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2002-05-03 | Studies of Vibrational Properties in Ga Stabilized d-Pu by Extended X-ray Absorption Fine Structure | Temperature dependent extended x-ray absorption fine structure (EXAFS)
spectra were measured for a 3.3 at% Ga stabilized Pu alloy over the range T= 20
- 300 K at both the Ga K-edge and the Pu L_III-edge. The temperature dependence
of the pair-distance distribution widths, \sigma(T) was accurately modeled
using a correlated-Debye model for the lattice vibrational properties,
suggesting Debye-like behavior in this material. We obtain pair- specific
correlated-Debye temperatures, \Theta_cD, of 110.7 +/- 1.7 K and 202.6 +/- 3.7
K, for the Pu-Pu and Ga-Pu pairs, respectively. These results represent the
first unambiguous determination of Ga-specific vibrational properties in PuGa
alloys, and indicate the Ga-Pu bonds are significantly stronger than the Pu-Pu
bonds. This effect has important implications for lattice stabilization
mechanisms in these alloys. | 0205074v1 |
2002-05-23 | Epitaxial growth in dislocation-free strained alloy films: Morphological and compositional instabilities | The mechanisms of stability or instability in the strained alloy film growth
are of intense current interest to both theorists and experimentalists. We
consider dislocation-free, coherent, growing alloy films which could exhibit a
morphological instability without nucleation. We investigate such strained
films by developing a nonequilibrium, continuum model and by performing a
linear stability analysis. The couplings of film-substrate misfit strain,
compositional stress, deposition rate, and growth temperature determine the
stability of film morphology as well as the surface spinodal decomposition. We
consider some realistic factors of epitaxial growth, in particular the
composition dependence of elastic moduli and the coupling between top surface
and underlying bulk of the film. The interplay of these factors leads to new
stability results. In addition to the stability diagrams both above and below
the coherent spinodal temperature, we also calculate the kinetic critical
thickness for the onset of instability as well as its scaling behavior with
respect to misfit strain and deposition rate. We apply our results to some real
growth systems and discuss the implications related to some recent experimental
observations. | 0205494v1 |
2002-05-28 | The role of the alloy structure in the magnetic behavior of granular systems | The effect of grain size, easy magnetization axis and anisotropy constant
distributions in the irreversible magnetic behavior of granular alloys is
considered. A simulated granular alloy is used to provide a realistic grain
structure for the Monte Carlo simulation of the ZFC-FC curves. The effect of
annealing and external field is also studied. The simulation curves are in good
agreement with the FC and ZFC magnetization curves measured on melt spun Cu-Co
ribbons. | 0205575v1 |
2002-07-25 | Ordering kinetics in an fcc A_3B binary alloy model: Monte Carlo studies | Using an atom-vacancy exchange algorithm, we investigate the kinetics of the
order-disorder transition in an fcc A_3B binary alloy model following a
temperature quench from the disordered phase. We observe two clearly distinct
ordering scenarios depending on whether the final temperature T_f falls above
or below the ordering spinodal T_{sp}, which is deduced from simulations at
equilibrium. For shallow quenches (T_f>T_{sp}) we identify an incubation time
tau_{inc} which characterizes the onset of ordering through the formation of
overcritical ordered nuclei. The algorithm we use together with experimental
information on tracer diffusion in Cu_3Au alloys allows us to estimate the
physical time scale connected with tau_{inc} in that material. Deep quenches,
T_f<T_{sp}, result in spinodal ordering. Coarsening processes at long times
proceed substantially slower than predicted by the Lifshitz-Allen-Cahn t^{1/2}
law. Structure factors related to the geometry of the two types of domain walls
that appear in our model are found to be consistent with Porod's law in one and
two dimensions. | 0207611v1 |
2002-07-31 | Electronic structure of GaAs1-xNx alloy by soft-X-ray absorption and emission: Origin of the reduced optical efficiency | The local electronic structure of N atoms in a diluted GaAs1-xNx (x=3%)
alloy, in view of applications in optoelectronics, is determined for the first
time using soft-X-ray absorption (SXA) and emission (SXE). Deviations from
crystalline GaN, in particular in the conduction band, are dramatic. Employing
the orbital character and elemental specificity of the SXE/SXA spectroscopies,
we identify a charge transfer from the N atoms at the valence band maximum,
reducing the overlap with the wavefunction in conduction band minimum, as the
main factor limiting the optical efficiency of GaAs1-xNx alloys. Moreover, a
k-conserving process of resonant inelastic x-ray scattering involving the L1
derived valence and conduction states is discovered. | 0207738v1 |
2002-08-03 | Relativity and Magnetism in Ni-Pd and Ni-Pt Alloys | We show that the differences in the magnetic properties of Ni-Pd and Ni-Pt
alloys arise mainly due to relativity. In particular, we find that the local
magnetic moment of Ni increases with the addition of Pd in Ni-Pd while it
decreases with the addition of Pt in Ni-Pt, as found experimentally, only if
relativity is present. Our analysis is based on the effects of relativity on
(i) the spin-polarized densities of states of Ni, (ii) the splitting of
majority and minority spin d-band centers of Ni, and (iii) the separation
between s-d band centers of Pd and Pt in Ni-Pd and Ni-Pt alloys. | 0208062v2 |
2002-08-08 | Dynamic structure in a molten binary alloy by ab initio Molecular Dynamics: Crossover from Hydrodynamics to the Microscopic Regime | The dynamic structure factor of the 7Li0.61Na0.39 liquid alloy at T=590 K has
been calculated by ab initio molecular dynamics simulations using 2000
particles. For small wavevectors, 0.15 <= q/A-1 <= 1.6, we find clear side
peaks in the partial dynamic structure factors. Whereas for q <= 0.25 A-1 the
peak frequencies correspond to the hydrodynamic sound dispersion of the binary
alloy, for greater q values we obtain two modes with phase velocities above and
below the hydrodynamic sound. A smooth transition between hydrodynamic sound
and the two collective modes is shown to take place in the range 0.25 <= q/A-1
<= 0.35. The mass ratio in this system, mNa/mLi = 3, is the smallest one so far
for which the fast mode is observed. We also predict that inelastic X-ray
scattering experiments would be able to detect the slow mode, and explain why
the inelastic neutron scattering experiments [P.R. Gartyrell-Mills et al,
Physica B 154, 1 (1988)] do not show any of these modes. | 0208158v1 |
2002-09-25 | A Current Induced Transition in atomic-sized contacts of metallic Alloys | We have measured conductance histograms of atomic point contacts made from
the noble-transition metal alloys CuNi, AgPd, and AuPt for a concentration
ratio of 1:1. For all alloys these histograms at low bias voltage (below 300
mV) resemble those of the noble metals whereas at high bias (above 300 mV) they
resemble those of the transition metals. We interpret this effect as a change
in the composition of the point contact with bias voltage. We discuss possible
explanations in terms of electromigration and differential diffusion induced by
current heating. | 0209577v3 |
2002-11-27 | Do we understand electron correlation effects in Gadolinium based intermetallic compounds? | Recognising the difficulties in systematic understanding of the physical
characteristics of strongly correlated f-electron systems, we considered it
worthwhile to subject the so-called "normal" f-electron systems like those of
Gd to careful investigations. We find that the spin-disorder contribution to
electrical resistivity ($\rho$) in the paramagnetic state, instead of remaining
contant, surprisingly increases with decreasing temperature (T) in the
paramagnetic state in some of the Gd alloys. In some cases, this "excess
resistance" is so large that a distinct minimum in the plot of $\rho$ versus T
can be seen, mimicking the behaviour of Kondo lattices. This excess resistance
can be suppressed by the application of a magnetic field, naturally resulting
in large magnetoresistance. In addition, these alloys are found to exhibit
heavy-fermion-like heat-capacity behavior. These unusual findings imply
hither-to-unexplored electron correlation effects even in Gd-based alloys. | 0211625v1 |
2003-01-13 | First-Principles Investigation of Perfect and Diffuse Anti-Phase Boundaries in HCP-Based Ti-Al Alloys | First-principles thermodynamic models based on the cluster expansion
formalism, monte-carlo simulations and quantum-mechanical total energy
calculations are employed to compute short-range-order parameters and
diffuse-antiphase-boundary energies in hcp-based $\alpha$-Ti-Al alloys. Our
calculations unambiguously reveal a substantial amount of SRO is present in
$\alpha$-Ti-6 Al and that, at typical processing temperatures concentrations,
the DAPB energies associated with a single dislocation slip can reach 25
mJ/m$^{2}$. We find very little anisotropy between the energies of DAPBs lying
in the basal and prism planes. Perfect antiphase boundaries in DO$_{19}$
ordered Ti$_3$Al are also investigated and their interfacial energies,
interfacial stresses and local displacements are calculated from first
principles through direct supercell calculations. Our results are discussed in
light of mechanical property measurements and deformation microstructure
strudies in $\alpha$ Ti-Al alloys. | 0301199v1 |
2003-02-18 | Entropy change at the martensitic transformation in ferromagnetic shape memory alloys Ni_{2+x}Mn_{1-x}Ga | The entropy change $\Delta S$ between the high-temperature cubic phase and
the low-temperature tetragonally-based martensitic phase of
Ni$_{2+x}$Mn$_{1-x}$Ga ($x = 0 - 0.20$) alloys was studied. The experimental
results obtained indicate that $\Delta S$ in the Ni$_{2+x}$Mn$_{1-x}$Ga alloys
increases with the Ni excess $x$. The increase of $\Delta S$ is presumably
accounted for by an increase of magnetic contribution to the entropy change. It
is suggested that the change in modulation of the martensitic phase of
Ni$_{2+x}$Mn$_{1-x}$Ga results in discontinuity of the composition dependence
of $\Delta S$. | 0302352v1 |
2003-04-18 | Theoretical Study of Electronic Structure and Superconductivity in Nb_(1-x)B_2 Alloys | Using the Korringa-Kohn-Rostoker coherent-potential approximation in the
atomic-sphere approximation (KKR-ASA CPA) we have studied the changes in the
electronic structure and the superconducting transition temperature T_{c} in
Nb_{1-x}B_{2} alloys as a function of x. We find that the variation in the
electronic structure of Nb_{1-x}B_{2} alloys as a function of x is consistent
with the rigid-band model. However, the variation of T_{c}, obtained using the
Allen-Dynes equation within the Gaspari-Gyorffy formalism to estimate the
electron-phonon matrix elements, does not follow the expected trend. We
associate this disagreement to the use of a constant \omega_{rms} in the
Allen-Dynes equation over the whole range of vacancy concentration, thereby
indicating the importance of lattice dynamical effects in these systems. | 0304437v1 |
2003-07-14 | Interplay between disorder, quantum and thermal fluctuations in ferromagnetic alloys: The case of UCu2Si(2-x)Ge(x) | We consider, theoretically and experimentally, the effects of structural
disorder, quantum and thermal fluctuations in the magnetic and transport
properties of certain ferromagnetic alloys.We study the particular case of
UCu2Si(2-x)Ge(x). The low temperature resistivity, rho(T,x), exhibits Fermi
liquid (FL) behavior as a function of temperature T for all values of x, which
can be interpreted as a result of the magnetic scattering of the conduction
electrons from the localized U spins. The residual resistivity, rho(0,x),
follows the behavior of a disordered binary alloy. The observed non-monotonic
dependence of the Curie temperature, Tc(x), with x can be explained within a
model of localized spins interacting with an electronic bath whose transport
properties cross-over from ballistic to diffusive regimes. Our results clearly
show that the Curie temperature of certain alloys can be enhanced due to the
interplay between quantum and thermal fluctuations with disorder. | 0307328v2 |
2003-09-03 | Charge Distributions in Metallic Alloys: a Charge Excess Functional theory approach | The distribution of local charge excesses (DLC) in metallic alloys,
previously obtained as a result of the analysis of order N electronic structure
calculations, is derived from a variational principle. A phenomenological
Charge Excess Functional (CEF) theory is obtained which is determined by three
concentration dependent, material specific, parameters that can be obtained
from {\it ab initio} calculations. The theory requires modest computational
efforts and reproduces with an excellent accuracy the DLC and the electrostatic
energies of ordered, substitutionally disordered or segregating metallic alloys
and, hence, can be considered an efficient approach alternative to conventional
electronic structure calculations. The substantial reduction of computing time
opens new perspectives for the understanding of metallic systems and their
mechanical properties. | 0309088v2 |
2003-09-17 | Effects of atomic short-range order on the properties of perovskite alloys in their morphotropic phase boundary | The effects of atomic short-range order on the properties of
Pb(Zr_{1-x}Ti_x)O_3 alloy in its morphotropic phase boundary (MPB) are
predicted by combining first-principles-based methods and annealing techniques.
