659 research outputs found
Size Distribution of Superparamagnetic Particles Determined by Magnetic Sedimentation
We report on the use of magnetic sedimentation as a means to determine the
size distribution of dispersed magnetic particles. The particles investigated
here are i) single anionic and cationic nanoparticles of diameter D = 7 nm and
ii) nanoparticle clusters resulting from electrostatic complexation with
polyelectrolytes and polyelectrolyte-neutral copolymers. A theoretical
expression of the sedimentation concentration profiles at the steady state is
proposed and it is found to describe accurately the experimental data. When
compared to dynamic light scattering, vibrating sample magnetometry and
cryogenic transmission electron microscopy, magnetic sedimentation exhibits a
unique property : it provides the core size and core size distribution of
nanoparticle aggregates.Comment: 9 pages - templated ACS - 7 figures - 1 tabl
Magnetic and vibrational properties of high-entropy alloys
The magnetic properties of high-entropy alloys based on equimolar FeCoCrNi were investigated using vibrating sample magnetometry to determine their usefulness in high-temperature magnetic applications. Nuclear resonant inelastic x-ray scattering measurements were performed to evaluate the vibrational entropy of the ^(57)Fe atoms and to infer chemical order. The configurational and vibrational entropy of alloying are discussed as they apply to these high-entropy alloys
Effects of chemical composition and B2 order on phonons in bcc Fe–Co alloys
The phonon density of states (DOS) gives insight into interatomic forces and provides the vibrational entropy, making it a key thermodynamic function for understanding alloy phase transformations. Nuclear resonant inelastic x-ray scattering and inelastic neutron scattering were used to measure the chemical dependence of the DOS of bcc Fe–Co alloys. For the equiatomic alloy, the A2→B2 (chemically disordered→chemically ordered) phase transformation caused measurable changes in the phonon spectrum. The measured change in vibrational entropy upon ordering was −0.02±0.02 k_B/atom, suggesting that vibrational entropy results in a reduction in the order–disorder transition temperature by 60±60 K. The Connolly–Williams cluster inversion method was used to obtain interaction DOS (IDOS) curves that show how point and pair variables altered the phonon DOS of disordered bcc Fe–Co alloys. These IDOS curves accurately captured the change in the phonon DOS and vibrational entropy of the B2 ordering transition
Positive Vibrational Entropy of Chemical Ordering in FeV
Inelastic neutron scattering and nuclear resonant inelastic x-ray scattering were used to measure phonon spectra of FeV as a B2 ordered compound and as a bcc solid solution. The two data sets were combined to give an accurate phonon density of states, and the phonon partial densities of states for V and Fe atoms. Contrary to the behavior of ordering alloys studied to date, the phonons in the B2 ordered phase are softer than in the solid solution. Ordering increases the vibrational entropy by +0.22±0.03k_B/atom, which stabilizes the ordered phase to higher temperatures. First-principles calculations show that the number of electronic states at the Fermi level increases upon ordering, enhancing the screening between ions, and reducing the interatomic force constants. The effect of screening is larger at the V atomic sites than at the Fe atomic sites
Absence of long-range chemical ordering in equimolar FeCoCrNi
Equimolar FeCoCrNi alloys have been the topic of recent research as "high-entropy alloys," where the name is derived from the high configurational entropy of mixing for a random solid solution. Despite their name, no systematic study of ordering in this alloy system has been performed to
date. Here, we present results from anomalous x-ray scattering and neutron scattering on quenched and annealed samples. An alloy of FeNi_3 was prepared in the same manner to act as a control. Evidence of long-range chemical ordering is clearly observed in the annealed FeNi_3 sample from both experimental techniques. The FeCoCrNi sample given the same heat treatment lacks long-range chemical order
Mass resolution optimization in a large isotopic composition experiment
A range-energy experiment was built to measure the isotopic composition of galactic cosmic rays. An enrichment of neutron rich isotopes, 22Ne and (25Mg + 26Mg) in particular, when compared to the solar composition is shown. A rich statistics measurement of these and other neutron-rich isotopes in the galactic flux yields information to the source of these particles. A computer simulation of the experiment was used to estimate the instrument resolution. The Cherenkov detector light collection efficiency, was calculated. Absorption of light in the radiator was considered to determine the optimum Cherenkov medium thickness. The experiment will determine the isotopic composition for the elements neon through argon in the energy range 300 to 800 MeV per nucleon
