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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,423 records · Page 79
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Simulation of thermal reset transitions in resistive switching memories including quantum effects

An in-depth study of reset processes in RRAMs (Resistive Random Access Memories) based on Ni/HfO{sub 2}/Si-n{sup +} structures has been performed. To do so, we have developed a physically based simulator where both ohmic and tunneling based conduction regimes are considered along with the thermal description of the devices. The devices under study have been successfully fabricated and measured. The experimental data are correctly reproduced with the simulator for devices with a single conductive filament as well as for devices including several conductive filaments. The contribution of each conduction regime has been explained as well as the operation regimes where these ohmic and tunneling conduction processes dominate.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Vacancy dynamic in Ni-Mn-Ga ferromagnetic shape memory alloys

Vacancies control any atomic ordering process and consequently most of the order-dependent properties of the martensitic transformation in ferromagnetic shape memory alloys. Positron annihilation spectroscopy demonstrates to be a powerful technique to study vacancies in NiMnGa alloys quenched from different temperatures and subjected to post-quench isothermal annealing treatments. Considering an effective vacancy type the temperature dependence of the vacancy concentration has been evaluated. Samples quenched from 1173 K show a vacancy concentration of 1100 ± 200 ppm. The vacancy migration and formation energies have been estimated to be 0.55 ± 0.05 eV and 0.90 ± 0.07 eV, respectively.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Origin of the current discretization in deep reset states of an Al{sub 2}O{sub 3}/Cu-based conductive-bridging memory, and impact on state level and variability

In this paper, we develop a Quantum-Point-Contact (QPC) model describing the state conduction in a W/Al{sub 2}O{sub 3}/TiW/Cu Conductive-Bridging Memory cell (CBRAM). The model allows describing both the voltage- and the temperature-dependence of the conduction. For deep current levels, a resistance component is added in series to the point-contact constriction to account for electron scattering in the residual filament. The fitting of single-particle perturbation also allowed to estimate the number and effective size of the conduction-controlling particles in the QPC constriction. The results clearly point to smaller particles for CBRAM (Cu particles) as compared to oxide-based resistive RAM involving oxygen-vacancy defects, which is discussed as a possible origin of deeper reset level obtained in CBRAM. We also evidence a beneficial impact of this smaller particle size on lower Random-Telegraph-Noise amplitude measured on CBRAM devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Voltage induced magnetostrictive switching of nanomagnets: Strain assisted strain transfer torque random access memory

A spintronic device, called the “strain assisted spin transfer torque (STT) random access memory (RAM),” is proposed by combining the magnetostriction effect and the spin transfer torque effect which can result in a dramatic improvement in the energy dissipation relative to a conventional STT-RAM. Magnetization switching in the device which is a piezoelectric-ferromagnetic heterostructure via the combined magnetostriction and STT effect is simulated by solving the Landau-Lifshitz-Gilbert equation incorporating the influence of thermal noise. The simulations show that, in such a device, each of these two mechanisms (magnetostriction and spin transfer torque) provides in a 90° rotation of the magnetization leading a deterministic 180° switching with a critical current significantly smaller than that required for spin torque alone. Such a scheme is an attractive option for writing magnetic RAM cells.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

GeTe sequences in superlattice phase change memories and their electrical characteristics

We studied GeTe structures in superlattice phase change memories (superlattice PCMs) with a [GeTe/Sb{sub 2}Te{sub 3}] stacked structure by X-ray diffraction (XRD) analysis. We examined the electrical characteristics of superlattice PCMs with films deposited at different temperatures. It was found that XRD spectra differed between the films deposited at 200 °C and 240 °C; the differences corresponded to the differences in the GeTe sequences in the films. We applied first-principles calculations to calculate the total energy of three different GeTe sequences. The results showed the Ge-Te-Ge-Te sequence had the lowest total energy of the three and it was found that with this sequence the superlattice PCMs did not run.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Effects of ZrO{sub 2} doping on HfO{sub 2} resistive switching memory characteristics