Clustering is found to lead to a compositional expansion of this boundary,
while the association of unlike atoms yields a contraction of this region.
Atomic short-range order can thus drastically affect properties of perovskite
alloys in their MPB, by inducing phase transitions. Microscopic mechanisms
responsible for these effects are revealed and discussed. | 0309413v1 |
2003-10-29 | Pareto-optimal alloys | Large databases that can be used in the search for new materials with
specific properties remain an elusive goal in materials science. The search
problem is complicated by the fact that the optimal material for a given
application is usually a compromise between a number of materials properties
and the price. In this letter we present a database consisting of the lattice
parameters, bulk moduli, and heats of formation for over 64,000 ordered
metallic alloys, which has been established by direct first-principles
density-functional-theory calculations. Furthermore, we use a concept from
economic theory, the Pareto-optimal set, to determine optimal alloy solutions
for the compromise between low compressibility, high stability and price. | 0310709v1 |
2003-12-25 | Influence of Fe and Co on Phase Transitions in Ni-Mn-Ga Alloys | Differential scanning calorimetry (DSC) and magnetic measurements were
performed to study the influence of ferromagnetic 3-d transition elements Fe
and Co on structural and magnetic properties of ferromagnetic shape memory
alloys Ni2MnGa. Addition of Fe or Co on the Ni sites decreases the temperature
of martensitic phase transition Tm, whereas addition of Co on the Mn sites
results in a considerable increase of Tm. Magnetic measurement revealed that
Curie temperature TC increases upon substitution of Fe or Co for Ni. This
observation is of importance for design of high temperature ferromagnetic shape
memory alloys. | 0312639v1 |
2003-12-26 | Breakdown by a magnetic field of the superconducting fluctuations in the normal state in Pb_{1-x}In_x alloys | The effects induced on the magnetization by coherent fluctuating Cooper pairs
in the normal state have been measured in Pb_{1-x}In_x alloys up to high
magnetic fields, of amplitudes above Hc2(0), the upper critical field
extrapolated to T=0K. Our results show that in dirty alloys these
superconducting fluctuation effects are, in the entire H-T phase diagram above
Hc2(T), independent of the amount of impurities and that they vanish when
H~1.1Hc2(0). These striking results are consistent with a phenomenological
estimate that takes into account the limits imposed by the uncertainty
principle to the shrinkage, when H increases, of the superconducting wave
function. | 0312647v1 |
2004-07-20 | Inelastic neutron scattering in random binary alloys : an augmented space approach | Combining the augmented space representation for phonons with a generalized
version of Yonezawa-Matsubara diagrammatic technique, we have set up a
formalism to seperate the coherent and incoherent part of the total intensity
of thermal neutron scattering from disordered alloys. This is done exacly
without taking any recourse to mean-field like approximation (as done
previously). The formalism includes disorder in masses, force constants and
scattering lengths. Implementation of the formalism to realistic situations is
performed by an augmented space Block recursion which calculates entire Green
matrix and self energy matrix which in turn is needed to evaluate the coherent
and incoherent intensities. we apply the formalism to NiPd and NiPt alloys.
Numerical results on coherent and incoherent scattering cross sections are
presented along the highest symmetry directions. Finally the incoherent
intensities are compared with the CPA and also with experiments. | 0407512v1 |
2004-09-01 | Cluster coherent potential approximation for electronic structure of disordered alloys | We extend the single-site coherent potential approximation (CPA) to include
the effects of non-local disorder correlations (alloy short-range order) on the
electronic structure of random alloy systems. This is achieved by mapping the
original Anderson disorder problem to that of a selfconsistently embedded
cluster. This cluster problem is then solved using the equations of motion
technique. The CPA is recovered for cluster size $N_{c}=1$, and the disorder
averaged density-of-states (DOS) is always positive definite. Various new
features, compared to those observed in CPA, and related to repeated scattering
on pairs of sites, reflecting the effect of SRO are clearly visible in the DOS.
It is explicitly shown that the cluster-CPA method always yields
positive-definite DOS. Anderson localization effects have been investigated
within this approach. In general, we find that Anderson localization sets in
before band splitting occurs, and that increasing partial order drives a
continuous transition from an Anderson insulator to an incoherent metal. | 0409031v1 |
2004-12-06 | Resonant X-Ray Scattering from the Surface of a Dilute Hg-Au Alloy | We present the first resonant x-ray reflectivity measurements from a liquid
surface. The surface structure of the liquid Hg-Au alloy system just beyond the
solubility limit of 0.14at% Au in Hg had previously been shown to exhibit a
unique surface phase characterized by a low-density surface region with a
complicated temperature dependence. In this paper we present reflectivity
measurements near the Au LIII edge, for 0.2at% Au in Hg at room temperature.
The data are consistent with a concentration of Au in the surface region that
can be no larger than about 30at%. These results rule out previous suggestions
that pure Au layers segregate at the alloy surface. | 0412113v1 |
2005-02-02 | Scanning Tunneling Spectroscopy of the superconducting proximity effect in a diluted ferromagnetic alloy | We studied the proximity effect between a superconductor (Nb) and a diluted
ferromagnetic alloy (CuNi) in a bilayer geometry. We measured the local density
of states on top of the ferromagnetic layer, which thickness varies on each
sample, with a very low temperature Scanning Tunneling Microscope. The measured
spectra display a very high homogeneity. The analysis of the experimental data
shows the need to take into account an additional scattering mechanism. By
including in the Usadel equations the effect of the spin relaxation in the
ferromagnetic alloy, we obtain a good description of the experimental data. | 0502050v2 |
2005-05-26 | Inverse magnetocaloric effect in ferromagnetic Ni-Mn-Sn alloys | The magnetocaloric effect (MCE) in paramagnetic materials has been widely
used for attaining very low temperatures by applying a magnetic field
isothermally and removing it adiabatically. The effect can be exploited also
for room temperature refrigeration by using recently discovered giant MCE
materials. In this letter, we report on an inverse situation in Ni-Mn-Sn
alloys, whereby applying a magnetic field adiabatically, rather than removing
it, causes the sample to cool. This has been known to occur in some
intermetallic compounds, for which a moderate entropy increase can be induced
when a field is applied, thus giving rise to an inverse magnetocaloric effect.
However, the entropy change found for some ferromagnetic Ni-Mn-Sn alloys is
just as large as that reported for giant MCE materials, but with opposite sign.
The giant inverse MCE has its origin in a martensitic phase transformation that
modifies the magnetic exchange interactions due to the change in the lattice
parameters. | 0505652v1 |
2005-09-09 | Electrons and phonons in the ternary alloy CaAl$_{2-x}$Si$_x$} as a function of composition | We report a detailed first-principles study of the structural, electronic and
vibrational properties of the superconducting C$_{32}$ phase of the ternary
alloy CaAl$_{2-x}$Si$_x$, both in the experimental range $0.6 \leq x \leq 1.2$,
for which the alloy has been synthesised, and in the theoretical limits of high
aluminium and high silicon concentration. Our results indicate that, in the
experimental range, the dependence of the electronic bands on composition is
well described by a rigid-band model, which breaks down outside this range.
Such a breakdown, in the (theoretical) limit of high aluminium concentration,
is connected to the appearance of vibrational instabilities, and results in
important differences between CaAl$_2$ and MgB$_2$. Unlike MgB$_2$, the
interlayer band and the out-of-plane phonons play a major role on the stability
and superconductivity of CaAlSi and related C$_{32}$ intermetallic compounds. | 0509242v1 |
2005-09-11 | Superconductivity in Metal-mixed Ion-Implanted Polymer Films | Ion-implantation of normally insulating polymers offers an alternative to
depositing conjugated organics onto plastic films to make electronic circuits.
We used a 50 keV nitrogen ion beam to mix a thin 10 nm Sn/Sb alloy film into
the sub-surface of polyetheretherketone (PEEK) and report the low temperature
properties of this material. We observed metallic behavior, and the onset of
superconductivity below 3 K. There are strong indications that the
superconductivity does not result from a residual thin-film of alloy, but
instead from a network of alloy grains coupled via a weakly conducting,
ion-beam carbonized polymer matrix. | 0509278v2 |
2005-09-20 | Giant Spin-splitting in the Bi/Ag(111) Surface Alloy | Surface alloying is shown to produce electronic states with a very large
spin-splitting. We discuss the long range ordered bismuth/silver(111) surface
alloy where an energy bands separation of up to one eV is achieved. Such strong
spin-splitting enables angular resolved photoemission spectroscopy to directly
observe the region close to the band edge, where the density of states shows
quasi-one dimensional behavior. The associated singularity in the local density
of states has been measured by low temperature scanning tunneling spectroscopy.
