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At least 1,171 records · Page 65
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Perpendicular magnetic anisotropy in Mn{sub 2}CoAl thin film

Heusler compound Mn{sub 2}CoAl (MCA) is attracting more attentions due to many novel properties, such as high resistance, semiconducting behavior and suggestion as a spin-gapless material with a low magnetic moment. In this work, Mn{sub 2}CoAl epitaxial thin film was prepared on MgO(100) substrate by magnetron sputtering. The transport property of the film exhibits a semiconducting-like behavior. Moreover, our research reveals that perpendicular magnetic anisotropy (PMA) can be induced in very thin Mn{sub 2}CoAl films resulting from Mn-O and Co-O bonding at Mn{sub 2}CoAl/MgO interface, which coincides with a recent theoretical prediction. PMA and low saturation magnetic moment could lead to large spin-transfer torque with low current density in principle, and thus our work may bring some unanticipated Heusler compounds into spintronics topics such as the domain wall motion and the current-induced magnetization reversal.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Changes in electrical properties of MOS transistor induced by single 14 MeV neutron

Neutron radiation causes significant changes in the characteristics of MOS devices by the creation of oxide-trapped charge and interface traps. The degradation of the current gain of the GF4936 dual n-channel depletion mode MOS transistor, caused by neutron displacement defects, was measured using in-situ method during neutron irradiation. The average degradation of the gain current is 35 mA at maximum fluence of 2.0 × 10{sup 10} n/cm2 while with an average of 25 mA at minimum fluence of 5.0 × 10{sup 8} n/cm{sup 2}. The change in channel current gain increased proportionally with neutron fluence, meanwhile drain saturation current decreased proportionally with the neutron fluence.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Acoustic spin pumping in magnetoelectric bulk acoustic wave resonator

We present the generation and detection of spin currents by using magnetoelastic resonance excitation in a magnetoelectric composite high overtone bulk acoustic wave (BAW) resonator (HBAR) formed by a Al-ZnO-Al-GGG-YIG-Pt structure. Transversal BAW drives magnetization oscillations in YIG film at a given resonant magnetic field, and the resonant magneto-elastic coupling establishes the spin-current generation at the Pt/YIG interface. Due to the inverse spin Hall effect (ISHE) this BAW-driven spin current is converted to a dc voltage in the Pt layer. The dependence of the measured voltage both on magnetic field and frequency has a resonant character. The voltage is determined by the acoustic power in HBAR and changes its sign upon magnetic field reversal. We compare the experimentally observed amplitudes of the ISHE electrical field achieved by our method and other approaches to spin current generation that use surface acoustic waves and microwave resonators for ferromagnetic resonance excitation, with the theoretically expected values.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Temperature dependence of perpendicular magnetic anisotropy in CoFeB thin films

We study perpendicular magnetic anisotropy in thin films of Ta/Co{sub 20}Fe{sub 60}B{sub 20}/MgO by ferromagnetic resonance and find a linear temperature dependence for the first and second order uniaxial terms from 5 to 300 K. Our data suggest the possible hybridization of Fe-O orbitals at the CoFeB/MgO interface for the origin of the first order anisotropy. However, we also find that non-interfacial contributions to the anisotropy are present. An easy-cone anisotropy is found for the entire temperature range in the narrow region of film thicknesses around the spin reorientation transition 1.2–1.35 nm.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Spin-orbit torque induced magnetization switching in Ta/Co{sub 20}Fe{sub 60}B{sub 20}/MgO structures under small in-plane magnetic fields

Spin-orbit torque (SOT)-induced magnetization switching under small in-plane magnetic fields in as-deposited and annealed Ta/CoFeB/MgO structures is studied. For the as-deposited samples, partial SOT-induced switching behavior is observed under an in-plane field of less than 100 Oe. Conversely, for the annealed samples, an in-plane field of 10 Oe is large enough to achieve full deterministic magnetization switching. The Dzyaloshinskii-Moriya interaction at the Ta/CoFeB interface is believed to be the main reason for the discrepancy of the requisite in-plane magnetic fields for switching in the as-deposited and annealed samples. In addition, asymmetric field dependence behavior of SOT-induced magnetization switching is observed in the annealed samples. Deterministic magnetization switching in the absence of an external magnetic field is obtained in the annealed samples, which is extremely important to develop SOT-based magnetoresistive random access memory.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Two symmetric n-type interfaces SrTiO{sub 3}/LaAlO{sub 3} in perovskite: Electronic properties from density functional theory

