97 research outputs found
Interaction between U/UO2 bilayers and hydrogen studied by in-situ X-ray diffraction
This paper reports experiments investigating the reaction of H with
uranium metal-oxide bilayers. The bilayers consist of 100 nm of
epitaxial -U (grown on a Nb buffer deposited on sapphire) with a
UO overlayer of thicknesses of between 20 and 80 nm. The oxides were made
either by depositing via reactive magnetron sputtering, or allowing the uranium
metal to oxidise in air at room temperature. The bilayers were exposed to
hydrogen, with sample temperatures between 80 and 200 C, and monitored via
in-situ x-ray diffraction and complimentary experiments conducted using
Scanning Transmission Electron Microscopy - Electron Energy Loss Spectroscopy
(STEM-EELS). Small partial pressures of H caused rapid consumption of the
U metal and lead to changes in the intensity and position of the diffraction
peaks from both the UO overlayers and the U metal. There is an
orientational dependence in the rate of U consumption. From changes in the
lattice parameter we deduce that hydrogen enters both the oxide and metal
layers, contracting the oxide and expanding the metal. The air-grown oxide
overlayers appear to hinder the H-reaction up to a threshold dose, but
then on heating from 80 to 140 C the consumption is more rapid than for the
as-deposited overlayers. STEM-EELS establishes that the U-hydride layer lies at
the oxide-metal interface, and that the initial formation is at defects or
grain boundaries, and involves the formation of amorphous and/or
nanocrystalline UH. This explains why no diffraction peaks from UH
are observed. {\textcopyright British Crown Owned Copyright 2017/AWE}Comment: Submitted for peer revie
The malleability of uranium: manipulating the charge-density wave in epitaxial films
We report x-ray synchrotron experiments on epitaxial films of uranium,
deposited on niobium and tungsten seed layers. Despite similar lattice
parameters for these refractory metals, the uranium epitaxial arrangements are
different and the strains propagated along the a-axis of the uranium layers are
of opposite sign. At low temperatures these changes in epitaxy result in
dramatic modifications to the behavior of the charge-density wave in uranium.
The differences are explained with the current theory for the electron-phonon
coupling in the uranium lattice. Our results emphasize the intriguing
possibilities of producing epitaxial films of elements that have complex
structures like the light actinides uranium to plutonium.Comment: 6 pages, 6 figure
A field induced modulated state in the ferromagnet PrPtAl
The theory of quantum order-by-disorder (QOBD) explains the formation of modulated magnetic states at the boundary between ferromagnetism and paramagnetism in zero field. PrPtAl has been argued to provide an archetype for this. Here, we report the phase diagram in magnetic field, applied along both the easy a axis and hard b axis. For field aligned to the b axis, we find that the magnetic transition temperatures are suppressed and at low temperature there is a single modulated fan state, separating an easy a axis ferromagnetic state from a field polarized state. This fan state is well explained with the QOBD theory in the presence of anisotropy and field. Experimental evidence supporting the QOBD explanation is provided by the large increase in the T^{2} coefficient of the resistivity and direct detection of enhanced magnetic fluctuations with inelastic neutron scattering, across the field range spanned by the fan state. This shows that the QOBD mechanism can explain field induced modulated states that persist to very low temperature
Field-Induced Modulated State in the Ferromagnet PrPtAl
The theory of quantum order-by-disorder (QOBD) explains the formation of modulated magnetic states at the boundary between ferromagnetism and paramagnetism in zero field. PrPtAl has been argued to provide an archetype for this. Here, we report the phase diagram in magnetic field, applied along both the easy a axis and hard b axis. For field aligned to the b axis, we find that the magnetic transition temperatures are suppressed and at low temperature there is a single modulated fan state, separating an easy a axis ferromagnetic state from a field polarized state. This fan state is well explained with the QOBD theory in the presence of anisotropy and field. Experimental evidence supporting the QOBD explanation is provided by the large increase in the T^{2} coefficient of the resistivity and direct detection of enhanced magnetic fluctuations with inelastic neutron scattering, across the field range spanned by the fan state. This shows that the QOBD mechanism can explain field induced modulated states that persist to very low temperature