The implications of this new class of materials for potential spintronics
applications as well as fundamental issues are discussed. | 0509509v1 |
2005-11-11 | Alloy disorder effects on the room temperature optical properties of GaInNAs quantum wells | The effect of alloy disorder on the optical density of states and the average
room temperature carrier statistics in GaInNAs quantum wells is discussed. A
red shift between the peak of the room temperature photoluminescence and the
surface photovoltage spectra, that systematically increases with the nitrogen
content within the quantum wells is observed. The relationship between this
Stokes' shift and the absorption linewidth in different samples suggests that
the photoexcited carriers undergo a continuous transition, from being in
quasi-thermal equilibrium with the lattice to being completely trapped by the
quantum dot-like potential fluctuations, as the nitrogen fraction in the alloy
is increased. The values of the 'electron temperature' inferred from the
photoluminescence spectra are found to be consistent with this interpretation. | 0511282v1 |
2006-01-10 | Engineering of spin-lattice relaxation dynamics by digital growth of diluted magnetic semiconductor CdMnTe | The technological concept of "digital alloying" offered by molecular-beam
epitaxy is demonstrated to be a very effective tool for tailoring static and
dynamic magnetic properties of diluted magnetic semiconductors. Compared to
common "disordered alloys" with the same Mn concentration, the spin-lattice
relaxation dynamics of magnetic Mn ions has been accelerated by an order of
magnitude in (Cd,Mn)Te digital alloys, without any noticeable change in the
giant Zeeman spin splitting of excitonic states, i.e. without effect on the
static magnetization. The strong sensitivity of the magnetization dynamics to
clustering of the Mn ions opens a new degree of freedom for spin engineering. | 0601194v1 |
2006-02-10 | Kinetic stabilization of Fe film on (4 by 2)-GaAs(100) | We grow Fe film on (4 by 2)-GaAs(100) at low temperature, (~ 130 K) and study
their chemical structure by photoelectron spectroscopy using synchrotron
radiation. We observe the effective suppression of As segregation and
remarkable reduction of alloy formation near the interface between Fe and
substrate. Hence, this should be a way to grow virtually pristine Fe film on
GaAs(100). Further, the Fe film is found stable against As segregation even
after warmed up to room temperature. There only forms very thin, ~ 8 angstrom
thick interface alloy. It is speculated that the interface alloy forms via
surface diffusion mediated by interface defects formed during the low
temperature growth of the Fe film. Further out-diffusion of both Ga and As are
suppressed because it should then proceed via inefficient bulk diffusion. | 0602270v1 |
2006-03-06 | Kinetic stabilization of Fe film on GaAs(100): An in situ x-ray reflectivity Study | We study the growth of the Fe films on GaAs(100) at a low temperature, 140 K,
by $in$-$situ$ UHV x-ray reflectivity using synchrotron radiation. We find
rough surface with the growth exponent, $\beta_S$ = 0.51$\pm$0.04. This
indicates that the growth of the Fe film proceeds via the restrictive
relaxation due to insufficient thermal diffusion of the adatoms. The XRR curves
are nicely fit by a model with a uniform Fe film, implying that the surface
segregation and interface alloying of both Ga and As are negligible. When the
Fe film is annealed to 300 K, however, the corresponding XRR can be fit only
after including an additional layer of 9 A thickness between the Fe film and
the substrate, indicating the formation of ultrathin alloy near the interface.
The confinement of the alloy near the interface derives from the fact that the
diffusion of Ga and As from the substrate should proceed via the inefficient
bulk diffusion, and hence the overlying Fe film is kinetically stabilized. | 0603123v1 |
2006-04-27 | Effect of heavy ion irradiation on microstructure and electron density distribution of zirconium alloy characterised by X-ray diffraction technique | Different techniques of the X-ray Diffraction Line Profile Analysis (XRDLPA)
have been used to assess the microstructure of the irradiated
Zr-1.0%Nb-1.0%Sn-0.1%Fe alloy. The domain size, microstrain, density of
dislocation and the stacking fault probabilities of the irradiated alloy have
been estimated as a function of dose by the Williamson-Hall Technique, Modified
Rietveld Analysis and the Double Voigt Method. A clear signature in the
increase in the density of dislocation with the dose of irradiated was
revealed. The analysis also estimated the average density of dislocation in the
major slip planes after irradiation. For the first time, we have established
the changes in the electron density distribution due to irradiation by X-ray
diffraction technique. We could estimate the average displacement of the atoms
and the lattice strain caused due to irradiation from the changes in the
electron density distribution as observed in the contour plots. | 0604614v1 |
2006-07-28 | Effect of short-range order on the electronic structure and optical properties of the CuZn alloy : an augmented space approach | In this work we have combined the generalized augmented space method
introduced by one of us with the recursion method of Haydock et al (GASR),
within the framework of the local density functional based linear muffin-tin
orbitals basis (TB-LMTO). Using this we have studied the effect of short-range
ordering and clustering on the density of states, optical conductivity and
reflectivity of 50-50 CuZn alloys. Our results are in good agreement with
alternative techniques. We argue that the TB-LMTO-GASR is a feasible, efficient
and quantitatively accurate computational technique for the study of
environmental effects in disordered binary alloys. | 0607767v1 |
2006-08-14 | Thermal relaxation of magnetic clusters in amorphous Hf_{57}Fe_{43} alloy | The magnetization processes in binary magnetic/nonmagnetic amorphous alloy
Hf_{57}Fe_{43} are investigated by the detailed measurements of magnetic
hysteresis loops, temperature dependence of magnetization, relaxation of
magnetization and magnetic ac susceptibility, including a nonlinear term.
Blocking of magnetic moments at lower temperatures is accompanied with the slow
relaxation of magnetization and magnetic hysteresis loops. All of the observed
properties are explained with the superparamagnetic behaviour of the single
domain magnetic clusters inside the nonmagnetic host, their blocking by the
anisotropy barriers and thermal fluctuation over the barriers accompanied by
relaxation of magnetization. From magnetic viscosity analysis based on thermal
relaxation over the anisotropy barriers it is found out that magnetic clusters
occupy the characteristic volume from 25 up to 200 nm3 . The validity of the
superparamagnetic model of Hf_{57}Fe_{43} is based on the concentration of iron
in the Hf_{100-x}Fe_{43} system that is just below the threshold for the long
range magnetic ordering. This work throws more light on magnetic behaviour of
other amorphous alloys, too. | 0608307v3 |
2006-10-07 | Study on the Formation of Nano tau 3 Phase by Mechanical Alloying | In the present investigation the pure elemental powder mixture of Al (70 at%)
Ni (15 at %), Cu (15 at %) was mechanically milled in an attritor ball mill for
10, 20, 40, 60, 80 and 100 hours in hexane medium at 400 rpm. The isothermal
annealing of 100 h mechanically milled powder has been done at 700 0C for 20,
40 and 60 hours. The mechanically alloyed powders are characterized using X-ray
diffraction, differential thermal analysis and transmission electron microscopy
techniques. It was observed that mechanical alloying led to the formation of
nano tau 3 phases in Al70 Cu15Ni15 after 80 h of milling. In the case of 100 h
MM and subsequent annealing at 700 0 C for 20, 40 and 60 h, powder exhibited
the formation of tau 3 phases with bigger grain sizes. The phase formation and
transformations in the above systems have been discussed based. | 0610202v1 |
2006-10-23 | Influence of external magnetic fields on growth of alloy nanoclusters | Kinetic Monte Carlo simulations are performed to study the influence of
external magnetic fields on the growth of magnetic fcc binary alloy
nanoclusters with perpendicular magnetic anisotropy. The underlying kinetic
model is designed to describe essential structural and magnetic properties of
CoPt_3-type clusters grown on a weakly interacting substrate through molecular
beam epitaxy. The results suggest that perpendicular magnetic anisotropy can be
enhanced when the field is applied during growth. For equilibrium bulk systems
a significant shift of the onset temperature for L1_2 ordering is found, in
agreement with predictions from Landau theory. Stronger field induced effects
can be expected for magnetic fcc-alloys undergoing L1_0 ordering. | 0610589v1 |
2006-10-23 | Modified embedded-atom method interatomic potentials for the Mg-Al alloy system | We developed new modified embedded-atom method (MEAM) interatomic potentials
for the Mg-Al alloy system using a first-principles method based on density
functional theory (DFT). The materials parameters, such as the cohesive energy,
equilibrium atomic volume, and bulk modulus, were used to determine the MEAM
parameters. Face-centered cubic, hexagonal close packed, and cubic rock salt
structures were used as the reference structures for Al, Mg, and MgAl,
respectively. The applicability of the new MEAM potentials to atomistic
simulations for investigating Mg-Al alloys was demonstrated by performing
simulations on Mg and Al atoms in a variety of geometries. The new MEAM
potentials were used to calculate the adsorption energies of Al and Mg atoms on
Al (111) and Mg (0001) surfaces. The formation energies and geometries of
various point defects, such as vacancies, interstitial defects and
substitutional defects, were also calculated. We found that the new MEAM
potentials give a better overall agreement with DFT calculations and
experiments when compared against the previously published MEAM potentials. | 0610602v4 |
2006-11-05 | Grain-boundary grooving and agglomeration of alloy thin films with a slow-diffusing species | We present a general phase-field model for grain-boundary grooving and
agglomeration of polycrystalline alloy thin films. In particular, we study the
effects of slow-diffusing species on grooving rate. As the groove grows, the
slow species becomes concentrated near the groove tip so that further grooving
is limited by the rate at which it diffuses away from the tip. At early times
the dominant diffusion path is along the boundary, while at late times it is
parallel to the substrate. This change in path strongly affects the
time-dependence of grain boundary grooving and increases the time to
agglomeration. The present model provides a tool for agglomeration-resistant
thin film alloy design. keywords: phase-field, thermal grooving, diffusion,
kinetics, metal silicides | 0611118v1 |
2006-11-10 | Evolution of the crystal-field splittings in the compounds CeX (X=P, As, Sb, Bi), \\CeY (Y=S, Se, Te) and their alloys CeX$_{1-x}$Y$_{x}$} | The crystal-field splittings of the monopnictides and monochalcogenides of
Cerium (CeX and CeY) and their alloys (CeX$_{1-x}$Y$_x$) are calculated by
means of an \emph{ab initio} many-body combined technique. The hybridization
functions of the 4f states of Cerium with the conduction band for each material
are obtained from first principles within the local density approximation (LDA)
and are used as input for the Anderson impurity model, which is solved within a
multi-orbital Non-Crossing Approximation (NCA). This realistic theoretical
approach (LDA-NCA) is able to reproduce the experimental results for the
crystal-field splittings of the CeX and CeY series in agreement with previous
theoretical calculations. It is also able to describe the non-linear evolution
of the splittings in the CeX$_{1-x}$Y$_x$ alloys as a function of x. An
analysis of the values of the crystal-field splittings in all the compounds can
be done in depth in this contribution, due to a detailed knowledge of the band
structure and crystal environment in combination with many-body physics. | 0611293v1 |
2006-11-13 | Magnetovolume effect in Ce(Ni{1-x}Cu{x})5 alloys | Magnetic susceptibility of the isostructural Ce(Ni{1-x}Cu{x})5 alloys (0< x
<0.9) was studied as a function of the hydrostatic pressure up to 2 kbar at
fixed temperatures 77.3 and 300 K, using a pendulum-type magnetometer. A
pronounced magnitude of the pressure effect is found to be negative in sign and
to depend strongly and non-monotonously on the Cu content, showing a sharp
maximum in vicinity of x = 0.4. The experimental results are discussed in terms
of the Ce valence change under pressure. It has been concluded that the
fractional occupation of the f-states, which corresponds to the half-integer
valence of Ce ion (3.5), is favorable for the valence instability in alloys
studied. For the reference CeNi5 compound the main contributions to magnetic
susceptibility and their volume dependence are calculated ab initio within the
local spin density approximation (LSDA), and appeared to be in close agreement
with experimental data. | 0611325v1 |
2006-11-20 | Using Cluster Dynamics to Model Electrical Resistivity Measurements in Precipitating Al-Sc Alloys | Electrical resistivity evolution during precipitation in Al-Sc alloys is
modeled using cluster dynamics. This mesoscopic modeling has already been shown
to correctly predict the time evolution of the precipitate size distribution.
