Publications (108)239.64 Total impact

Article: Electronphonon coupling in Nibased binary alloys with application to displacement cascade modeling
[Show abstract] [Hide abstract] ABSTRACT: Energy transfer between lattice atoms and electrons is an important channel of energy dissipation during displacement cascade evolution in irradiated materials. On the assumption of small atomic displacements, the intensity of this transfer is controlled by the strength of electronphonon (elph) coupling. The elph coupling in concentrated Nibased alloys was calculated using electronic structure results obtained within the coherent potential approximation. It was found that Ni0.5Fe0.5, Ni0.5Co0.5 and Ni0.5Pd0.5 are ordered ferromagnetically, whereas Ni0.5Cr0.5 is nonmagnetic. Since the magnetism in these alloys has a Stonertype origin, the magnetic ordering is accompanied by a decrease of electronic density of states at the Fermi level, which in turn reduces the elph coupling. Thus, the elph coupling values for all alloys are approximately 50% smaller in the magnetic state than for the same alloy in a nonmagnetic state. As the temperature increases, the calculated coupling initially increases. After passing the Curie temperature, the coupling decreases. The rate of decrease is controlled by the shape of the density of states above the Fermi level. Introducing a twotemperature model based on these parameters in 10 keV molecular dynamics cascade simulation increases defect production by 1020% in the alloys under consideration.  [Show abstract] [Hide abstract] ABSTRACT: Equiatomic alloys (e.g. high entropy alloys) have recently attracted considerable interest due to their exceptional properties, which might be closely related to their extreme disorder induced by the chemical complexity. In order to understand the effects of chemical complexity on their fundamental physical properties, a family of (eight) Nibased, facecentercubic (FCC), equiatomic alloys, extending from elemental Ni to quinary high entropy alloys, has been synthesized, and their electrical, thermal, and magnetic properties are systematically investigated in the range of 4–300 K by combining experiments with ab initio KorringKohnRostoker coherentpotentialapproximation (KKRCPA) calculations. The scattering of electrons is significantly increased due to the chemical (especially magnetic) disorder. It has weak correlation with the number of elements but strongly depends on the type of elements. Thermal conductivities of the alloys are largely lower than pure metals, primarily because the high electrical resistivity suppresses the electronic thermal conductivity. The temperature dependence of the electrical and thermal transport properties is further discussed, and the magnetization of five alloys containing three or more elements is measured in magnetic fields up to 4 T.

Article: Differences in the accumulation of ionbeam damage in Ni and NiFe explained by atomistic simulations
[Show abstract] [Hide abstract] ABSTRACT: Following lowdose irradiation with a 3 MeV beam of Au ions, i.e. less than one displacement per atom, a lower number of defects were experimentally observed in NiFe than in pure Ni. At higher doses, more damage is observed in NiFe than in pure Ni. Also, at these high doses, defect structures are observed deep in the material, far from the region where ions are implanted, more so in Ni than in NiFe. In this study, these experimental results are explained using atomistic modeling. Sequences of overlapping displacement cascades with intervening defect aging are simulated. Evidence is provided that nanosecond aging at 900 K can be used as a surrogate for longtime, roomtemperature aging. Then, using this procedure, it is shown that the low defect diffusivity of NiFe leads to less aggregation and recombination events between each displacement cascade in a given volume than in Ni. Variations in the local defect chemistry in NiFe produces a broad spectrum of defect formation energy, leading to the trapping of defects at energetically favorable sites: this explains the low defect diffusivity. Also, this low diffusivity explains why, at high dose, defects in NiFe do not propagate as deeply in the material than in pure Ni.  [Show abstract] [Hide abstract] ABSTRACT: We used molecular dynamics modeling of atomic displacement cascades to characterize the nature of primary radiation damage in 3C–SiC. We demonstrated that the most commonly used interatomic potentials are inconsistent with ab initio calculations of defect energetics. Both the Tersoff potential used in this work and a modified embeddedatom method potential reveal a barrier to recombination of the carbon interstitial and carbon vacancy which is much higher than the density functional theory (DFT) results. The barrier obtained with a newer potential by Gao and Weber is closer to the DFT result. This difference results in significant differences in the cascade production of point defects. We have completed both 10 keV and 50 keV cascade simulations in 3C–SiC at a range of temperatures. In contrast to the Tersoff potential, the GaoWeber potential produces almost twice as many C vacancies and interstitials at the time of maximum disorder (∼0.2 ps) but only about 25% more stable defects at the end of the simulation. Only about 20% of the carbon defects produced with the Tersoff potential recombine during the in–cascade annealing phase, while about 60% recombine with the GaoWeber potential. The GaoWeber potential appears to give a more realistic description of cascade dynamics in SiC, but still has some shortcomings when the defect migration barriers are compared to the ab initio results.