Structural and magnetic properties of ErTbmultilayers
Abstract.: We have investigated the structural and magnetic properties of Er|Tb multilayers by different scattering methods. Diffuse X-ray scattering under grazing incidence reveals the interface structure in Er|Tb bilayers and trilayers, indicating vertically correlated roughness between the Er and Tb interfaces. The magnetic properties of ErnEr|TbnTb superlattices have been studied as a function of the superlattice composition (indices denote the number of atomic layers). Coupled ferromagnetic structures exist in all investigated samples. The phase transition temperature varies with the Tb layer thickness. Modulated magnetic order is short range for all samples beside the Er20|Tb5 superlattice, the sample with the smallest Tb layer thickness. We observe dipolar antiferromagnetic coupling between single ferromagnetic Tb layers in all samples, with the onset of this ordering depending on the Tb layer thickness. Due to competing interactions, exchange coupling is limited to the interface near region. Therefore long range modulated magnetic order is observed in the Er20|Tb5 superlattice only, where the interface regions overlap. The distinct differences to the magnetic structure of an Er0.8Tb0.2 alloy film are explained by a highly anisotropic arrangement of neighbouring atoms due to the correlated roughnes
Distinct order of Gd 4f and Fe 3d moments coexisting in GdFe4Al8
Single crystals of flux-grown tetragonal GdFe4Al8 were characterized by
thermodynamic, transport, and x-ray resonant magnetic scattering measurements.
In addition to antiferromagnetic order at TN ~ 155 K, two low-temperature
transitions at T1 ~ 21 K and T2 ~ 27 K were identified. The Fe moments order at
TN with an incommensurate propagation vector (tau,tau,0) with tau varying
between 0.06 and 0.14 as a function of temperature, and maintain this order
over the entire T<TN range. The Gd 4f moments order below T2 with a
ferromagnetic component mainly out of plane. Below T1, the ferromagnetic
components are confined to the crystallographic plane. Remarkably, at low
temperatures the Fe moments maintain the same modulation as at high
temperatures, but the Gd 4f moments apparently do not follow this modulation.
The magnetic phase diagrams for fields applied in [110] and [001] direction are
presented and possible magnetic structures are discussed.Comment: v2: 14 pages, 12 figures; PRB in prin
Antiferromagnetic order and domains in Sr3Ir2O7 probed by x-ray resonant scattering
This article reports a detailed x-ray resonant scattering study of the
bilayer iridate compound, Sr3Ir2O7, at the Ir L2 and L3 edges. Resonant
scattering at the Ir L3 edge has been used to determine that Sr3Ir2O7 is a
long-range ordered antiferromagnet below TN 230K with an ordering wavevector,
q=(1/2,1/2,0). The energy resonance at the L3 edge was found to be a factor of
~30 times larger than that at the L2. This remarkable effect has been seen in
the single layer compound Sr2IrO4 and has been linked to the observation of a
Jeff=1/2 spin-orbit insulator. Our result shows that despite the modified
electronic structure of the bilayer compound, caused by the larger bandwidth,
the effect of strong spin-orbit coupling on the resonant magnetic scattering
persists. Using the programme SARAh, we have determined that the magnetic order
consists of two domains with propagation vectors k1=(1/2,1/2,0) and
k2=(1/2,-1/2,0), respectively. A raster measurement of a focussed x-ray beam
across the surface of the sample yielded images of domains of the order of 100
microns size, with odd and even L components, respectively. Fully relativistic,
monoelectronic calculations (FDMNES), using the Green's function technique for
a muffin-tin potential have been employed to calculate the relative intensities
of the L2,3 edge resonances, comparing the effects of including spin-orbit
coupling and the Hubbard, U, term. A large L3 to L2 edge intensity ratio (~5)
was found for calculations including spin-orbit coupling. Adding the Hubbard,
U, term resulted in changes to the intensity ratio <5%.Comment: 9 pages, 9 figure
Simultaneous dynamic electrical and structural measurements of functional materials
A new materials characterization system developed at the XMaS beamline, located at the European Synchrotron Radiation Facility in France, is presented. We show that this new capability allows to measure the atomic structural evolution (crystallography) of piezoelectric materials whilst simultaneously measuring the overall strain characteristics and electrical response to dynamically (ac) applied external stimuli
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