In this work, we show that it leads too to resistivity predictions in
quantitative agreement with experimental data. We only assume that all clusters
contribute to the resistivity and that each cluster contribution is
proportional to its area. One interesting result is that the resistivity excess
observed during coarsening mainly arises from large clusters and not really
from the solid solution. As a consequence, one cannot assume that resistivity
asymptotic behavior obeys a simple power law as predicted by LSW theory for the
solid solution supersaturation. This forbids any derivation of the precipitate
interface free energy or of the solute diffusion coefficient from resistivity
experimental data in a phase-separating system like Al-Sc supersaturated
alloys. | 0611524v1 |
2006-12-07 | Influence of Similar Atom Substitution on Glass Formation in (La-Ce)-Al-Co Bulk Metallic Glasses | The glass-formation range of bulk metallic glasses (BMGs) based on lanthanum
and cerium was pinpointed in La-Al-Co, Ce-Al-Co and pseudo-ternary
(La-Ce)-Al-Co system respectively by copper mold casting. Through the stepwise
substitution of La for solvent Ce in (LaxCe1-x)65Al10Co25 alloys (0<x<1), the
fully glassy rods of the (La0.7Ce0.3)65Al10Co25 alloy can be successfully
produced up to 25 mm in diameter by tilt-pour casting. Comparing with the
glass-forming ability (GFA) of single-lanthanide based alloys, La65Al10Co25 and
Ce65Al10Co25, the coexistence of La and Ce with similar atomic size and various
valence electronic structure can obviously improve the GFA of
(LaxCe1-x)65Al10Co25 BMGs, which can't be explained by the former GFA criteria
for BMGs, e.g. atomic size mismatch and negative heats of mixing. A
thermodynamic model was proposed to evaluate this substitution effect, which
gives a reasonable explanation for the obvious improvement of GFA induced by
the coexistence of similar atoms. | 0612185v1 |
2007-03-02 | A generic method for modelling the behavior of anisotropic metallic materials : application to recrystallized zirconium alloys | A simplified polycrystalline model (the so-called RL model) is proposed to
simulate the anisotropic viscoplastic behavior of metallic materials. A generic
method is presented that makes it possible to build a simplified anisotropic
material texture, based on the principal features of the pole figures. The
method is applied to a recrystallized zirconium alloy, used as clad material in
the fuel rods of nuclear power plants. An important database consisting in
mechanical tests performed on Zircaloy tubes is collected. Only a small number
of tests (pure tension, pure shear) are used to identify the material
parameters, and the texture parameters. It is shown that six crystallographic
orientations (6 "grains") are sufficient to describe the large anisotropy of
such hcp alloy. The identified crystallographic orientations match the
experimental pole figures of the material, not used in the identification
procedure. Special attention is paid to the predictive ability of the model,
i.e., its ability to simulate correctly experimental tests not belonging to the
identification database. These predictive results are good, thanks to an
identification procedure that enables to consider the contribution of each slip
system in each crystallographic orientation. | 0703057v1 |
2007-03-02 | Ab-initio design of half-metallic fully-compensated ferrimagnets: the case of Cr$_2$MnZ (Z= P, As, Sb, Bi) compounds | Electronic structure calculations from first-principles are employed to
design some new half-metallic fully-compensated ferrimagnets (or as they are
widely known half-metallic antiferromagnets) susceptible of finding
applications in spintronics. Cr$_2$MnZ (Z= P, As, Sb, Bi) compounds have 24
valence electrons per unit cell and calculations show that their total spin
moment is approximately zero for a wide range of lattice constants in agreement
with the Slater-Pauling behavior for ideal half-metals. Simultaneously, the
spin magnetic moments of Cr and Mn atoms are antiparallel and the compounds are
ferrimagnets. Mean-field approximation is employed to estimate their Curie
temperature, which exceeds room temperature for the alloy with Sb. Our findings
suggest that Cr$_2$MnSb is the compound of choice for further experimental
investigations. Contrary to the alloys mentioned above half-metallic
antiferromagnetism is unstable in the case of the Cr$_2$FeZ (Z= Si, Ge, Sn)
alloys. | 0703078v1 |
2007-03-15 | Mesoscopic Magnetic States in Metallic Alloys with Strong Electronic Correlations: A Percolative Scenario for CeNi$_{1-x}$Cu$_{x}$ | We present evidence for the existence of magnetic clusters of approximately
20 \AA in the strongly correlated alloy system CeNi$_{1-x}$Cu$_{x}$ (0.7 $\le$
x $\le$ 0.2) based on small angle neutron scattering experiments as well as the
occurrence of staircase-like hysteresis cycles during very low temperature (100
mK) magnetization measurements. An unusual feature is the observation of
long-range ferromagnetic order below the cluster-glass transition without any
indication of a sharp transition at a Curie temperature. These observations
strongly support a phenomenological model where a percolative process connects
the cluster-glass state observed at high temperatures with the long-range
ferromagnetic order observed by neutron diffraction experiments at very low
temperatures. The model can account for all the puzzling macroscopic and
microscopic data previously obtained in this system, providing a new
perspective with regard to the magnetic ground state of other alloyed compounds
with small magnetic moments or weak ferromagnetism with intrinsic disorder
effects. | 0703399v1 |
2007-03-18 | Universal behavior of $\rm CePd_{1-x}Rh_x$ Ferromagnet at Quantum Critical Point | The heavy-fermion metal $\rm CePd_{1-x}Rh_x$ can be tuned from ferromagnetism
at $x=0$ to non-magnetic state at some critical concentration $x_c$. The
non-Fermi liquid behavior (NFL) at $x\simeq x_c$ is recognized by power low
dependence of the specific heat $C(T)$ given by the electronic contribution,
magnetic susceptibility $\chi(T)$ and volume expansion coefficient $\alpha(T)$
at low temperatures: $C/T\propto\chi(T)\propto\alpha(T)/T\propto1/\sqrt{T}$. We
also demonstrate that the behavior of normalized effective mass $M^*_N$
observed in $\rm CePd_{1-x}Rh_x$ at $x\simeq 0.8$ agrees with that of $M^*_N$
observed in paramagnetic $\rm CeRu_2Si_2$ and conclude that these alloys
exhibit the universal NFL thermodynamic behavior at their quantum critical
points. We show that the NFL behavior of $\rm CePd_{1-x}Rh_x$ can be accounted
for within frameworks of quasiparticle picture and fermion condensation quantum
phase transition, while this alloy exhibits a universal thermodynamic NFL
behavior which is independent of the characteristic features of the given alloy
such as its lattice structure, magnetic ground state, dimension etc. | 0703474v1 |
2007-03-23 | On the Kondo problem and thermodynamics of dilute magnetic alloys | An argument is given showing that Coulomb attraction between conduction
electrons and impurity ions in a dilute magnetic alloy (DMA) can be
disregarded, provided the system's inverse temperature beta is replaced by an
effective inverse temperature t < beta. This replacement allows to remove the
singularity in Kondo's expression for DMA impurity resistivity and extend his
theory to 0 K. The extended Kondo formula agrees with experimental data on
resistivity of CuFe in the range of low temperatures and in the neighbourhood
of the resistivity minimum.
Using an asymptotic solution of the thermodynamics of a dilute s-d system at
inverse temperature t, the impurity thermodynamic functions are derived and
shown to provide good agreement with experimental data on CuFe, CuCr and
(LaCe)Al_2 alloys in the low-temperature range. The magnitude of these
functions agrees with experiment and does not require rescaling as in previous
s-d theories. Nonlinear dependence of CuFe heat capacity on impurity
concentration has been accounted for the first time. | 0703617v1 |
2007-02-28 | Simulation of Phase Combinations in Shape Memory Alloys Patches by Hybrid Optimization Methods | In this paper, phase combinations among martensitic variants in shape memory
alloys patches and bars are simulated by a hybrid optimization methodology. The
mathematical model is based on the Landau theory of phase transformations. Each
stable phase is associated with a local minimum of the free energy function,
and the phase combinations are simulated by minimizing the bulk energy. At low
temperature, the free energy function has double potential wells leading to
non-convexity of the optimization problem. The methodology proposed in the
present paper is based on an initial estimate of the global solution by a
genetic algorithm, followed by a refined quasi-Newton procedure to locally
refine the optimum. By combining the local and global search algorithms, the
phase combinations are successfully simulated. Numerical experiments are
presented for the phase combinations in a SMA patch under several typical
mechanical loadings. | 0702168v1 |
2006-11-20 | A linear Wegner estimate for alloy type Schroedinger operators on metric graphs | We study spectra of alloy-type random Schr\"odinger operators on metric
graphs. For finite edge subsets of general graphs we prove a Wegner estimate
which is linear in the volume (i.e. the number of edges) and the length of the
considered energy interval. The single site potential of the alloy-type model
needs to have fixed sign, but the considered metric graph does not need to have
a periodic structure. The second result we obtain is an exhaustion construction
of the integrated density of states for ergodic random Schr\"odinger operators
on metric graphs with a $\ZZ^{\nu}$-structure. For certain models the two above
results together imply the Lipschitz continuity of the integrated density of
states. | 0611609v1 |
2005-07-19 | Towards analytical solutions of the alloy solidification problem | In this paper, an analytical solution of alloy solidification problem is
presented. We develop a special method to obtain an exact analytical solution
for mushy zone problem. The main key of this method is a requirement that
thermal diffusivity in the mushy zone to be constant. From such condition we
obtain an ordinary differential equation for liquid fraction function. Thus the
method can be examine as ''a model'' to achive analytical solution of some
unrealistic problems.
An example of solutions is presented: the noneutectic titanium-based alloy
solidification. We provide the comparison of numerical simulation results with
obtained exact solutions. It shown that very simple apparent capacity-based
numerical scheme is provided a good agreement with exact positions of the
solidus and liquidus isoterms, and with temperature profiles also.
Finally, some extensions of the method are outlined. | 0507046v3 |
2007-04-10 | Cooling and heating by adiabatic magnetization in the Ni$_{50}$Mn$_{34}$In$_{16}$ magnetic shape memory alloy | We report on measurements of the adiabatic temperature change in the inverse
magnetocaloric Ni$_{50}$Mn$_{34}$In$_{16}$ alloy. It is shown that this alloy
heats up with the application of a magnetic field around the Curie point due to
the conventional magnetocaloric effect. In contrast, the inverse magnetocaloric
effect associated with the martensitic transition results in the unusual
decrease of temperature by adiabatic magnetization. We also provide
magnetization and specific heat data which enable to compare the measured
temperature changes to the values indirectly computed from thermodynamic
relationships. Good agreement is obtained for the conventional effect at the
second-order paramagnetic-ferromagnetic phase transition. However, at the first
order structural transition the measured values at high fields are lower than
the computed ones. Irreversible thermodynamics arguments are given to show that
such a discrepancy is due to the irreversibility of the first-order martensitic
transition. | 0704.1241v1 |
2007-06-07 | Electronic, magnetic and optical properties of random Fe-Cr alloys | In this communication we have studied the electronic structure, magnetic and
optical properties of bcc \fecr alloys in the ferromagnetic phase. We have used
the augmented space recursion technique coupled with tight-binding linearized
muffin-tin orbital technique (TB-LMTO-ASR) as well as the coherent-potential
approximation based on the Korringa-Kohn-Rostocker method (KKR-CPA). Also the
plane wave projector augmented wave (PAW) method has been used with the
disorder simulated by the special quasi-random structure method f or
configuration averaging (SQS). This was to provide a comparison between the
different methods in common use for random alloys. Moreover, using the
self-consistent potential parameters from TB-LMTO-ASR ca lculations we obtained
the spin resolved optical conductivity using the generalized recursion
technique proposed by M\"uller and Vishwanathan. | 0706.1023v2 |
2007-12-05 | Melting of alloys along grain boundaries | We discuss melting of alloys along grain boundaries as a free boundary
problem for two moving solid-liquid interfaces. One of them is the melting
front and the other is the solidification front. The presence of the triple
junction plays an important role in controlling the velocity of this process.
