. The carbon doping effects on electrical and structural properties of Ge2Sb2Te5 are studied by in situ resistance and x-ray diffraction measurements as well as optical spectroscopy. C atoms are found to significantly enhance the thermal stability of amorphous Ge2Sb2Te5 by increasing the degree of disorder of the amorphous phase. The reversible electrical switching capability of the phase-change memory cells is improved in terms of power consumption with carbon addition. The endurance of ∼2.1 × 104 cycles suggests that C-doped Ge2Sb2Te5 film will be a potential phase-change material for high-density storage application.

Influence of asymmetry on vortex nucleation and annihilation in submicroscaled permalloy disk array

Magnetic properties of vortex nucleation, annihilation, and switching field distribution (SFD) in NiFe disk arrays, where the elements are with 300nm diameter and different degrees of asymmetry, were investigated through measurements and simulations of hysteresis loop. The nucleation and annihilation of vortex state show strong dependences on the asymmetry. More interestingly, the width of SFD, the crucial factor for high-density storage application, oscillating with the degree of asymmetry is observed. The simulation results agree well with the experimental data.

Coherent hole-burnings induced by a bichromatic laser field

In a Doppler-broadened three-level Λ-type system driven simultaneously by a coupling laser and two equal-amplitude saturating laser fields with a frequency difference 2δ, the absorption spectrum of a weak probe laser exhibits multiple deep spectral holes through coherent hole-burning CHB with controllable numbers, widths, depths, and positions. More significant, changing δ or lasers directions, CHBs can degenerate into narrower and deeper spectral holes where the slope of the refractive index is very steep. The multiple narrow spectral holes in a single absorption profile are expected to have potential applications in high density storage, optical information processes, and slow-light.

All-Electrical Control of Single Ion Spins in a Semiconductor

We propose a method for all-electrical manipulation of single ion spins substituted into a semiconductor. Mn ions with a bound hole in GaAs form a natural example. Direct electrical manipulation of the ion spin is possible, because electric fields manipulate the orbital wave function of the hole, and through the spin-orbit coupling the spin is reoriented as well. Coupling ion spins can be achieved using gates to control the size of the hole wave function. Coherent manipulation of ionic spins may find applications in high-density storage and in scalable coherent or quantum information processing.

Characteristics of nano-thickness lead zirconate titanate thin film for high-density storage applications

We successfully fabricated lead zirconate titanate (PZT) films with film thickness of 70 nm using a modified solution combined with lead titanate (PTO) seed layer. Throughout various approaches, we have found that the microstructure of PZT film plays an important role in determining the hysteretic properties particularly below the thickness of 100 nm. We modified the ligands of precursors to improve the microstructure of thin PZT film. In addition, we also adopted a thin PTO seed layer to enhance the initial nucleation density. Finally, we could obtain good electric properties similar to those of 250 nm PZT. The hysteretic properties such as Q– V, and hysteresis loop seem to be excellent enough to operate at as low as 2 V, which is a prerequisite property for high-density storage applications.