Memory Storage Devices Research Articles

Microwave magnetization dynamics in ferromagnetic spherical nanoshells

Ferromagnetic particles of nano- and micrometer-size range are in high demand owing to their promising technological applications in magnetic memory storage, biomedical treatment, and microwave devices. New absorbing materials are required to possess the properties of low density and strong absorption in a broad frequency band. The dynamic susceptibility spectra of nanosized spherical shells supporting onion and vortex magnetization configurations are studied as a function of particle radius $40--125\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$ and shell thickness $10--115\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$ by means of numerical micromagnetic simulation. For the saturated shell, the frequencies of higher-order spin-wave modes are found to rapidly approach the subterahertz frequency regime ($200--300\phantom{\rule{0.16em}{0ex}}\mathrm{GHz}$) when the geometry is transformed from the single-domain sphere to the spherical shell. The frequency of each mode is proportional to ${R}_{2}^{\ensuremath{-}2}$ when the sphere radius outer ${R}_{2}$ is below 30 nm in reasonable agreement with the exchange approximation. Thickness-dependent vortex core gyrotropic modes were studied in the absence of an external bias field in the frequency range $0.1--30\phantom{\rule{0.16em}{0ex}}\mathrm{GHz}$. The size dependence of the vortex spin excitations is complicated by mixing of the modes at different particle sizes. For shells with large thickness and outer radii above ${R}_{2}\ensuremath{\ge}100\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$ the amplitudes of both $n=0$ and $n=1$ vortex flexural modes are less intense than even higher-frequency resonances. However, the $n=0$ mode becomes the dominant mode in the dynamic susceptibility when either the radius or thickness of the particle becomes sufficiently small.

Spin Filtering and Rectification in Lateral Heterostructures of Zigzag-Edge BC3 and Graphene Nanoribbons: Implications for Switching and Memory Devices

Heterostructures of different atomically thin nanomaterials provide an ideal platform of admixing their unique properties to achieve new functionalities. Here, we study the lateral heterostructures of graphene and BC3 nanoribbons with zigzag edge terminations within first-principles calculations. These heterostructures show localization of one kind of spin along the open graphene zigzag edge, whereas the other edge does not show any spin polarization. Therefore, the systems stabilize in the ferromagnetic ground state, inducing net magnetization and broken time reversal symmetry in each valley. Owing to this asymmetric spin distribution, the system shows spin-polarized current that is confirmed by further first-principles-based quantum transport calculations. The conduction behavior can be tuned further by changing the polarity of the side-gates in either sides of the ribbon edges. We observe higher spin polarization of current under forward gate bias but lower spin polarization of current under reverse gate bias. This functionality provides a unique way of spin filtering and rectification by changing the electrode polarity, which can find major applications in switching and memory storage devices.

The observation of resistive switching characteristics using transparent and biocompatible Cu2+-doped salmon DNA composite thin film

For the potential switching bio-memory device application using DNA composite thin film, we fabricated and characterized the transparent and biocompatible resistive switching random access memory (RRAM) device within the structure stacking of Pt/Cu2+ doped salmon DNA/FTO, where Cu2+ doping into salmon DNA was solution-processed. The device shows good bipolar switching characteristics with SET and RESET processes at negative and positive sweeps, respectively, with switching memory window greater than 103 ratios. The device was observed to be in low resistance state as its pristine state and an initial RESET state was necessary to achieve programmable SET and RESET cycles. Based on the electrical characteristics of the Cu2+-doped salmon DNA-based RRAM device we propose a switching mechanism with the formation and rupture of conductive filaments due to the migration of Cu2+ during the electrical stress. Our understanding could contribute to the engineering of biomaterial memory switching medium for the environmentally benign, biocompatible and biodegradable memory storage devices.