A resistive switching (RS) random access memory device with ZrO{sub 2}-doped HfO{sub 2} exhibits better RS performance than that with pure HfO{sub 2}. In particular, I{sub res}, V{sub res}, and V{sub set} are reduced by approximately 58%, 38%, and 39%, respectively, when HfO{sub 2} is doped with ZrO{sub 2} (9 at. %). In addition, the ZrO{sub 2} doping in HfO{sub 2} makes the distribution of most parameters steeper. Transmission electron microscopy (TEM) analysis reveals that the deposited zirconium-doped hafnium oxide (HZO) (9 at. %) is polycrystalline. Elemental mapping results by scanning TEM–energy dispersive spectroscopy also prove that ZrO{sub 2} is uniformly distributed in the HZO (9 at. %) film. The possible mechanism for the improvement in the RS characteristics is also suggested on the basis of the X-ray photoelectron spectroscopy results and filamentary RS mechanism. These results suggest that the ZrO{sub 2} doping into HfO{sub 2} likely not only will reduce power consumption but also will improve cyclic endurance by controlling the nonstoichiometric phase.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Nanometer-scale temperature imaging for independent observation of Joule and Peltier effects in phase change memory devices

This paper reports a technique for independent observation of nanometer-scale Joule heating and thermoelectric effects, using atomic force microscopy (AFM) based measurements of nanometer-scale temperature fields. When electrical current flows through nanoscale devices and contacts the temperature distribution is governed by both Joule and thermoelectric effects. When the device is driven by an electrical current that is both periodic and bipolar, the temperature rise due to the Joule effect is at a different harmonic than the temperature rise due to the Peltier effect. An AFM tip scanning over the device can simultaneously measure all of the relevant harmonic responses, such that the Joule effect and the Peltier effect can be independently measured. Here we demonstrate the efficacy of the technique by measuring Joule and Peltier effects in phase change memory devices. By comparing the observed temperature responses of these working devices, we measure the device thermopower, which is in the range of 30 ± 3 to 250 ± 10 μV K{sup −1}. This technique could facilitate improved measurements of thermoelectric phenomena and properties at the nanometer-scale.

42 ENGINEERING↗
DOE OSTI2014

Excess Ni-doping induced enhanced room temperature magneto-functionality in Ni-Mn-Sn based shape memory alloy

Present work reports on the observation of large magnetoresistance (∼−30% at 80 kOe) and magnetocaloric effect (∼12 J·kg{sup −1}·K{sup −1} for 0–50 kOe) near room temperature (∼290 K) on the Ni-excess ferromagnetic shape memory alloy Ni{sub 2.04}Mn{sub 1.4}Sn{sub 0.56}. The sample can be thought of being derived from the parent Ni{sub 2}Mn{sub 1.4}Sn{sub 0.6} alloy, where excess Ni was doped at the expense of Sn. Such Ni doping enhances the martensitic transition temperature and for the Ni{sub 2.04}Mn{sub 1.4}Sn{sub 0.56} it is found to be optimum (288 K). The doped alloy shows enhanced magneto-functional properties as well as reduced saturation magnetization as compared to the undoped counterpart at low temperature. A probable increment of antiferromagnetic correlation between Mn-atoms on Ni substitution can be accounted for the enhanced magneto-functional properties as well as reduction in saturation moment.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Resistive switching phenomena in TiO{sub x} nanoparticle layers for memory applications

Electrical characteristics of a Co/ TiO{sub x}/Co resistive memory device, fabricated by two different methods, are reported. In addition to crystalline TiO{sub 2} layers fabricated via conventional atomic layer deposition (ALD), an alternative method has been examined, where TiO{sub x} nanoparticle layers were fabricated via sol-gel. The different devices have shown different hysteresis loops with a unique crossing point for the sol-gel devices. A simple qualitative model is introduced to describe the different current-voltage behaviours by suggesting only one active metal-oxide interface for the ALD devices and two active metal-oxide interfaces for the sol-gel devices. Furthermore, we show that the resistive switching behaviour could be easily tuned by proper interface engineering and that despite having a similar active material, different fabrication methods can lead to dissimilar resistive switching properties.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2014

Ferromagnetic interactions and martensitic transformation in Fe doped Ni-Mn-In shape memory alloys