The first principles study of the (001) two symmetric n-type interfaces between two insulating perovskites, the nonpolar SrTiO{sub 3} (STO), and the polar LaAlO{sub 3} (LAO) was performed. We have analyzed the formation of metallic interface states between the STO and LAO heterointerfaces by using the all-electron full-potential linearized augmented plane-wave approach based on the density functional theory, within the local density approximation, the Perdew-Burke-Ernzerhof generalized gradient approximation (PBE-GGA), and the Engel-Vosko GGA (EVGGA) formalism. It has been found that some bands cross the Fermi energy level (E{sub F}), forming a metallic nature of two symmetric n-type 6.5STO/1.5LAO interfaces with density of states at E{sub F}, N(E{sub F}) of about 3.56 (state/eV/unit cell), and bare electronic specific heat coefficient (γ) of about 0.62 mJ/(mol cell K{sup 2}). The electronic band stature and the partial density of states in the vicinity of E{sub F} are mainly originated from Ti1,2,3,4-3dxy orbitals. These bands are responsible for the metallic behavior and the forming of the Fermi surface of the two symmetric n-type 6.5STO/1.5LAO interfaces. To obtain a clear map of the valence band electronic charge density distribution of the two symmetric n-type 6.5STO/1.5LAO interfaces, we have investigated the bond's nature and the interactions between the atoms. It reveals that the charge is attracted towards O atoms as it is clear that the O atoms are surrounded by uniform blue spheres which indicate the maximum charge accumulation.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Tunneling magnetoresistance and electroresistance in Fe/PbTiO{sub 3}/Fe multiferroic tunnel junctions

We perform first-principles electronic structure and spin-dependent transport calculations for a Fe/PbTiO{sub 3}/Fe multiferroic tunnel junction with asymmetric TiO{sub 2}- and PbO-terminated interfaces. We demonstrate that the interfacial electronic reconstruction driven by the in situ screening of ferroelectric polarization, in conjunction with the intricate complex band structure of barrier, play a decisive role in controlling the spin-dependent tunneling. Reversal of ferroelectric polarization results in a transition from insulating to half-metal-like conducting state for the interfacial Pb 6p{sub z} orbitals, which acts as an atomic-scale spin-valve by releasing the tunneling current in antiparallel magnetization configuration as the ferroelectric polarization pointing to the PbO-terminated interface. This effect produces large change in tunneling conductance. Our results open an attractive avenue in designing multiferroic tunnel junctions with excellent performance by exploiting the interfacial electronic reconstruction originated from the in situ screening of ferroelectric polarization.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Interface-mediated ferroelectric patterning and Mn valency in nano-structured PbTiO{sub 3}/La{sub 0.7}Sr{sub 0.3}MnO{sub 3}

We employed a multitechnique approach using piezo-force response microscopy and photoemission microscopy to investigate a self-organizing polarization domain pattern in PbTiO{sub 3}/La{sub 0.7}Sr{sub 0.3}MnO{sub 3} (PTO/LSMO) nanostructures. The polarization is correlated with the nanostructure morphology as well as with the thickness and Mn valence of the LSMO template layer. On the LSMO dots, the PTO is upwards polarized, whereas outside the nanodots, the polarization appears both strain and interface roughness dependent. The results suggest that the electronic structure and strain of the PTO/LSMO interface contribute to determining the internal bias of the ferroelectric layer.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Enhancement of the open circuit voltage of Cu 2 O/Ga 2 O 3 heterojunction solar cells through the mitigation of interfacial recombination

Thin film solar cells were fabricated using cuprous oxide (Cu 2 O) absorber layers grown by chemical vapor deposition (CVD) and gallium oxide (Ga 2 O 3 ) buffer layers grown by atomic layer deposition (ALD) on the cuprous oxide CVD films. The in-situ formation of heterojunction in the same deposition system without exposure to oxygen-rich ambient was found to be effective in mitigating the creation of detrimental cupric oxide (CuO) at the interface, resulting in a pristine photovoltaic junction capable of delivering an enhanced open-circuit voltage of 1.78 V. Numerical device simulations of a novel two-layer absorber architecture (CVD-Cu 2 O on ECD-Cu 2 O) showed promising possibilities (theoretical 13.2 % efficiency) for a solar cell combining in-situ junction formation with electrochemical deposition of the absorber layer.