Enhancement of room temperature ferromagnetic behavior of Co-doped ZnO nanoparticles synthesized via sol–gel technique

Zn1−xCoxO (x = 0, 0.001, 0.005, 0.01, 0.02, 0.04, and 0.06) nanoparticles (NPs) were synthesized by sol–gel method. Ingestion of Co2+ ions in the ZnO crystal lattice created defects without disturbing the original crystal structure of ZnO. This is confirmed by the X-ray diffraction (XRD) studies and is also observed in the photoluminescence (PL) emission study where the Co-doped ZnO NPs have shown emissions in the green and yellow-orange regions of visible spectrum. XRD and transmission electron microscopic (TEM) analysis showed reduction in crystallite size from 29 to 14.5 nm and from 51 to 36 nm, respectively, with increase in Co2+ ions concentration. Optical absorption spectroscopic studies (UV-visible spectroscopy) of these samples show significant peak shift towards higher energy (blue shift), which is in accordance with the Kubo’s theory. Magnetic properties were studied using quantum designed vibrating sample magnetometry (VSM), which confirmed that Co-doped ZnO NPs exhibit room temperature ferromagnetism (RTFM) for the samples having high Co concentration. With these notable properties, this study suggests that Co-doped ZnO NPs may be useful in the field of optoelectronic devices and memory-storage devices as well.

Electronic, electrical and magnetic behaviours of reduced graphene-oxide functionalized with silica coated gold nanoparticles

We have synthesized graphene-oxide (GO) by the modified hummer's process and subsequently reduced it with gold-nanoparticles (Au-NPs) using silica coated colloidal Au-NPs and hydrazine monohydrate solutions to form r-GO:Au-NPs nanocomposites. We have studied the microstructure, electronic, electrical and magnetic properties of r-GO and r-GO:Au-NPs nanocomposites. We have observed from the Raman spectroscopy that the intensity of D-peak (disorder) gets reduced with respect to G (graphite-cluster) in r-GO:Au-NPs composites. The reduction of ID/IG ratio obtained from Raman spectra [r-GO: 1.22 → r-GO:(Au-NPs)4.88: 0.98] clearly indicates that the sp2-cluster is reduced in r-GO:Au-NPs nanocomposites. The reduction of sp2-cluster and/or enhancement of sp3-cluster is due to replacement of sp2-cluster by the Au-NPs. This observation also observed from the X-ray absorption near edge structure (XANES) spectroscopy, X-ray photoelectron spectroscopy (XPS) measurements that are consistence with reduction of conductivity as we observe from the current (I) - voltage (V) characteristics of the nanocomposites. Magnetic M-H hysteresis loops show the magnetization is enhanced in r-GO:Au-NPs nanocomposites. We believe that the reduction of conductivity and enhancement of magnetization of r-GO:Au nanocomposites would be most suitable for ferro-electro-magnetic materials for the memory storage device applications.

The magnetoelectric effect of the CFO thin film by coupling a P(VDF-co-TrFE) piezoelectric layer

The CoFe2O4 (CFO) thin films are prepared by the sol-gel method combined with the spin-casting process, and the related SEM and XRD results show that a 9-layer spinning of the original sol together with an annealing temperature of 700 °C is the appropriate condition for obtaining this CFO film with an excellent magnetic performance. The CFO films are subsequently compounded with a ferroelectric copolymer poly(vinylidene fluoride-co-trifluoroethylene) [P(VDF-co-TrFE)]. After polarizing the obtained CFO/P(VDF-co-TrFE) double-layer composite film under a high electrical field of 275 MV/m, the P(VDF-co-TrFE) layer shows the character of normal ferroelectricity that has a large dielectric permittivity (ɛ33/ɛ0 = 9.5) and a high piezoelectric value (d31 = 20 pC/N). Interestingly, we also find that the magnetization of this electrically poled CFO/P(VDF-co-TrFE) film increases from 85.58 to 99.15 emu/cm3 at a magnetic field of 1.8 T since the electrostrictive effect may lead to the re-orientation of magnetic domains and thereby resulting in a magnetization easy-axis reorientation effect in the out-of-plane direction of the CFO thin film. Consequently, a large magnetoelectric coupling coefficient (αE31) of 508.5 MV/cm Oe is obtained through calculation provided the interfaces between CFO and P(VDF-co-TrFE) have an appropriate compatible coefficient of about 0.5. The results indicate that the CFO/P(VDF-co-TrFE) double-layer film is a potential multiferroic composite as an attractive candidate for memory storage and spintronic devices.