The structure, magnetic, and martensitic properties of Fe doped Ni-Mn-In magnetic shape memory alloys have been studied by differential scanning calorimetry, magnetization, resistivity, X-ray diffraction (XRD), and EXAFS. While Ni{sub 2}MnIn{sub 1−x}Fe{sub x} (0 ≤ x ≤ 0.6) alloys are ferromagnetic and non martensitic, the martensitic transformation temperature in Ni{sub 2}Mn{sub 1.5}In{sub 1−y}Fe{sub y} and Ni{sub 2}Mn{sub 1.6}In{sub 1−y}Fe{sub y} increases for lower Fe concentrations (y ≤ 0.05) before decreasing sharply for higher Fe concentrations. XRD analysis reveals presence of cubic and tetragonal structural phases in Ni{sub 2}MnIn{sub 1−x}Fe{sub x} at room temperature with tetragonal phase content increasing with Fe doping. Even though the local structure around Mn and Ni in these Fe doped alloys is similar to martensitic Mn rich Ni-Mn-In alloys, presence of ferromagnetic interactions and structural disorder induced by Fe affect Mn-Ni-Mn antiferromagnetic interactions resulting in suppression of martensitic transformation in these Fe doped alloys.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

A bio-inspired memory device based on interfacing Physarum polycephalum with an organic semiconductor

The development of devices able to detect and record ion fluxes is a crucial point in order to understand the mechanisms that regulate communication and life of organisms. Here, we take advantage of the combined electronic and ionic conduction properties of a conducting polymer to develop a hybrid organic/living device with a three-terminal configuration, using the Physarum polycephalum Cell (PPC) slime mould as a living bio-electrolyte. An over-oxidation process induces a conductivity switch in the polymer, due to the ionic flux taking place at the PPC/polymer interface. This behaviour endows a current-depending memory effect to the device.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Role of interfacial layer on complementary resistive switching in the TiN/HfO{sub x}/TiN resistive memory device

The role of the bottom interfacial layer (IL) in enabling stable complementary resistive switching (CRS) in the TiN/HfO{sub x}/IL/TiN resistive memory device is revealed. Stable CRS is obtained for the TiN/HfO{sub x}/IL/TiN device, where a bottom IL comprising Hf and Ti sub-oxides resulted from the oxidation of TiN during the initial stages of atomic-layer deposition of HfO{sub x} layer. In the TiN/HfO{sub x}/Pt device, where formation of the bottom IL is suppressed by the inert Pt metal, no CRS is observed. Oxygen-ion exchange between IL and the conductive path in HfO{sub x} layer is proposed to have caused the complementary bipolar switching behavior observed in the TiN/HfO{sub x}/IL/TiN device.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Effect of niobium addition on the martensitic transformation and magnetocaloric effect in low hysteresis NiCoMnSn magnetic shape memory alloys

The effect of Nb substitution for Ni in Ni{sub 45}Co{sub 5}Mn{sub 40}Sn{sub 10} magnetic shape memory alloys on their magnetic properties, martensitic transformation characteristics, transformation hysteresis, and magnetocaloric properties was studied using wavelength-dispersive X-ray spectroscopy, differential scanning calorimetry, and the temperature and field dependence of the magnetization. Ni{sub 45}Co{sub 5}Mn{sub 40}Sn{sub 10} alloy has a very low transformation hysteresis; however, the martensitic transformation temperatures are notably above room temperature, which is not desirable for magnetic refrigeration applications. In this study, small quantities of Nb substitution were shown to drastically shift the transformation temperatures to lower temperatures, at a rate of 68 K/at. % Nb, which is needed for household refrigeration. The austenite Curie temperature also decreased with increasing Nb content. However, a decrease in the latent heat of the martensitic transition was observed, which negatively affects the magnetic field-induced adiabatic temperature change capability. Still, the relatively large transformation entropy and the low transformation hysteresis make the Nb-doped Ni{sub 45}Co{sub 5}Mn{sub 40}Sn{sub 10} alloys potential candidates for solid state refrigeration near room temperature.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Plasma enhanced multistate storage capability of single ZnO nanowire based memory