The interfaces strongly interact via the diffusion field in the thin liquid
layer between them. In the liquid film migration (LFM) mechanism the system
chooses a more efficient kinetic path, which is controlled by diffusion in the
liquid film on relatively short distances. However, only weak coherency strain
energy is the effective driving force for LFM in the case of melting of
one-phase alloys.
The process with only one melting front would be controlled by the very slow
diffusion in the mother solid phase on relatively large distances. | 0712.0751v1 |
2008-01-06 | Kinetics of crystallization of FeB-based amorphous alloys studied by neutron thermo-diffractometry | Kinetics of crystallization of two amorphous alloys, Fe70Cr10B20 and
Fe80Zr10B10, have been followed up by neutron thermodiffractometry experiments
performed in the two axis diffractometer D20 (ILL, Grenoble). The structural
changes are directly correlated with the temperature dependence of the
magnetization. Fe70Cr10B20 crystallizes following a two-step process: an
eutectic crystallization of alfa-Fe (bcc) and the metastable tetragonal phase
(Fe0.8Cr0.2)3B followed by another eutectic transformation to the stable phase
(Fe0.75Cr0.25)2B and more segregation of alfa-Fe. These tetragonal phases are
magnetically anisotropic, giving rise to a large increase of the coercivity.
This behaviour is similar to that of Fe80B20 alloys, with Cr atoms replacing
the Fe positions in both crystalline phases. Fe80Zr10B10 shows also a two-step
process in which two polymorphic transformations take place. | 0801.0869v1 |
2008-03-08 | Magnetism of Ru and Rh thin films on Ag(001) substrate | In a very recent x-ray magnetic circular dichroism experiment concerning with
Ru and Rh impurities and metal films on Ag(001) substrate, no local magnetic
momen ts were displayed in direct contradiction with previous theoretical
works. It is thought that there can be three main reasons for this
inconsistency: relaxation, alloying and many-body effects. Some of the
above-mentioned systems are studie d by using a first-principles method in
which relaxation and alloying are taken into account, even so magnetism is
still obtained. For low-coverage systems, hig h magnetic moments in both Ru
($\sim$ 2.49 $\mu_{B}$) and Rh ($\sim$ 2.00 $\mu_{B}$) are obtained. Naturally,
as the coverage is increased the magnetic moments are approached to zero. Also,
it is noticed that the relaxation distances are in creased by magnetism, which
in turn is decreased by alloying. The behavior of th e magnetic properties is
explained in terms of Stoner model. | 0803.1257v1 |
2008-06-11 | Magnetism in small bimetallic Mn-Co clusters | Effects of alloying on the electronic and magnetic properties of
Mn$_{x}$Co$_{y}$ ($x+y$=$n$=2-5; $x$=0-$n$) and Mn$_2$Co$_{11}$ nanoalloy
clusters are investigated using the density functional theory (DFT). Unlike the
bulk alloy, the Co-rich clusters are found to be ferromagnetic and the magnetic
moment increases with Mn-concentration, and is larger than the moment of pure
Co$_n$ clusters of same size. For a particular sized cluster the magnetic
moment increases by 2 $\mu_B$/Mn-substitution, which is found to be independent
of the size and composition. All these results are in good agreement with
recent Stern-Gerlach (SG) experiments [Phys. Rev. B {\bf 75}, 014401 (2007) and
Phys. Rev. Lett. {\bf 98}, 113401 (2007)]. Likewise in bulk Mn$_x$Co$_{1-x}$
alloy, the local Co-moment decreases with increasing Mn-concentration. | 0806.1979v1 |
2008-08-26 | Theoretical prediction and experimental study of a ferromagnetic shape memory alloy: Ga_2MnNi | We predict the existence of a new ferromagnetic shape memory alloy Ga_2MnNi
using density functional theory. The martensitic start temperature (T_M) is
found to be approximately proportional to the stabilization energy of the
martensitic phase (deltaE_tot) for different shape memory alloys. Experimental
studies performed to verify the theoretical results show that Ga_2MnNi is
ferromagnetic at room temperature and the T_M and T_C are 780K and 330K,
respectively. Both from theory and experiment, the martensitic transition is
found to be volume conserving that is indicative of shape memory behavior. | 0808.3469v1 |
2008-09-01 | An embedded-atom method model for liquid Co, Nb, Zr and supercooled binary alloys | The parameters of many-body potentials for Co, Nb and Zr metals, based on the
embedded-atom method, have been systematically derived. The analytical
potential scheme allows us to reproduce correctly the cohesive energies and
structural properties of the pure metals and selected alloys making use of a
small set of parameters. With a pair potential going smoothly to zero for a
sufficient cutoff radius, radial partial and bond angular distribution
functions for Co, Nb, Zr and alloys are computed using molecular dynamics
simulations that ensure good quantitative agreement with the available
experimental data up to the melting point. Atomic short range order is analysed
in the light of consecutive Gaussian function decomposition and
Honeycutt-Andersen indices. | 0809.0198v1 |
2008-11-13 | Pressure and alloying effects on the metal to insulator transition in NiS{2-x}Se{x} studied by infrared spectroscopy | The metal to insulator transition in the charge transfer NiS{2-x}Se{x}
compound has been investigated through infrared reflectivity. Measurements
performed by applying pressure to pure NiS2 (lattice contraction) and by
Se-alloying (lattice expansion) reveal that in both cases an anomalous metallic
state is obtained. We find that optical results are not compatible with the
linear Se-alloying vs Pressure scaling relation previously established through
transport, thus pointing out the substantially different microscopic origin of
the two transitions. | 0811.2154v3 |
2009-03-02 | Unconventional Fermi surface spin textures in the Bi_xPb_{1-x}/Ag(111) surface alloy | The Fermi and Rashba energies of surface states in the Bi_xPb_{1-x}/Ag(111)
alloy can be tuned simultaneously by changing the composition parameter x. We
report on unconventional Fermi surface spin textures observed by spin and
angle-resolved photoemission spectroscopy {that are correlated with a
topological transition of the Fermi surface occurring at x=0.5. We show that
the surface states remain fully spin polarized upon alloying and that the spin
polarization vectors are approximately tangential to the constant energy
contours. We discuss the implications of the topological transition for the
transport of spin. | 0903.0233v1 |
2009-03-13 | Optical phonon scattering and theory of magneto-polarons in a quantum cascade laser in a strong magnetic field | We report a theoretical study of the carrier relaxation in a quantum cascade
laser (QCL) subjected to a strong magnetic field. Both the alloy (GaInAs)
disorder effects and the Frohlich interaction are taken into account when the
electron energy differences are tuned to the longitudinal optical (LO) phonon
energy. In the weak electron-phonon coupling regime, a Fermi's golden rule
computation of LO phonon scattering rates shows a very fast non-radiative
relaxation channel for the alloy broadened Landau levels (LL's). In the strong
electron-phonon coupling regime, we use a magneto-polaron formalism and compute
the electron survival probabilities in the upper LL's with including increasing
numbers of LO phonon modes for a large number of alloy disorder configurations.
Our results predict a nonexponential decay of the upper level population once
electrons are injected in this state. | 0903.2404v1 |
2009-04-25 | Electronic structure of Ba(Fe,Ru)2As2 and Sr(Fe,Ir)2As2 alloys | The electronic structures of Ba(Fe,Ru)$_2$As$_2$ and Sr(Fe,Ir)$_2$As$_2$ are
investigated using density functional calculations. We find that these systems
behave as coherent alloys from the electronic structure point of view. In
particular, the isoelectronic substitution of Fe by Ru does not provide doping,
but rather suppresses the spin density wave characteristic of the pure Fe
compound by a reduction in the Stoner enhancement and an increase in the band
width due hybridization involving Ru. The electronic structure near the Fermi
level otherwise remains quite similar to that of BaFe$_{2}$As$_{2}$. The
behavior of the Ir alloy is similar, except that in this case there is
additional electron doping. | 0904.3984v1 |
2009-06-13 | Ab initio study of atomic ordering and spin-glass transition in dilute CuMn alloys | An archetypical spin-glass metallic alloy, Cu0.83Mn0.17, is studied by means
of an ab-initio based approach. First-principles calculations are employed to
obtain effective chemical, strain-induced and magnetic exchange interactions,
as well as static atomic displacements, and the interactions are subsequently
used in thermodynamic simulations. It is shown that the calculated atomic and
magnetic short-range order accurately reproduces the results of
neutron-scattering experiments. In particular, it is confirmed that the alloy
exhibits a tendency toward ordering and the corresponding ordered phase is
revealed. The magnetic structure is represented by spin-spiral clusters
accompanied by weaker ferromagnetic short-range correlations. The spin-glass
transition temperature obtained in Monte Carlo simulations by a finite-size
scaling technique, 57 K, is in reasonable agreement with experimental data, 78
K. | 0906.2492v1 |
2009-09-16 | A computational study of the configurational and vibrational contributions to the thermodynamics of substitutional alloys: the Ni3Al case | We have developed a methodology to study the thermodynamics of order-disorder
transformations in n-component substitutional alloys that combines
nonequilibrium methods, which can efficiently compute free energies, with Monte
Carlo simulations, in which configurational and vibrational degrees of freedom
are simultaneously considered on an equal footing basis. Furthermore, by
appropriately constraining the system, we were able to compute the
contributions to the vibrational entropy due to bond proportion, atomic size
mismatch, and bulk volume effects. We have applied this methodology to
calculate configurational and vibrational contributions to the entropy of the
Ni3Al alloy as functions of temperature. We found that the bond proportion
effect reduces the vibrational entropy at the order-disorder transition, while
the size mismatch and the bond proportion effects combined do not change the
vibrational entropy at the transition. We also found that the volume increase
at the order-disorder transition causes a vibrational entropy increase of 0.08
kB/atom, which is significant when compared to the configurational entropy
increase of 0.27 kB/atom. Our calculations indicate that the inclusion of
vibrations reduces in about 30 percent the order-disorder transition
temperature determined solely considering the configurational degrees of
freedom. | 0909.3104v1 |
2009-11-16 | Lattice dynamics and structural stability of ordered Fe3Ni, Fe3Pd and Fe3Pt alloys | We investigate the binding surface along the Bain path and phonon dispersion
relations for the cubic phase of the ferromagnetic binary alloys Fe3X (X = Ni,
Pd, Pt) for L12 and DO22 ordered phases from first principles by means of
density functional theory. The phonon dispersion relations exhibit a softening
of the transverse acoustic mode at the M-point in the L12-phase in accordance
with experiments for ordered Fe3Pt. This instability can be associated with a
rotational movement of the Fe-atoms around the Ni-group element in the
neighboring layers and is accompanied by an extensive reconstruction of the
Fermi surface. In addition, we find an incomplete softening in [111] direction
which is strongest for Fe3 Ni. We conclude that besides the valence electron
density also the specific Fe-content and the masses of the alloying partners
should be considered as parameters for the design of Fe-based functional
magnetic materials. | 0911.3115v2 |
2009-11-30 | Stochastic statistical theory of nucleation and evolution of nano-sized precipitates in alloys with application to precipitation of copper in iron | The consistent and computationally efficient stochastic statistical approach
(SSA) is suggested to study kinetics of nucleation and evolution of nano-sized
precipitates in alloys. An important parameter of the theory is the size of
locally equilibrated regions at the nucleation stage which is estimated using
the "maximum thermodynamic gain" principle suggested.