Switchable Rashba effect by dipole moment switching in an Ag2Te monolayer

Because of the surface depolarization field, there is a critical thickness for ferroelectricity in ultrathin ferroelectric films, hindering miniaturization of high-density nonvolatile memory storage devices. A controllable Rashba effect by external electric field via switchable dipole moment could be a promising way to control and manipulate the spin degrees of freedom in spintronics. Here, based on first principles calculations, we show that non-planar Ag2Te monolayer, which has been recently predicted to be a topological insulator, possess a switchable out-of-plane electric dipole moment. The switching of the dipole can be realized by the penetration of Te atoms through the hexagonal Ag-plane. Additionally, non-planar Ag2Te shows a giant Rashba spin-splitting ( eV Å) due to the out-of-plane electric dipole moment. Our tight binding model indicates that the origin of such large is the large inversion symmetry breaking term ( eV), which is one order of magnitude larger in non-planar Ag2Te monolayer compared with other Rashba materials. Interestingly, the Rashba effect can be turned on/off by the phase transition from non-planar to planar structure via Te displacement. Moreover, the spin-texture can be completely reversed because of switchable electric dipole moment. Our work shows a new way to realize ferroelectric-like dipole moment switching and consequently switchable Rashba spin-splitting, which may facilitate a nonvolatile electrical control of the spin degrees of freedom, down to the monolayer thickness, promising potential applications to electrically controlled spintronic devices.

Fabrication of polymer composite films with carbon composite nanofibers doped MWNTs-OH for multilevel memory device application

Novel carbon composite nanofibers with hierarchical microstructure were synthesized using electro-spinning and carbonization (SC) techniques, and were combined into polyimide matrix based on in-situ polymerization method. The resulted polyimide composites exhibit multilevel resistive switching behavior (MRSB) for memory storage applications, with a favorable ON/OFF current ratio and a high retention time at an operating voltage less than 3 V. A hierarchical microstructure can be observed by SEM, TEM and SAXS tests, which was formed with different sizes between the hydroxyl multi-walled carbon nanotubes (MWNTs-OH), carbon nanofibers and PI matrix, influencing on the MRSB of the composite films. A physical model based on the molecular orbit energy level was utilized to investigate how the microstructures acting as charge transmission layers affect the MRSB. This result provides a new strategy for further improving the high density storage of polymer multilevel switching memory storage device.

Effects of low pressure plasma irradiation on electrical resistivity of perovskite oxide Eu1-xSrxMnO3

The use of perovskite type compounds, has gained a tremendous applications in various fields like memory storage devices, bolometers, superconductors, solar cells, etc. One of the very important uses of perovskite material is in the construction of bolometers. Bolometers are extensively used in fusion devices for temperature measurement and also in space application. In this paper we have presented the experimental behaviour of the electrical resistivity of the perovskite oxide Eu1-x SrxMnO3 after plasma exposure for different composition. It is found that the compound Eu1-x SrxMnO3 behaves as an insulator for all the values of x, since the effective single electron bandwidth is very narrow. The observed decrease in the resistivity below TMI may be due to the lack of delocalized carriers that contributes for the metallic nature of the sample.

Study the effect of dip in reaction temperature on thermal and electrical properties of ZnO nanoparticles

The present study reports thermal and electrical properties of Zinc oxide (ZnO) nanoparticles prepared using co-precipitation route. Sudden dip in reaction temperature observed during the synthesis process created defects in the crystal lattice of ZnO which leads to reduction in crystallite size from 33 nm to 28 nm with increase in reaction temperature. This is confirmed by the X-ray diffraction studies. Thermal analysis of the samples shows better thermal stability for smaller nanoparticles. Temperature-dependent current-voltage characteristics of the ZnO samples show reduction in the conductivity and increase in dielectric constant with respect to rise in reaction temperature. Increase in dielectric constant with decrease in size of nanoparticles may be useful in the field of nanoelectronics like memory-storage devices, etc.

Effects of Oxidizing/Reducing Agent Ratio on Phase Purity, Crystallinity, and Magnetic Behavior of Solution-Combustion-Grown BiFeO3 Submicroparticles.

