742 research outputs found
Simulation and analysis of solenoidal ion sources
We present a detailed analysis and simulation of solenoidal, magnetically confined electron bombardment ion sources, aimed at molecular beam detection. The aim is to achieve high efficiency for singly ionized species while minimizing multiple ionization. Electron space charge plays a major role and we apply combined ray tracing and finite element simulations to determine the properties of a realistic geometry. The factors controlling electron injection and ion extraction are discussed. The results from simulations are benchmarked against experimental measurements on a prototype source
Determining the Anisotropic Exchange Coupling of CrO_2 via First-Principles Density Functional Theory Calculations
We report a study of the anisotropic exchange interactions in bulk CrO_2
calculated from first principles within density functional theory. We determine
the exchange coupling energies, using both the experimental lattice parameters
and those obtained within DFT, within a modified Heisenberg model Hamiltonian
in two ways. We employ a supercell method in which certain spins within a cell
are rotated and the energy dependence is calculated and a spin-spiral method
that modifies the periodic boundary conditions of the problem to allow for an
overall rotation of the spins between unit cells. Using the results from each
of these methods, we calculate the spin-wave stiffness constant D from the
exchange energies using the magnon dispersion relation. We employ a Monte Carlo
method to determine the DFT-predicted Curie temperature from these calculated
energies and compare with accepted values. Finally, we offer an evaluation of
the accuracy of the DFT-based methods and suggest implications of the competing
ferro- and antiferromagnetic interactions.Comment: 10 pages, 13 figure
Texture, twinning and metastable "tetragonal" phase in ultrathin films of HfO<sub>2</sub> on a Si substrate
Thin HfO<sub>2</sub> films grown on the lightly oxidised surface of (100) Si wafers have been examined using dark-field transmission electron microscopy and selected area electron diffraction in plan view. The polycrystalline film has a grain size of the order of 100 nm and many of the grains show evidence of twinning on (110) and (001) planes. Diffraction studies showed that the film had a strong [110] out-of-plane texture, and that a tiny volume fraction of a metastable (possibly tetragonal) phase was retained. The reasons for the texture, twinning and the retention of the metastable phase are discussed
Medium range structural order in amorphous tantala spatially resolved with changes to atomic structure by thermal annealing
Amorphous tantala (a-Ta2O5) is an important technological material that has
wide ranging applications in electronics, optics and the biomedical industry.
It is used as the high refractive index layers in the multi-layer dielectric
mirror coatings in the latest generation of gravitational wave interferometers,
as well as other precision interferometers. One of the current limitations in
sensitivity of gravitational wave detectors is Brownian thermal noise that
arises from the tantala mirror coatings. Measurements have shown differences in
mechanical loss of the mirror coatings, which is directly related to Brownian
thermal noise, in response to thermal annealing. We utilise scanning electron
diffraction to perform Fluctuation Electron Microscopy (FEM) on Ion Beam
Sputtered (IBS) amorphous tantala coatings, definitively showing an increase in
the medium range order (MRO), as determined from the variance between the
diffraction patterns in the scan, due to thermal annealing at increasing
temperatures. Moreover, we employ Virtual Dark-Field Imaging (VDFi) to
spatially resolve the FEM signal, enabling investigation of the persistence of
the fragments responsible for the medium range order, as well as the extent of
the ordering over nm length scales, and show ordered patches larger than 5 nm
in the highest temperature annealed sample. These structural changes directly
correlate with the observed changes in mechanical loss.Comment: 22 pages, 5 figure
Synthesis of H<sub>x</sub>Li<sub>1-x</sub>LaTiO<sub>4</sub> from quantitative solid-state reactions at room temperature
The layered perovskite HLaTiO4 reacts stoichiometrically with LiOH·H2O at room temperature to give targeted compositions in the series HxLi1-xLaTiO4. Remarkably, the Li+ and H+ ions are quantitatively exchanged in the solid state and this allows stoichiometric control of ion exchange for the first time in this important series of compounds
Epitaxial Co2Cr0.6Fe0.4Al thin films and magnetic tunneling junctions
Epitaxial thin films of the theoretically predicted half metal
Co2Cr0.6Fe0.4Al were deposited by dc magnetron sputtering on different
substrates and buffer layers. The samples were characterized by x-ray and
electron beam diffraction (RHEED) demonstrating the B2 order of the Heusler
compound with only a small partition of disorder on the Co sites. Magnetic
tunneling junctions with Co2Cr0.6Fe0.4Al electrode, AlOx barrier and Co counter
electrode were prepared. From the Julliere model a spin polarisation of
Co2Cr0.6Fe0.4Al of 54% at T=4K is deduced. The relation between the annealing
temperature of the Heusler electrodes and the magnitude of the tunneling
magnetoresistance effect was investigated and the results are discussed in the
framework of morphology and surface order based of in situ STM and RHEED
investigations.Comment: accepted by J. Phys. D: Appl. Phy
Influence of s-d interfacial scattering on the magnetoresistance of magnetic tunnel junctions
We propose the two-band s-d model to describe theoretically a diffuse regime
of the spin-dependent electron transport in magnetic tunnel junctions (MTJ's)
of the form F/O/F where F's are 3d transition metal ferromagnetic layers and O
is the insulating spacer. We aim to explain the strong interface sensitivity of
the tunneling properties of MTJ's and investigate the influence of electron
scattering at the nonideal interfaces on the degradation of the TMR magnitude.