For several realistic models of iron-copper alloys studied, the results of
the SSA-based simulations of precipitation kinetics agree well with the kinetic
Monte Carlo simulation results for all main characteristics of microstructure.
The approach developed is also used to study kinetics of nucleation and changes
in microstructural evolution under variations of temperature or concentration. | 0911.5558v2 |
2009-12-01 | Observation of spin-triplet superconductivity in Co-based Josephson Junctions | We have measured a long-range supercurrent in Josephson junctions containing
Co (a strong ferromagnetic material) when we insert thin layers of either PdNi
or CuNi weakly-ferromagnetic alloys between the Co and the two superconducting
Nb electrodes. The critical current in such junctions hardly decays for Co
thicknesses in the range of 12-28 nm, whereas it decays very steeply in similar
junctions without the alloy layers. The long-range supercurrent is controllable
by the thickness of the alloy layer, reaching a maximum for a thickness of a
few nm. These experimental observations provide strong evidence for induced
spin-triplet pair correlations, which have been predicted to occur in
superconducting/ferromagnetic hybrid systems in the presence of certain types
of magnetic inhomogeneity. | 0912.0205v1 |
2009-12-06 | Quantum Griffiths Phase in the weak itinerant ferromagnetic alloy Ni$_{1-x}$V$_x$ | We present magnetization ($M$) data of the $d$-metal alloy Ni$_{1-x}$V$_x$ at
vanadium concentrations close to $x_c \approx 11.4%$ where the onset of
long-range ferromagnetic (FM) order is suppressed to zero temperature. Above
$x_c$, the temperature ($T$) and magnetic field ($H$) dependencies of the
magnetization are best described by simple nonuniversal power laws. The
exponents of $M/H \sim T^{-\gamma}$ and $M \sim H^\alpha$ are related by
$1-\gamma=\alpha$ for wide temperature ($10K < T \leq 300K$) and field ($H \leq
5T$) ranges. $\gamma$ is strongly $x$ dependent, decreasing from 1 at $x\approx
x_c$ to $\gamma < 0.1$ for x=15%. This behavior is not compatible with either
classical or quantum critical behavior in a clean 3D FM. Instead it closely
follows the predictions for a quantum Griffiths phase associated with a quantum
phase transition in a disordered metal. Deviations at the lowest temperatures
hint at a freezing of large clusters and the onset of a cluster glass phase,
presumably due to RKKY interactions in this alloy. | 0912.1146v2 |
2010-03-09 | Dielectric function of (ZnxCd1-x)3P2 alloy system in the region of direct optical transitions | The band structure of (ZnxCd1-x)3P2 alloy system is considered within the
framework of Kildal's band model. Frequency dependencies of real and imaginary
parts of dielectric function were received and analyzed in terms of direct band
to band transitions. Theoretical calculations were performed for light
polarized both parallel and perpendicular to the c- axis of the crystal. In
calculations the selection rules for optical transitions were applied. The
frequency dependence of real part of dielectric function is described by a
maximum in hv=(1,2 - 1,5)Eg energy region. In high energy region hv>>Eg the
imaginary part of dielectric function has a plateau. Longitudinal dielectric
function is less than the transverse dielectric function for all compositions x
of (ZnxCd1-x)3P2 alloy system both for real and imaginary parts. When turning
from Zn3P2 to Cd3P2 the reduction of dielectric function values occurs. | 1003.1887v1 |
2010-03-17 | Origin of the anomalous piezoelectric response in wurtzite Sc$_x$Al$_{1-x}$N alloys | The origin of the anomalous, 400% increase of the piezoelectric coefficient
in Sc$_x$Al$_{1-x}$N alloys is revealed. Quantum mechanical calculations show
that the effect is intrinsic. It comes from a strong change in the response of
the internal atomic coordinates to strain and pronounced softening of C$_{33}$
elastic constant. The underlying mechanism is the flattening of the energy
landscape due to a competition between the parent wurtzite and the so far
experimentally unknown hexagonal phases of the alloy. Our observation provides
a route for the design of materials with high piezoelectric response. | 1003.3353v1 |
2010-05-20 | Stabilization of antiferromagnetism in CeFe2 alloys: Effects of chemical and hydrostatic pressure | Effects of Al, Mn and Sb dopings in CeFe2 and effect of applied pressure have
been investigated. Al doping gives rise to the FM-AFM transition and a
reduction in the magnetic moment and TC values, clearly indicating the growth
of the AFM component. Mn and Sb dopings only cause a reduction in TC value. It
is found that in general external pressure enhances the antiferromagnetism in
both the pure and the doped alloys. Enhancement of the Ce 4f- Fe 3d
hybridization as a result of dopings and with the external pressure may be the
reason for the stabilization of antiferromagnetism in these alloys. | 1005.3669v1 |
2010-06-17 | Recent progress in exploring magnetocaloric materials | Magnetic refrigeration based on the magnetocaloric effect (MCE) of materials
is a potential technique that has prominet advantages over the currently used
gas compression-expansion technique in the sense of its high efficiency and
environment friendship. In this article, our recent progress in explorating
effective MCE materials is reviewed with the emphasis on the MCE in the
LaFe13-xSix-based alloys with a first order magnetic transition discovered by
us. These alloys show large entropy changes in a wide temperature range near
room temperature. Effects of magnetic rare-earth doping, interstitial atom, and
high pressure on the MCE have been systematically studied. Special issues such
as appropriate approaches to determining the MCE associated with the
first-order magnetic transition, the depression of magnetic and thermal
hystereses, and the key factors determining the magnetic exchange in alloys of
this kind are discussed. The applicability of the giant MCE materials to the
magnetic refrigeration near ambient temperature is evaluated. A brief review of
other materials with significant MCE is also presented in the article. | 1006.3415v1 |
2010-06-23 | Open theoretical problems in the physics of aperiodic systems | Quasicrystals have intrigued and stimulated research in a large number of
disciplines. Mathematicians, physicists, chemists, metallurgists and materials
scientists have found in them a fertile ground for new insights and
discoveries. In the quarter century that has ensued since the publication of
the experimental observation of a quasiperiodic Al-Mn alloy \cite{shecht}, many
different kinds of quasiperiodic alloys have been manufactured and studied. The
physical properties of quasicrystals are no less interesting than the unusual
structural properties that led to their discovery in 1984. In this review, I
present some of the properties that characterize quasicrystals, briefly discuss
several types of theories that have been put forward, and describe some new
behaviors that might be investigated by experiment. | 1006.4514v1 |
2010-10-08 | A unified first-principles study of Gilbert damping, spin-flip diffusion and resistivity in transition metal alloys | Using a formulation of first-principles scattering theory that includes
disorder and spin-orbit coupling on an equal footing, we calculate the
resistivity $\rho$, spin flip diffusion length $l_{sf}$ and the Gilbert damping
parameter $\alpha$ for Ni$_{1-x}$Fe$_x$ substitutional alloys as a function of
$x$. For the technologically important Ni$_{80}$Fe$_{20}$ alloy, permalloy, we
calculate values of $\rho = 3.5 \pm 0.15$ $\mu$Ohm-cm, $l_{sf}=5.5 \pm 0.3$ nm,
and $\alpha= 0.0046 \pm 0.0001$ compared to experimental low-temperature values
in the range $4.2-4.8$ $\mu$Ohm-cm for $\rho$, $5.0-6.0$ nm for $l_{sf}$, and
$0.004-0.013$ for $\alpha$ indicating that the theoretical formalism captures
the most important contributions to these parameters. | 1010.1626v3 |
2010-10-18 | Significant elastic anisotropy in Ti$_{1-x}$Al$_x$N alloys | Strong compositional-dependent elastic properties have been observed
theoretically and experimentally in Ti$_{1-x}$Al$_x$ N alloys. The elastic
constant, C$_{11}$, changes by more than 50% depending on the Al-content.
Increasing the Al-content weakens the average bond strength in the local
octahedral arrangements resulting in a more compliant material. On the other
hand, it enhances the directional (covalent) nature of the nearest neighbor
bonds that results in greater elastic anisotropy and higher sound velocities.