Multiple-state storage (MSS) is common for resistive random access memory, but the effects of plasma treatment on the MSS and the switching properties have been scarcely investigated. We have demonstrated a stable four-state storage capability of single zinc oxide nanowire (ZnO NW) treated by argon plasma. The electrical switching is attributed to the electron trapping and detrapping from the oxygen vacancies (V{sub o}s). The MSS relates to the electrical-thermal induced distribution of the V{sub o}s which determines electron transport behavior to show different resistance states. Additionally, programming (set and reset) voltages decrease with plasma treatment due to the thickness modulation of the interface barrier.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Exploring electrical conductivity anomalies across the martensite transition in Fe{sub 7}Pd{sub 3} ferromagnetic shape memory alloys: Experiments and ab-initio calculations

Conductivity in Fe{sub 7}Pd{sub 3} is characterized by an anomalous increase when traversing the face–centered–cubic (fcc) austenite to face–centered–tetragonal (fct) martensite transition, contrary to most other conventional and ferromagnetic shape memory alloys. Experiments on molecular– beam–epitaxy–grown single crystals indicate a resistivity change of ≈20% during the transformation on top of a quadratic temperature dependence reaching up to room temperature. The physical foundations of residual resistivity changes along the full Bain path are addressed by a Kubo– Greenwood approach within the framework of density functional theory. To do so, a concept to reliably extract the DC conductivities is proposed that yields reproducible results consistent with experiments. Finding that conductivity peaks in the fct phase, we identify a large density of states paired with high velocities at the Fermi level in the majority spin sub–bands in presence of minimum s–d electron scattering as underlying physical origin.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Optical and magneto-optical studies of martensitic transformation in Ni-Mn-Ga magnetic shape memory alloys

Optical and magneto-optical properties of single crystal of Ni{sub 50.1}Mn{sub 28.4}Ga{sub 21.5} magnetic shape memory alloy during its transformation from martensite to austenite phase were systematically studied. Crystal orientation was approximately along (100) planes of parent cubic austenite. X-ray reciprocal mapping confirmed modulated 10 M martensite phase. Temperature depended measurements of saturation magnetization revealed the martensitic transformation at 335 K during heating. Magneto-optical spectroscopy and spectroscopic ellipsometry were measured in the sample temperature range from 297 to 373 K and photon energy range from 1.2 to 6.5 eV. Magneto-optical spectra of polar Kerr rotation as well as the spectra of ellipsometric parameter Ψ exhibited significant changes when crossing the transformation temperature. These changes were assigned to different optical properties of Ni-Mn-Ga in martensite and austenite phases due to modification of electronic structure near the Fermi energy during martensitic transformation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Investigation of chemical distribution in the oxide bulk layer in Ti/HfO{sub 2}/Pt memory devices using x-ray photoelectron spectroscopy

Resistive switching (RS) of Ti/HfO{sub 2}/Pt memory devices was studied using X-ray photoelectron spectroscopy. Hf{sup 4+} monotonously decreases with depth increasing towards to HfO{sub 2}/Pt interface in low resistance state, while a fluctuation distribution of Hf{sup 4+} is shown in high resistance state (HRS) and in the pristine Ti/HfO{sub 2}/Pt devices (without any SET or RESET process). It is explained by the existence of locally accumulated oxygen vacancies (clusters) in the oxide bulk layer in HRS and pristine states. A dynamic model of RS processes was proposed that the oxygen vacancy clusters dominantly determines the resistivity by the connecting/rupture between the neighbor cluster sites in the bulk.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Resistance controllability and variability improvement in a TaO{sub x}-based resistive memory for multilevel storage application

In order to obtain reliable multilevel cell (MLC) characteristics, resistance controllability between the different resistance levels is required especially in resistive random access memory (RRAM), which is prone to resistance variability mainly due to its intrinsic random nature of defect generation and filament formation. In this study, we have thoroughly investigated the multilevel resistance variability in a TaO{sub x}-based nanoscale (<30 nm) RRAM operated in MLC mode. It is found that the resistance variability not only depends on the conductive filament size but also is a strong function of oxygen vacancy concentration in it. Based on the gained insights through experimental observations and simulation, it is suggested that forming thinner but denser conductive filament may greatly improve the temporal resistance variability even at low operation current despite the inherent stochastic nature of resistance switching process.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