 [Show abstract] [Hide abstract] ABSTRACT: A grand challenge in materials research is to understand complex electronic correlation and nonequilibrium atomic interactions, and how such intrinsic properties and dynamic processes affect energy transfer and defect evolution in irradiated materials. Here we report that chemical disorder, with an increasing number of principal elements and/or altered concentrations of specific elements, in singlephase concentrated solid solution alloys can lead to substantial reduction in electron mean free path and orders of magnitude decrease in electrical and thermal conductivity. The subsequently slow energy dissipation affects defect dynamics at the early stages, and consequentially may result in less deleterious defects. Suppressed damage accumulation with increasing chemical disorder from pure nickel to binary and to more complex quaternary solid solutions is observed. Understanding and controlling energy dissipation and defect dynamics by altering alloy complexity may pave the way for new design principles of radiationtolerant structural alloys for energy applications.
 [Show abstract] [Hide abstract] ABSTRACT: Timedependent density functional theory and Ehrenfest dynamics are used to calculate the electronic excitations produced by a moving Ni ion in a Ni crystal in the case of energetic MeV range (electronic stopping power regime), as well as thermal energy meV range (electronphonon interaction regime). Results at high energy compare well to experimental databases of stopping power, and at low energy the electronphonon interaction strength determined in this way is very similar to the linear response calculation and experimental measurements. This approach to electronphonon interaction as an electronic stopping process provides the basis for a unified framework to perform classical molecular dynamics of ionsolid interaction with ab initio type nonadiabatic terms in a wide range of energies. © 2015 American Physical Society.
 [Show abstract] [Hide abstract] ABSTRACT: Ferromagnetic shape memory alloys (FSMAs) have shown great potential as active components in next generation smart devices due to their exceptionally large magneticfieldinduced strains and fast response times. During application of magnetic fields in FSMAs, as is common in several magnetoelastic smart materials, there occurs simultaneous rotation of magnetic moments and reorientation of twin variants, resolving which, although critical for design of new materials and devices, has been difficult to achieve quantitatively with current characterization methods. At the same time, theoretical modeling of these phenomena also faced limitations due to uncertainties in values of physical properties such as magnetocrystalline anisotropy energy (MCA), especially for offstoichiometric FSMA compositions. Here, in situ polarized neutron diffraction is used to measure directly the extents of both magnetic moments rotation and crystallographic twinreorientation in an FSMA single crystal during the application of magnetic fields. Additionally, highresolution neutron scattering measurements and firstprinciples calculations based on fully relativistic density functional theory are used to determine accurately the MCA for the compositionally disordered alloy of Ni2Mn1.14Ga0.86. The results from these stateoftheart experiments and calculations are selfconsistently described within a phenomenological framework, which provides quantitative insights into the energetics of magnetostructural coupling in FSMAs. Based on the current model, the energy for magnetoelastic twin boundaries propagation for the studied alloy is estimated to be ∼150 kJ/m3 .
 [Show abstract] [Hide abstract] ABSTRACT: High energy vibrational scattering in the binary systems UC and US is measured using timeofflight inelastic neutron scattering. A clear set of welldefined peaks equally separated in energy is observed in UC, corresponding to harmonic oscillations of the light C atoms in a cage of heavy U atoms. The scattering is much weaker in US and only a few oscillator peaks are visible. We show how the difference between the materials can be understood by considering the neutron scattering lengths and masses of the lighter atoms. Monte Carlo ray tracing is used to simulate the scattering, with near quantitative agreement with the data in UC, and some differences with US. The possibility of observing anharmonicity and anisotropy in the potentials of the light atoms is investigated in UC. Overall the observed data is well accounted for by considering each light atom as a single atom isotropic quantum harmonic oscillator.