The strong dependence of the elastic properties on the Al-content offers new
insight into the detailed understanding of the spinodal decomposition and age
hardening in Ti$_{1-x}$Al$_x$N alloys. | 1010.3678v1 |
2010-10-26 | Spin Hall effect of alloys: Extrinsic and intrinsic contribution | A fully relativistic description of the spin-orbit induced spin Hall effect
is presented that is based on Kubo's linear response formalism. Using an
appropriate operator for the spin current density a Kubo-St\v{r}eda-like
equation for the spin Hall conductivity (SHC) is obtained. An implementation
using the Korringa-Kohn-Rostoker (KKR) band structure method in combination
with the Coherent Potential Approximation (CPA) allow detailed investigations
on various alloy systems. A decomposition of the SHC into intrinsic and
extrinsic contributions is suggested. Accompanying calculations for the
skew-scattering contribution of the SHC using the Boltzmann equation
demonstrate the equivalence to the Kubo formalism in the dilute alloy regime
and support the suggested decomposition scheme. | 1010.5520v1 |
2010-12-07 | Ultrafast optical manipulation of atomic arrangements in chalcogenide alloy memory materials | A class of chalcogenide alloy materials that shows significant changes in
optical properties upon an amorphous-to-crystalline phase transition has lead
to development of large data capacities in modern optical data storage. Among
chalcogenide phase-change materials, Ge2Sb2Te5 (GST) is most widely used
because of its reliability. We use a pair of femtosecond light pulses to
demonstrate the ultrafast optical manipulation of atomic arrangements from
tetrahedral (amorphous) to octahedral (crystalline) Ge-coordination in GST
superlattices. Depending on the parameters of the second pump-pulse, ultrafast
nonthermal phase-change occurred within only few-cycles (~ 1 ps) of the
coherent motion corresponding to a GeTe4 local vibration. Using the ultrafast
switch in chalcogenide alloy memory could lead to a major paradigm shift in
memory devices beyond the current generation of silicon-based flash-memory. | 1012.1408v2 |
2010-12-14 | Spin spirals in ordered and disordered solids | A scheme to calculate the electronic structure of systems having a spiral
magnetic structure is presented. The approach is based on the KKR
(Korringa-Kohn-Rostoker) Green's function formalism which allows in combination
with CPA (Coherent Potential Approximation) alloy theory to deal with
chemically disordered materials. It is applied to the magnetic random alloys
Fe$_x$Ni$_{1-x}$, Fe$_x$Co$_{1-x}$ and Fe$_x$Mn$_{1-x}$. For these systems the
stability of their magnetic structure was analyzed. For Fe$_x$Ni$_{1-x}$ the
spin stiffness for was determined as a function of concentration that was found
in satisfying agreement with experiment. Performing spin spiral calculations
the longitudinal momentum-dependent magnetic susceptibility was calculated for
pure elemental systems (Cr, Ni) being in non-magnetic state as well as for
random alloys (Ag$_x$Pt$_{1-x}$). The obtained susceptibility was used to
analyze the stability of the paramagnetic state of these systems. | 1012.3054v1 |
2010-12-14 | A unified cluster expansion method applied to the configurational thermodynamics of cubic TiAlN | We study the thermodynamics of cubic Ti1-xAlxN using a unified cluster
expansion approach for the alloy problem. The purely configurational part of
the alloy Hamiltonian is expanded in terms of concentration and volume
dependent effective cluster interactions. By separate expansions of the
chemical fixed-lattice, and local lattice relaxation terms of the ordering
energies, we demonstrate how the screened generalized perturbation method can
be fruitfully combined with a concentration dependent Connolly-Williams cluster
expansion method. Utilising the obtained Hamiltonian in Monte Carlo simulations
we access the free energy of Ti1-xAlxN alloys and construct the isostructural
phase diagram. The results show surprising similarities with the previously
obtained mean-field results: The metastable c-TiAlN is subject to coherent
spinodal decomposition over a larger part of the concentration range, e.g. from
x >= 0.33 at 2000 K. | 1012.3120v1 |
2010-12-15 | Electron-phonon coupling and two-band superconductivity of Al- and C-doped MgB2 | We have studied the electron-phonon and superconducting properties of the
Mg{1-x}AlxB2 and MgB{2(1-y)}C{2y} alloys within the framework of density
functional theory using the self-consistent virtual-crystal approximation. For
both alloys, the Eliashberg spectral functions and the electron-phonon coupling
constants have been calculated in the two-band model for several concentrations
up to x(Al)=0.55 and y(C)=0.175. We solved numerically the two-band Eliashberg
gap equations without considering interband scattering. Using a single
parameter for the Coulomb pseudopotential, which was determined for the undoped
compound, we were able to reproduce the experimental doping dependence of
Delta_sigma, Delta_pi, and T_c for both alloys on a quantitative level. In
particular, the observed differences in the doping range of superconductivity
between Al and C doping indicate a pronounced influence of the doping site,
which can be explained naturally in the present approach without the need to
invoke interband scattering, suggesting that this factor plays only a minor
role. | 1012.3330v1 |
2011-03-24 | Theoretical and numerical study of lamellar eutectic three-phase growth in ternary alloys | We investigate lamellar three-phase patterns that form during the directional
solidification of ternary eutectic alloys in thin samples. A distinctive
feature of this system is that many different geometric arrangements of the
three phases are possible, contrary to the widely studied two-phase patterns in
binary eutectics. Here, we first analyze the case of stable lamellar coupled
growth of a symmetric model ternary eutectic alloy, using a Jackson-Hunt type
calculation in thin film morphology, for arbitrary configurations, and derive
expressions for the front undercooling as a function of velocity and spacing.
Next, we carry out phase-field simulations to test our analytic predictions and
to study the instabilities of the simplest periodic lamellar arrays. For large
spacings, we observe different oscillatory modes that are similar to those
found previously for binary eutectics and that can be classified using the
symmetry elements of the steady-state pattern. For small spacings, we observe a
new instability that leads to a change in the sequence of the phases. Its onset
can be well predicted by our analytic calculations. Finally, some preliminary
phase-field simulations of three-dimensional growth structures are also
presented. | 1103.4806v1 |
2011-04-07 | Icosahedral quasicrystal and 1/1 cubic approximant in Au-Al-Yb alloys | A P-type icosahedral quasicrystal is formed in Au-Al-Yb alloy of which
6-dimensional lattice parameter a6D = 7.448 {\AA}. The composition of the
quasicrystal was analyzed to be Au51Al34Yb15. This quasicrystal is formed in
as-cast alloys, and is regarded as metastable because of decomposition into
other crystalline phases by annealing at 700 \degree C. Among Tsai-type
quasicrystals, this quasicrystal is situated just between Zn-Sc group with
smaller a6D and larger Cd-Yb group. Intermediate valence of Yb recently
observed in this quasicrystal may be due to this unique situation, namely
smaller major component Au-Al than in Cd-Yb.
The predominant phase in the annealed specimen is a 1/1 cubic approximant
with lattice parameter a = 14.500 {\AA} belonging to the space group Im-3 .
This phase is stable at the composition Au51Al35Yb14 at 700 \degree C. Rietveld
structural analysis indicated that the crystal structure is understood as
periodic arrangement of Tsai-type clusters each including four Au-Al atoms at
their centers. Chemical ordering of Au and Al is characteristics of this
approximant. | 1104.1226v1 |
2011-05-09 | Unified derivation of phase-field models for alloy solidification from a grand-potential functional | In the literature, two quite different phase-field formulations for the
problem of alloy solidification can be found. In the first, the material in the
diffuse interfaces is assumed to be in an intermediate state between solid and
liquid, with a unique local composition. In the second, the interface is seen
as a mixture of two phases that each retain their macroscopic properties, and a
separate concentration field for each phase is introduced. It is shown here
that both types of models can be obtained by the standard variational procedure
if a grand-potential functional is used as a starting point instead of a
free-energy functional. The dynamical variable is then the chemical potential
instead of the composition. In this framework, a complete analogy with
phase-field models for the solidification of a pure substance can be
established. This analogy is then exploited to formulate quantitative
phase-field models for alloys with arbitrary phase diagrams. The precision of
the method is illustrated by numerical simulations with varying interface
thickness. | 1105.1670v1 |
2011-07-14 | Discrete Schrödinger operators with random alloy-type potential | We review recent results on localization for discrete alloy-type models based
on the multiscale analysis and the fractional moment method, respectively. The
discrete alloy-type model is a family of Schr\"odinger operators $H_\omega = -
\Delta + V_\omega$ on $\ell^2 (\ZZ^d)$ where $\Delta$ is the discrete Laplacian
and $V_\omega$ the multiplication by the function $V_\omega (x) = \sum_{k \in
\ZZ^d} \omega_k u(x-k)$. Here $\omega_k$, $k \in \ZZ^d$, are i.i.d. random
variables and $u \in \ell^1 (\ZZ^d ; \RR)$ is a so-called single-site
potential. Since $u$ may change sign, certain properties of $H_\omega$ depend
in a non-monotone way on the random parameters $\omega_k$. This requires new
methods at certain stages of the localization proof. | 1107.2800v1 |
2011-07-22 | Magnetic configurations in cubic Bi2MnFeO6 alloys from first-principles | We expand our study on cubic BiFeO$_3$ alloys presented in [K. Koumpouras and
I. Galanakis, \textit{J. Magn. Magn. Mater} 323, 2328 (2011)] to include also
the BiMnO$_3$ and Bi$_2$MnFeO$_6$ alloys. For the latter we considered three
different cases of distribution of the Fe-Mn atoms in the lattice and six
possible magnetic configurations. We show that Fe and Mn atoms in all cases
under study retain a large spin magnetic moment, the magnitude of which exceeds
the 3 $\mu_B$. Their electronic and magnetic properties are similar to the ones
in the parent BiMnO$_3$ and BiFeO$_3$ compounds. Thus oxygen atoms which are
the nearest-neighbors of Fe(Mn) atoms play a crucial role since they mediate
the magnetic interactions between the transition metal atoms and screen any
change in their environment. Finally, we study the effect of lattice
contraction on the magnetic properties of Bi$_2$MnFeO$_6$. | 1107.4448v1 |
2011-07-30 | Structural, Microchemistry, and Hydrogenation Properties of TiMn0.4Fe0.2V0.4,TiMn0.1Fe0.2V0.7 and Ti0.4Zr0.6Mn0.4Fe0.2V0.4 Metal Hydrides | In this work, TiFe-based alloys have been developed according to the
stoichiometry Ti1-xAxFe1-yBy (A \equiv Zr; B \equiv Mn, V). The hydrogen
solubility properties have been investigated to develop dynamic hydrides of
Ti-based alloys for hydrogen storage applications. The hydrogenation behavior
of these alloys has been studied, and their hydrogen storage capacities and
kinetics have been evaluated. Several activation modes, including activation at
high temperatures under hydrogen pressure, have been attempted for the
as-milled powders. In order to clarify the structural / microstructural
characteristics, and chemical composition before and after hydrogenation, X-Ray
Diffraction (XRD), EDAX - Mapping Analysis and Scanning Electron Microscopy
(SEM), have been carried out for the samples. Modeling of the isotherms has
been performed by using MATLAB programming. The maximum gravimetric density of
4.3 wt%, has been obtained on the sample with the BCC main phase. The
calculated enthalpy of reaction ({\Delta}H) is found to be about 4 kJ/mol. | 1108.0108v3 |
2011-09-11 | Mössbauer study of the field induced uniaxial anisotropy in electro-deposited FeCo alloy films | Thin ferromagnetic films with in-plane magnetic anisotropy are promising
materials for obtaining high microwave permeability. The paper reports on the
M\"ossbauer study of the field induced in-plane uniaxial anisotropy in
electro-deposited $FeCo$ alloy films. The $FeCo$ alloy films have been prepared
by electro-deposition method with and without external magnetic field applied
parallel to the film plane during deposition. The vibrating sample magnetometry
and M\"ossbauer spectroscopy measurements at room temperature indicate that the
film deposited in external field shows an in-plane uniaxial anisotropy with an
easy direction coincides with the external field direction and a hard direction
perpendicular to the field direction, whereas the film deposited without
external field doesn't show any in-plane anisotropy. M\"ossbauer spectra taken
in three geometric arrangements show that the magnetic moments are almost
constrained in the film plane for the film deposited with applied magnetic
field. And the magnetic moments are tend to align in the direction of the
applied external magnetic field during deposition, indicating that the observed
anisotropy should be attributed to directional ordering of atomic pairs. | 1109.2286v1 |
2011-09-27 | A robust but disordered collapsed-volume phase in a cerium alloy under the application of pulsed magnetic fields | We report synchrotron x-ray powder diffraction measurements of
Ce0.8La0.1Th0.1 subject to pulsed magnetic fields as high as 28 Tesla. This
alloy is known to exhibit a continuous volume collapse on cooling at ambient
pressure, which is a modification of the gamma -> alpha transition in elemental
cerium. Recently, it has been suggested on the basis of field-cooled
resistivity and pulsed field magnetization measurements that the volume
collapse in this alloy can be suppressed by the application of magnetic fields.