Fabrication of phase-pure well-crystalline BiFeO3 submicroparticles in large scale is of great importance for the utilization of this rhombohedrally distorted perovskite material in applications such as memory storage and spintronic devices and visible photocatalyst for the degradation of organic pollutants. In fact, because of the narrow temperature range of phase stabilization, the fabrication of phase-pure BiFeO3 in large scale remained elusive. We present the synthesis of phase-pure BiFeO3 particles of submicrometric dimensions (246-330 nm average size) through the adjustment of oxidizing/reducing agent ratio in solution combustion process utilizing glycine as reducing agent and nitrate precursors as oxidizing agent. Utilizing X-ray diffraction and Raman spectroscopy, we demonstrate that the BiFeO3 submicroparticles synthesized at equivalence ratio (Φe) close to 0.5 do not contain undesired impurities such as Bi2Fe4O9 and Bi24Fe2O39. Moreover, the submicroparticles are highly crystalline, possessing high room temperature magnetic moment and stable antiferromagnetic behavior across a wide temperature range. The superparamagnetic behavior at low magnetic field manifested by impurities attached to the BiFeO3 submicroparticles might lead to their use as effective magnetically separable photocatalysts.

Efficient block cipher mode for NVM

The usage of non-volatile memory (NVM) storage devices is rapidly increasing in consumer and enterprise systems, providing high performance and low energy consumption compared to hard disk drives. Lately it is observed that NVM storage devices are widely used in cloud computing as well. However, existing disk encryption methods are designed for hard disk devices and an efficient block cipher mode of operation for NVM storage devices has not yet been studied in detail. In this work we have analyzed and evaluated performance of CBC, CTR and XTS-AES block cipher modes for whole disk encryption solutions and found that CTR and XTS-AES modes outperform CBC mode under the same scenarios.

Effect of Sm3+ and Zr4+ codoping on the magnetic, ferroelectric and magnetodielectric properties of sonochemically synthesized BiFeO3 nanorods

The practical application of the existing multiferroic materials is crucially dependent on research aimed at improving their multiferroic and magnetodielectric properties. Here we explore the effect of Sm3+ and Zr4+ codoping on the magnetic, ferroelectric and magnetodielectric properties of nanocrystalline BiFeO3. The formation of phase pure undoped and Sm3+/Zr4+ doped BiFeO3 via sonochemical synthesis has been confirmed using powder X ray diffraction (XRD). The morphology of the samples has been verified using electron microscopy techniques like SEM and TEM. The presence of the dopant ions has been established from energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). Room temperature ferromagnetism has been observed in Sm3+/Zr4+ doped BiFeO3 which is a marked advancement from the antiferromagnetic behaviour displayed by bulk BiFeO3. This has been attributed to size effects and lattice distortions in the sample. Doping of Sm3+/Zr4+ also decreases the leakage current and considerably improves its ferroelectric properties. Careful evaluation of the results obtained suggest that the suppression of oxygen vacancy related defects is more effective with Sm3+ doping in Bi3+ site compared to acceptor doping of Zr4+ in Fe3+ site of BiFeO3. The observation of enhanced magnetodielectric properties in the Sm3+/Zr4+ doped BiFeO3 suggests potential application of these types of material in memory storage devices in future.

Thermomchromic Reaction-Induced Reversible Upconversion Emission Modulation for Switching Devices and Tunable Upconversion Emission Based on Defect Engineering of WO3:Yb3+,Er3+ Phosphor.

Reversible luminescence modulation of upconversion phosphors has the potential applications as photoswitches and optical memory and data storage devices. Previously, the photochromic reaction was extensively used for the realization of reversible luminescence modulation. It is very necessary to develop other approaches such as thermomchromic reaction to obtain the reversible upconversion luminescence modulation. In this work, the WO3:Yb3+,Er3+ phosphors with various colors were prepared at various temperatures, exhibiting tunable upconversion luminescence attributed to the formation of oxygen vacancies in the host. Upon heat treatment in the reducing atmosphere or air, the WO3:Yb3+,Er3+ phosphors show a reversible thermomchromic property. The reversible upconversion luminescence modulation of WO3:Yb3+,Er3+ phosphors was observed based on thermomchromic reaction. Additionally, the upconversion luminescence modulation is maintained after several cycles, indicating its excellent stability. The WO3:Yb3+,Er3+ phosphors with reversible upconversion luminescence and excellent reproducibility have potential applications as the photoswitches and optical memory and data storage devices.