36 MATERIALS SCIENCE↗
DOE OSTI2014

A new estimate of the chondrule cooling rate deduced from an analysis of compositional zoning of relict olivine

Compositional zoning in chondrule phenocrysts records the crystallization environments in the early solar nebula. We modeled the growth of olivine phenocrysts from a silicate melt and proposed a new fractional crystallization model that provides a relation between the zoning profile and the cooling rate. In our model, we took elemental partitioning at a growing solid-liquid interface and time-dependent solute diffusion in the liquid into consideration. We assumed a local equilibrium condition, namely, that the compositions at the interface are equal to the equilibrium ones at a given temperature. We carried out numerical simulations of the fractional crystallization in one-dimensional planar geometry. The simulations revealed that under a constant cooling rate the growth velocity increases exponentially with time and a linear zoning profile forms in the solid as a result. We derived analytic formulae of the zoning profile, which reproduced the numerical results for wide ranges of crystallization conditions. The formulae provide a useful tool to estimate the cooling rate from the compositional zoning. Applying the formulae to low-FeO relict olivine grains in type II porphyritic chondrules observed by Wasson and Rubin, we estimate the cooling rate to be ∼200-2000 K s{sup –1}, which is greater than that expected from furnace-based experiments by orders of magnitude. Appropriate solar nebula environments for such rapid cooling conditions are discussed.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2019

Three-dimensional character of the Fermi surface in ultrathin LaTiO 3 /SrTiO 3 heterostructures

LaTiO 3 films on SrTiO 3 single crystal substrates exhibit metallic behavior attributed to the LaTiO 3 film, the interface as well as part of the SrTiO 3 . In the limit of ultrathin LaTiO 3 films on SrTiO 3 , the contribution to the metallicity from strain-induced electronic structure modification of the LaTiO 3 film is minimized so that the dominant contribution to metallicity is from the interface and part of the SrTiO 3 due to charge transfer of 3d electrons from LaTiO 3 to SrTiO 3 . In such a limit, we observe quantum oscillations whose angular dependence indicates a three-dimensional Fermi surface. Finally, such angular dependence is observed in two sets of quantum oscillations—one low frequency and one high frequency—that we have attributed to an inner and outer Fermi surface associated with a Rashba-like spin split hybridized d xz+yz band.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Accurate band alignment at the amorphous Al 2 O 3 /p-Ge(100) interface determined by hard x-ray photoelectron spectroscopy and density functional theory

Here, we have investigated the band alignment at the interface of amorphous aluminum oxide (am-Al 2 O 3 ) grown by atomic layer deposition on p-Ge(100) and the effects of postgrowth annealing using hard x-ray photoelectron spectroscopy and density function theory (DFT). Accurate determination of the valence-band offsets was obtained by comparing the experimentally measured valence bands with DFT-calculated densities of states. The am-Al 2 O 3 density of states calculated from a weighted ensemble of crystalline Al 2 O 3 structures gives excellent agreement with experiment, sufficiently capturing the nonlinear shape of the valence-band edge. We report a valence-band offset of 2.60 ± 0.1 eV for am-Al 2 O 3 /p-Ge, which is reduced by 0.20 eV upon annealing as interfacial GeOx is formed.

47 OTHER INSTRUMENTATION↗
DOE OSTI2015

Compact modeling of total ionizing dose and aging effects in MOS technologies

This paper presents a physics-based compact modeling approach that incorporates the impact of total ionizing dose (TID) and stress-induced defects into simulations of metal-oxide-semiconductor (MOS) devices and integrated circuits (ICs). This approach utilizes calculations of surface potential (ψs) to capture the charge contribution from oxide trapped charge and interface traps and to describe their impact on MOS electrostatics and device operating characteristics as a function of ionizing radiation exposure and aging effects. The modeling approach is demonstrated for bulk and silicon-on-insulator (SOI) MOS device. The formulation is verified using TCAD simulations and through the comparison of model calculations and experimental I-V characteristics from irradiated devices. The presented approach is suitable for modeling TID and aging effects in advanced MOS devices and ICs.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Comparative Study of the Lateral Photovoltaic Effect in Fe{sub 3}O{sub 4}/SiO{sub 2}/n-Si and Fe{sub 3}O{sub 4}/SiO{sub 2}/p-Si Structures

The results of a comparative study of the lateral photovoltaic effect in Fe{sub 3}O{sub 4}/SiO{sub 2}/n-Si and Fe{sub 3}O{sub 4}/SiO{sub 2}/p-Si structures are presented. The lateral photovoltage reaches its maximum near the measurement contacts in both structures, but the signs of this voltage differ. As the light spot moves away from the contacts, the photovoltage varies linearly in Fe{sub 3}O{sub 4}/SiO{sub 2}/n-Si and decreases exponentially in Fe{sub 3}O{sub 4}/SiO{sub 2}/p-Si. It is found that interface states at the SiO{sub 2}/Si interface induce the photovoltage polarity inversion associated with a change in the conductivity type of silicon. An extreme thickness dependence of the photovoltage with an optimum Fe{sub 3}O{sub 4} film thickness of ~50 nm is observed in both structures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