Conversely, our direct diffraction measurements show a robust collapsed phase,
which persists in magnetic fields as high as 28 Tesla. We also observe
nanoscale disorder in the collapsed phase, which increasingly contaminates the
high temperature phase on thermal cycling. | 1109.5986v1 |
2011-10-06 | Composition dependence of elastic constants in wurtzite AlGaInN alloys | In this paper, we analyze the dependence of elastic constants c_ij on
composition for random wurtzite quater- nary AlGayInN alloy in the whole
concentration range. The study takes as its starting point the c_ij parameters
for zinc blende phase calculated earlier by the authors on the basis of valence
force field model. To obtain the wurtzite constants from cubic material
parameters the Martin transformation is used. The de- viations from linear
Vegard-like dependence of c_ij on composition are analyzed and accurate
quadratic fits to calculated moduli are presented. The influence of nonlinear
internal strain term in the Martin transformation is also investigated. Our
general results for quaternary AlGaInN alloys are compared with the recent ab
initio calculations for ternaries GaInN and AlInN (Gorczyca and Lepkowski,
Phys. Rev. B 83 203201, 2011) and good qualitative agreement is found. | 1110.1346v2 |
2011-11-06 | Thermal conductivity and diffusion-mediated localization in Fe_{1-x}Cr_{x} Alloys | We apply a new Kubo-Greenwood type formula combined with a generalized
Feynman diagram- matic technique to report a first principles calculation of
the thermal transport properties of disordered Fe_{1-x}Cr_{x} alloys. The
diagrammatic approach simplifies the inclusion of disorder-induced scattering
effects on the two particle correlation functions and hence renormalizes the
heat current operator to calculate configuration averaged lattice thermal
conductivity and diffusivity. The thermal conductivity K(T) in the present case
shows an approximate quadratic T-dependence in the low temperature regime (T <
20 K), which subsequently rises smoothly to a T-independent saturated value at
high T . A numerical estimate of mobility edge from the thermal diffusivity
data yields the fraction of localized states. It is concluded that the complex
disorder scattering processes, in force-constant dominated disorder alloys such
as Fe-Cr, tend to localize the vibrational modes quite significantly. | 1111.1403v1 |
2011-11-07 | Effect of interface alloying and band-alignment on the Auger recombination of heteronanocrystals | We report a numerical study of the effect of interface alloying and
band-alignment on the Auger recombination processes of core/shell nanocrystals.
Numerical calculations are carried out using a two-band Kane Hamiltonian.
Smooth interfaces are found to suppress Auger recombination, the strength of
the suppression being very sensitive to the core size. The use of type-II
structures constitutes an additional source of suppression, especially when the
shell confines electrons rather than holes. We show that "magic" sizes leading
to negligible Auger recombination [Cragg and Efros, Nano Letters 10 (2010) 313]
should be easier to realize experimentally in nanocrystals with extended
interface alloying and wide band gap. | 1111.1526v1 |
2011-12-08 | With no Color and Scent (part II): Metal and Alloy Microstructures-Handmade Replicas of Natural Objects | As a continuation of work on metal and alloy "plants" synthesis on porous
membranes by means of pulsed current electroplating volume metallic
microstructures resembling such natural objects as shells, cabbage leaves,
mushrooms are grown and presented in their modest elegance. Such structures are
formed from PdNi and PdCo alloys as well as Ag, Cu and Ni in conditions defined
by the shape of membrane pores and the parameters of the pulsed current. It is
shown that the obtained complex structures are formed by layers of metallic
nanowires as a result of their self-assembly while growing during the pulsed
current electroplating process. Depending on the shape of the membrane and the
regime of the pulsed current electroplating either one type of shell-like
structures or various structures can be grown. | 1112.1894v1 |
2011-12-13 | Onsager approach to 1D solidification problem and its relation to phase field description | We give a general phenomenological description of the steady state 1D front
propagation problem in two cases: the solidification of a pure material and the
isothermal solidification of two component dilute alloys.
The solidification of a pure material is controlled by the heat transport in
the bulk and the interface kinetics.
The isothermal solidification of two component alloys is controlled by the
diffusion in the bulk and the interface kinetics.
We find that the condition of positive-definiteness of the symmetric Onsager
matrix of interface kinetic coefficients still allows an arbitrary sign of the
slope of the velocity-concentration line near the solidus in the alloy problem
or of the velocity-temperature line in the case of solidification of a pure
material. This result offers a very simple and elegant way to describe the
interesting phenomenon of a possible non-single-value behavior of velocity
versus concentration which has previously been discussed by different
approaches. We also discuss the relation of this Onsager approach to the thin
interface limit of the phase field description. | 1112.2849v1 |
2012-02-12 | Structural Properties and Relative Stability of (Meta)Stable Ordered, Partially-ordered and Disordered Al-Li Alloy Phases | We resolve issues that have plagued reliable prediction of relative phase
stability for solid-solutions and compounds. Due to its commercially important
phase diagram, we showcase Al-Li system because historically density-functional
theory (DFT) results show large scatter and limited success in predicting the
structural properties and stability of solid-solutions relative to ordered
compounds. Using recent advances in an optimal basis-set representation of the
topology of electronic charge density (and, hence, atomic size), we present DFT
results that agree reasonably well with all known experimental data for the
structural properties and formation energies of ordered, off-stoichiometric
partially-ordered and disordered alloys, opening the way for reliable study in
complex alloys. | 1202.2500v1 |
2012-03-03 | Order-disorder Effects on Equation of State in FCC Ni-Al Alloys | Order-disorder effects on equation of state (EOS) properties of
substitutional binary alloys are investigated with the cluster variation method
(CVM) based on ab initio effective cluster interactions (ECI). Calculations are
applied to the fcc based Ni-Al system. Various related quantities are shown to
vary with concentration around stoichiometry with a surprising "W shape", such
as the thermal expansion coefficient, the heat capacity and the Gruneisen
parameter, due to configurational ordering effects. Analysis shows that this
feature originates from the dominated behavior of some elements of the inverse
of Hessian matrix. For the first time we point out that the strong
compositional variation of these properties might be partially responsible for
local fractures in alloys and mineral crystals under heating, highlighting the
importance of subtle thermodynamic behavior of order-disorder systems. | 1203.0646v1 |
2012-03-03 | Cluster Expansion of Electronic Excitations: Application to fcc Ni-Al Alloys | The cluster expansion method is applied to electronic excitations and a set
of effective cluster density of states (ECDOS) are defined, analogous to
effective cluster interactions (ECI). The ECDOS are used to generate alloy
thermodynamic properties as well as equation of state (EOS) of electronic
excitations for the fcc Ni-Al systems. When parent clusters with small size,
the convergence of the expansion is not so good but the electronic density of
state (DOS) is well reproduced. However, the integrals of the DOS such as the
cluster expanded free energy, entropy and internal energy associated with
electronic excitations are well described at the level of the
tetrahedron-octahedron cluster approximation, indicating the ECDOS is
applicable to produce electronic ECI for cluster variation method or Monte
Carlo calculations. On the other hand, the Gruneisen parameter, calculated with
first-principles methods, is not any longer a constant and implies that the
whole DOS profile should be considered for EOS of electronic excitations, where
ECDOS adapts very well for disordered alloys and solid solutions. | 1203.0647v1 |
2012-03-29 | Optimization of the magnetic properties of FePd alloys by severe plastic deformation | A FePd alloy was nanostructured by severe plastic deformation following two
different routes: ordered and disordered states were processed by high pressure
torsion (HPT). A grain size in a range of 50 to 150 nm is obtained in both
cases. Severe plastic deformation induces some significant disordering of the
long range ordered L10 phase. However, Transmission Electron Microscopy (TEM)
data clearly show that few ordered nanocrystals remain in the deformed state.
The deformed materials were annealed to achieve nanostructured long range
ordered alloys. The transformation proceeds via a first order transition
characterized by the nucleation of numerous ordered domains along grain
boundaries. The influence of the annealing conditions (temperature and time) on
the coercivity was studied for both routes. It is demonstrated that starting
with the disorder state prior to HPT and annealing at low temperature
(400\degree C) leads to the highest coercivity (about 1.8 kOe). | 1203.6456v1 |
2012-04-16 | First-principles modeling of temperature and concentration dependent solubility in the phase separating Fe$_x$Cu$_{1-x}$ alloy system | We present a novel cluster-expansion (CE) approach for the first-principles
modeling of temperature and concentration dependent alloy properties. While the
standard CE method includes temperature effects only via the configurational
entropy in Monte Carlo simulations, our strategy also covers the
first-principles free energies of lattice vibrations. To this end, the
effective cluster interactions of the CE have been rendered genuinely
temperature dependent, so that they can include the vibrational free energies
of the input structures. As a model system we use the phase-separating alloy
Fe-Cu with our focus on the Fe-rich side. There, the solubility is derived from
Monte Carlo simulations, whose precision had to be increased by averaging
multiple CEs. We show that including the vibrational free energy is absolutely
vital for the correct first-principles prediction of Cu solubility in the bcc
Fe matrix: The solubility tremendously increases and is now in quantitative
agreement with experimental findings. | 1204.3484v1 |
2012-05-04 | Neutron diffraction study of stability and phase transitions in Cu-Sn-In alloys as alternative Pb-free solders | In this work we present an experimental study of structure and phase
stability in ternary Cu-Sn-In alloys around 55 at.% Cu in the temperature range
100^{\circ}C < T < 550^{\circ}C. We have followed in real-time the sequence of
phase transformations in succesive heating and cooling ramps, using
state-of-the-art neutron powder thermodiffractometry. These experiments were
complemented with calorimetric studies of the same alloys. Our results give
experimental support to the current assessment of the ternary phase diagram in
this composition and temperature range, yielding the sequence of transitions
\eta -> \eta + L -> \epsilon + L with transformation temperatures of
210^{\circ}C and 445^{\circ}C, respectively. The use of neutrons allowed to
overcome common difficulties in phase identification with powder XRD due to
absorption and preferred orientation issues. Even the transitions to liquid
phases could be successfully identified and monitored in situ, turning this
technique into a valuable tool for phase diagram studies of emerging lead-free
solder candidates. | 1205.0965v1 |
2012-05-11 | First-principles computed electronic and magnetic properties of zincblende alkaline-earth pnictides | Employing first-principle electronic structure calculations, we study the
magnetic and electronic properties of the XY (X= Mg, Ca, Sr and Y= N, P, As,
Sb) compounds crystallizing in the zincblende structure. The Ca and Sr
alkaline-earth metal monopnictides are found to be half-metallic with a total
spin magnetic moment per formula unit of 1.0 $\mu_B$. In the case of the Mg
alloys the p-d hybridization effect is much weaker and only MgN is a
half-metal. Electron counting of the bands explains the Slater-Pauling behavior
exhibited by the total spin magnetic moment. We also study for these alloys the
effect of deformation taking into account both the cases of hydrostatic
pressure and tetragonalization keeping constant either the in-plane lattice
parameters or the unit cell volume. Even large degrees of deformation only
marginally affect the electronic and magnetic properties of these alloys.
Finally, we show that this stands also for the rocksalt structure. Our results
suggest that alkaline-earth metal monopnictides are promising materials for
magnetoelectronic applications. | 1205.2722v1 |
2012-06-29 | Light-induced magnetization reversal of high-anisotropy TbCo alloy films | Magnetization reversal using circularly polarized light provides a new way to
control magnetization without any external magnetic field and has the potential
to revolutionize magnetic data storage. However, in order to reach ultra-high
density data storage, high anisotropy media providing thermal stability are
needed. Here, we evidence all-optical magnetization switching for different
TbxCo1-x ferrimagnetic alloy composition and demonstrate all-optical switching
for films with anisotropy fields reaching 6 T corresponding to anisotropy
constants of 3x106 ergs/cm3. Optical magnetization switching is observed only
for alloys which compensation temperature can be reached through sample
heating. | 1206.6978v1 |
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