Crystalline phase dependent electrical properties of Mg incorporated tetragonal phase stabilized ZrO2 high-κ dielectric layer in Si based MOS capacitors

Electron beam evaporated Mg doped ZrO2 (Mg:ZrO2) high-k thin films on p-type Si(100) substrate and their metal oxide semiconductor (MOS) capacitors were structurally and electrically studied. Thicknesses of the dielectric films were in the range from 85 to 240 nm with RMS roughness between 1.4 and 2.3 nm. The targets for e-beam evaporation were made with well characterized pure ZrO2 with monoclinic phase and with 3%Mg doped tetragonal phase stabilized ZrO2 nanopowders. Structural phase analysis revealed that the phases of Mg:ZrO2 thin films deposited have retained their respective phases of zirconia targets used. The incorporation of Mg ions in ZrO2 dielectric gate layer had increased the width of the hysteresis window in the C-V characteristics. Various leakage current conduction mechanisms in the accumulation region of the MOS capacitors have been used to understand the leakage current mechanism and found that traps present in the oxide layer played a major role in explaining the leakage current. In particular, the conduction mechanisms such as Poole-Frenkel emission, Space Charge Limited Conduction and Trap-assisted tunneling were found to be dominant in these devices. A high dielectric constant of 28.9 was achieved in the Mg doped tetragonal phase stabilized ZrO2 gate oxide films with a leakage current density of 3.32 × 10−6 A/cm2 at − 1 V. Hence, the tetragonal phase stabilized Mg:ZrO2 gate oxide layer showed improved electrical characteristics making them potential for memory storage devices.

Інформаційно-вимірювальна система на базі датчиків з тензорезисторами на основі мікрокристалів кремнію

One of the promising directions of development of information and measuring systems for monitoring and diagnostics is the use of intelligent sensors of various physical quantities, in particular pressure, temperature, deformation, acceleration, etc. The main functional feature that distinguishes such sensors is the possibility of signal processing directly in the measuring zone, which involves the temperature compensation of the output signal, linearization of the transformation function. Along with primary converters, intelligent sensors include analog-to-digital and digital-to-analog converters, microcontroller, memory-storage device, input/output interfaces. However, the technologies used today for the creation of existing microelectronic sensors are quite complex and require special process equipment and materials, that leads to an increase in their value. The aim of this work is to develop an information and measurement system for use in conjunction with mechanical sensors based on strain gauges made of silicon whiskers in terms to provide a higher efficiency. The deformation and temperature characteristics of sensors of mechanical quantities (pressure, force and deformation) with strain gauges on the basis of p-type Si whiskers (ρ=0.005—0.02 Ω∙cm) in the temperature range from –60 to +60°C were analyzed. It has been established that at a deformation level of ±6∙10–4 relative units, the high values of sensitivity and linearity of strain gauge characteristics are maintained, and the hysteresis effect due to the characteristics of elastic elements is shown to the smallest extent. It is shown that the temperature dependence of relative change in the resistance of strain gauge resistors based on p-Si whiskers with resistivity of 0.005 Ω∙cm is characterized by the smallest nonlinearity. This simplifies the problem of compensation of the temperature errors, which are typical for such sensors. The temperature coefficient of resistance for strain gauges was found to be 0.15%/°Ñ. The information and measuring system was developed on the basis of pressure sensor with strain gauges made of silicon whiskers (ρ=0.005 Ω∙cm). This pressure sensor provides the simultaneous measurement of pressure and temperature in the ranges 0...100 kPa and –60...+60°Ñ, respectively. The measuring channel of the developed system was based on the AVR ATmega328P microcontroller, which provides the ability to create modern high-precision distributed data gathering and display systems. As a result of testing, satisfactory results were obtained regarding stability, sensitivity and measurement ranges of the developed information and measuring system. The main measurement error did not exceed 0.1%. The measuring circuit can be easily adapted to a new task without making any significant changes to its hardware, the function of the device is easily adjusted by changing the work program.