Electromagnetically induced transparency in cold 85Rb atoms trapped in the ground hyperfine F = 2 state
We report electromagnetically induced transparency (EIT) in cold 85Rb atoms,
trapped in the lower hyperfine level F = 2, of the ground state 5
(Tiwari V B \textit{et al} 2008 {\it Phys. Rev.} A {\bf 78} 063421). Two steady
state -type systems of hyperfine energy levels are investigated using
probe transitions into the levels F = 2 and F = 3 of the
excited state 5 in the presence of coupling transitions F = 3
F = 2 and F = 3 F = 3, respectively. The
effects of uncoupled magnetic sublevel transitions and coupling field's Rabi
frequency on the EIT signal from these systems are studied using a simple
theoretical model.Comment: 12 pages, 7 figure
Phonons and elasticity of cementite through the Curie temperature
Phonon partial densities of states (pDOS) of ^(57)Fe_3 C were measured from cryogenic temperatures through the Curie transition at 460 K using nuclear resonant inelastic x-ray scattering. The cementite pDOS reveal that low-energy acoustic phonons shift to higher energies (stiffen) with temperature before the magnetic transition. This unexpected stiffening suggests strongly nonharmonic vibrational behavior that impacts the thermodynamics and elastic properties of cementite. Density functional theory calculations reproduced the anomalous stiffening observed experimentally in cementite by accounting for phonon-phonon interactions at finite temperatures. The calculations show that the low-energy acoustic phonon branches with polarizations along the [010] direction are largely responsible for the anomalous thermal stiffening. The effect was further localized to the motions of the Fe_(II) site within the orthorhombic structure, which participates disproportionately in the anomalous phonon stiffening
Temperature and Pressure Dependence of the Fe-specific Phonon Density of States in Ba(Fe(1-x)Co(x))2As2
The {57}Fe-specific phonon density of states of Ba(Fe(1-x)Co(x))2As2 single
crystals (x=0.0, 0.08) was measured at cryogenic temperatures and at high
pressures with nuclear-resonant inelastic x-ray scattering. Measurements were
conducted for two different orientations of the single crystals, yielding the
orientation-projected {57}Fe-phonon density of states (DOS) for phonon
polarizations in-plane and out-of-plane with respect to the basal plane of the
crystal structure. In the tetragonal phase at 300 K, a clear stiffening was
observed upon doping with Co. Increasing pressure to 4 GPa caused a marked
increase of phonon frequencies, with the doped material still stiffer than the
parent compound. Upon cooling, both the doped and undoped samples showed a
stiffening, and the parent compound exhibited a discontinuity across the
magnetic and structural phase transition. These findings are generally
compatible with the changes in volume of the system upon doping, increasing
pressure, or increasing temperature, but an extra softening of high-energy
modes occurs with increasing temperature. First-principles computations of the
phonon DOS were performed and showed an overall agreement with the experimental
results, but underestimate the Grueneisen parameter. This discrepancy is
explained in terms of a magnetic Grueneisen parameter, causing an extra phonon
stiffening as magnetism is suppressed under pressure
Temperature and pressure dependence of the Fe-specific phonon density of states in Ba(Fe_(1−x)Co_x)_2As_2
The ^(57)Fe-specific phonon density of states (DOS) of Ba(Fe_(1−x)Co_x)_2As_2 single crystals (x=0.0,0.08) was measured at cryogenic temperatures and at high pressures with nuclear-resonant inelastic x-ray scattering. Measurements were conducted for two different orientations of the single crystals, yielding the orientation-projected ^(57)Fe-phonon density of states for phonon polarizations in-plane and out-of-plane with respect to the basal plane of the crystal structure. In the tetragonal phase at 300 K, a clear stiffening was observed upon doping with Co. Increasing pressure to 4 GPa caused a marked increase of phonon frequencies, with the doped material still stiffer than the parent compound. Upon cooling, both the doped and undoped samples showed a stiffening and the parent compound exhibited a discontinuity across the magnetic and structural phase transitions. These findings are generally compatible with the changes in volume of the system upon doping, increasing pressure, or increasing temperature, but an extra softening of high-energy modes occurs with increasing temperature. First-principles computations of the phonon DOS were performed and showed an overall agreement with the experimental results, but underestimate the Grüneisen parameter. This discrepancy is explained in terms of a magnetic Grüneisen parameter, causing an extra phonon stiffening as magnetism is suppressed under pressure
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