The generalized Kubo formalism and the Green's functions method were used to
calculate the conductance of the system. The vertex corrections to the
conductivity were found with the use of "ladder" approximation combined with
the coherent-potential approximation (CPA) that allowed to consider the case of
strong electron scattering. It is shown that the Ward identity is satisfied in
the framework of this approximation that provides the necessary condition for a
conservation of a tunneling current. Based on the known results of ab-initio
calculations of the TMR for ballistic junctions, we assume that exchange split
quasi-free s-like electrons with the density of states being greater for the
majority spin sub-band give the main contribution to the TMR effect. We show
that, due to interfacial inter-band scattering, the TMR can be substantially
reduced even down to zero value. This is related to the fact that delocalized
quasi-free electrons can scatter into the strongly localized d sub-band with
the density of states at the Fermi energy being larger for minority spins
compared to majority spins. It is also shown that spin-flip electron scattering
on the surface magnons within the interface leads to a further decrease of the
TMR at finite temperature.Comment: REVTeX4, 20 pages, 9 figures, 1 table, submitted to Phys.Rev.B; In
Version 2 the text is substantially improved, the main results and
conclusions left the sam
Linewidths in bound state resonances for helium scattering from Si(111)-(1x1)H
Helium-3 spin-echo measurements of resonant scattering from the Si(111)–(1 × 1)H surface, in the energy range 4–14 meV, are presented. The measurements have high energy resolution yet they reveal bound state resonance features with uniformly broad linewidths. We show that exact quantum mechanical calculations of the elastic scattering, using the existing potential for the helium/Si(111)–(1 × 1)H interaction, cannot reproduce the linewidths seen in the experiment. Further calculations rule out inelastic and other mechanisms that might give rise to losses from the elastic scattering channels. We show that corrugation in the attractive part of the atom–surface potential is the most likely origin of the experimental lineshapes
Quantifying farm sustainability through the lens of ecological theory
The achievements of the Green Revolution in meeting the nutritional needs of a growing global population have been won at the expense of unintended consequences for the environment. Some of these negative impacts are now threatening the sustainability of food production through the loss of pollinators and natural enemies of crop pests, the evolution of pesticide resistance, declining soil health and vulnerability to climate change. In the search for farming systems that are sustainable both agronomically and environmentally, alternative approaches have been proposed variously called ‘agroecological’, ‘conservation agriculture’, ‘regenerative’ and ‘sustainable intensification’. While the widespread recognition of the need for more sustainable farming is to be welcomed, this has created etymological confusion that has the potential to become a barrier to transformation. There is a need, therefore, for objective criteria to evaluate alternative farming systems and to quantify farm sustainability against multiple outcomes. To help meet this challenge, we reviewed the ecological theories that explain variance in regulating and supporting ecosystem services delivered by biological communities in farmland to identify guiding principles for management change. For each theory, we identified associated system metrics that could be used as proxies for agroecosystem function. We identified five principles derived from ecological theory: (i) provide key habitats for ecosystem service providers; (ii) increase crop and non-crop habitat diversity; (iii) increase edge density: (iv) increase nutrient-use efficiency; and (v) avoid extremes of disturbance. By making published knowledge the foundation of the choice of associated metrics, our aim was to establish a broad consensus for their use in sustainability assessment frameworks. Further analysis of their association with farm-scale data on biological communities and/or ecosystem service delivery would provide additional validation for their selection and support for the underpinning theories
Observation of band structure and density of states effects in Co-based magnetic tunnel junctions
Utilizing Co/AlO/Co magnetic tunnel junctions (MTJs) with Co
electrodes of different crystalline phases, a clear relationship between
electrode structure and junction transport properties is presented. For
junctions with one fcc(111) textured and one polycrystalline (poly-phase and
poly-directional) Co electrode, a strong asymmetry is observed in the
magnetotransport properties, while when both electrodes are polycrystalline the
magnetotransport is essentially symmetric. These observations are successfully
explained within a model based on ballistic tunneling between the calculated
band structures (DOS) of fcc-Co and hcp-Co.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Let
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