Effect of molecular weight of sodium polyacrylates on the size and morphology of nickel nanoparticles synthesized by the modified polyol method and their magnetic properties

Nickel nanoparticles were synthesized by the reduction of nickel chloride with hydrazine hydrate in a polyol medium in the presence of sodium polyacrylates (Na-PA) having molecular weights (Mw) of 1200, 5100 and 8000. The size and morphology of the resulting nickel nanoparticles were characterized by X-ray diffraction, scanning and transmission electron microscopy. Polymers having lower Mw values were found to be more efficient in reducing the nickel particle size. A decrease in the polymer concentrations yielded the smaller particles. Magnetic measurements showed that the as-prepared powders are ferromagnetic and their saturation magnetization and coercivity are size-dependent. Compared with bulk nickel, the nanoparticles exhibit an enhanced coercivity which is due to their small size and a decreased saturation magnetization resulted from the surface oxidation of the powder. The synthesis procedure offers a simple approach to preparing nickel nanopowders on a large scale which could be used as magnetic recording materials, including high-density memory storage devices.

Structural, Microstructural, Magnetic, and Ferroelectric Properties of Ba 2 + -Doped BiFeO3 Nanocrystalline Multifferroic Material

Multiferroic barium-substituted nanocrystalline samples having the general formula Bi1−x Ba x FeO3 (where x = 0.00, 0.05, 0.10, 0.15, 0.20, 0.25) were prepared by cost-effective sol–gel method using metal nitrates as oxidants. The prepared samples were annealed at 650 ∘C for 4 h using a muffle furnace. The influence of Ba2+ doping on the structural, microstructural, magnetic, and ferroelectric properties of bismuth ferrite (BFO) was studied systematically by standard techniques. All the samples were characterized by X-ray diffraction (XRD) technique, and the average crystallite size was calculated using Scherrer’s formula. The surface morphology of the prepared nanocrystalline samples was studied using field emission scanning electron microscopy (FE-SEM) technique. FTIR spectra were obtained to confirm the formation of the perovskite structures. The elemental compositional details obtained were confirmed by energy dispersive spectrum (EDS) analysis. Room temperature magnetic measurements were carried out up to a field of 15 kOe, and the parent sample (BFO) shows weak ferromagnetism. Further on, Ba2+ substitution in BFO shows an increase in magnetization. P–E loops were obtained to study the ferroelectric properties of all the samples. Obtained enhancement in magnetic and ferroelectric properties at room temperature is of great interest within the research community due to its wide range of applications in memory storage, spintronics, sensors, and multi-functional devices.

Micromagnetic study of domain wall depinning driven by nanosecond current pulse in notched Permalloy nanowires

The complete understanding of domain wall (DW) dynamics is important in the design of future spintronic devices. The characteristics of faster time-scale and lower current amplitude to move DW along nanowire are crucial in fabrication upgrade. In this study, we have investigated depinning behavior of magnetic domain wall triggered by nanosecond current pulse in notched Permalloy nanowires by means of micromagnetic simulation. We introduced double-triangular notch as the constrictions in the nanowire. The non-adiabaticity of the spin-transfer-torque is considered in simulation by varying the non-adiabatic constant (β) value. We observed that the depinning current density (Jd) was not significantly affected by β for notch size (s) < 50 nm. Interestingly, we found that the depinning time (td) for β ≥ 0.04 was slightly constant for all the cases with s > 70 nm, where the DW structure was kept to be a transverse structure during the depinning process. The broadly applicable depinning behavior is considered to contribute to the development of high-speed memory storage devices based on magnetic domain wall.

Correction to “High-Contrast On/Off Fluorescence Switching via Reversible E–Z Isomerization of Diphenylstilbene Containing the α-Cyanostilbenic Moiety”

Fluorescence switching or modulation of π-conjugated molecules is of great interest in the area of optoelectronic materials science and technology because of their promising applications. However, the “concentration fluorescence quenching” problems commonly associated with conventional π-conjugated molecules in the solid state were occasionally an obstacle to use of π-conjugated molecules in high-contrast memory storage devices. We designed and synthesized a novel class of fluorescence switching π-conjugated molecule, (Z)-CN-MBE, that exhibits a reversible photo/thermal E/Z isomerization process, accompanied by a remarkable fluorescence on/off switching in the solid state. This may provide a promising way to construct high-contrast optical binary memory storage devices. (Z)-CN-MBE is virtually nonfluorescent but exhibits an extraordinary Z to E isomerization upon external light/thermal stimuli even in the solid state with a concomitant drastic fluorescence enhancement owing to the unique “aggregation-indu...