Article: Journal of Alloys and Compounds
[Show abstract] [Hide abstract] ABSTRACT: High entropy alloys (HEA) have unique properties including the potential to be radiation tolerant. These materials with extreme disorder could resist damage because disorder, stabilized by entropy, is the equilibrium thermodynamic state. Disorder also reduces electron and phonon conductivity keeping the damage energy longer at the deposition locations, eventually favoring defect recombination. In the short timescales related to thermal spikes induced by collision cascades, phonons become the relevant energy carrier. In this work, we perform a systematic study of phonon thermal conductivity in multiple component solid solutions represented by LennardJones (LJ) potentials. We explore the conditions that minimize phonon mean free path via extreme alloy complexity, by varying the composition and the elements (differing in mass, atomic radii, and cohesive energy). We show that alloy complexity can be tailored to modify the scattering mechanisms that control energy transport in the phonon subsystem. Our analysis provides a qualitative guidance for the selection criteria used in the design of HEA alloys with low phonon thermal conductivity. 
Article: Thermodynamic approach to the stability of multiphase systems: Application to the Y2O3Fe system
[Show abstract] [Hide abstract] ABSTRACT: Oxidemetal systems are important in many practical applications, and they are undergoing extensive study using a wide range of techniques. The most accurate theoretical approaches are based on density functional theory (DFT), which is limited to ~10(2) atoms. Multiscale approaches, e.g. DFT + Monte Carlo, are often used to model oxide metal systems at the atomic level. These approaches can qualitatively describe the kinetics of some processes but not the overall stability of individual phases. In this article, we propose a thermodynamic approach to study equilibrium in multiphase systems, which can be sequentially enhanced by considering different defects and microstructures. We estimate the thermodynamic equilibrium by minimization of the free energy of the whole multiphase system using a limited set of defects and microstructural objects for which the properties are calculated by DFT. As an example, we consider Y2O3 + bcc Fe with vacancies in both the Y2O3 and bcc Fe phases, Y substitutions and O interstitials in Fe, Fe impurities, and antisite defects in Y2O3. The output of these calculations is the thermal equilibrium concentration of all the defects for a particular temperature and composition. The results obtained confirmed the high temperature stability of yttria in iron. Model development toward more accurate calculations is discussed. 
Article: Ferromagnetism and Nonmetallic Transport of ThinFilm αFeSi 2 : A Stabilized Metastable Material
[Show abstract] [Hide abstract] ABSTRACT: A metastable phase αFeSi_{2} was epitaxially stabilized on a silicon substrate using pulsed laser deposition. Nonmetallic and ferromagnetic behaviors are tailored on αFeSi_{2} (111) thin films, while the bulk material of αFeSi_{2} is metallic and nonmagnetic. The transport property of the films renders two different conducting states with a strong crossover at 50 K, which is accompanied by the onset of a ferromagnetic transition as well as a substantial magnetoresistance. These experimental results are discussed in terms of the unusual electronic structure of αFeSi_{2} obtained within density functional calculations and Boltzmann transport calculations with and without strain. Our finding sheds light on achieving ferromagnetic semiconductors through both their structure and doping tailoring, and provides an example of a tailored material with rich functionalities for both basic research and practical applications. 
 [Show abstract] [Hide abstract] ABSTRACT: Recently, interest in alnico magnetic alloys has been rekindled due to their potential to substitute for rareearth based permanent magnets provided modest improvements in their coercivity can be achieved without loss of saturation magnetization. Recent experimental studies have indicated that atomic and magnetic structure of the two phases (one AlNibased, the other FeCobased) that comprise these spinodally decomposed alloy is not as simple as previously thought. A key issue that arises is the distribution of Fe, Co, and Ti within the AlNibased matrix phase. In this paper, we report the results of firstprinciples calculations of the site preference of ternary alloying additions in DO3 Fe3 Al, Co3 Al, and Ni 3 Al alloys, as models for the aluminide phase. For compound compositions that are Al rich, which correspond to experimental situation, Ti and Fe are found to occupy the α sites, while Co and Ni prefer the γ sites of the DO3 lattice. An important finding is that the magnetic moments of transition metals in Fe3 Al and Co3 Al are ordered ferromagnetically, whereas the Ni 3 Al were found to be nonmagnetic unless the Fe or Co is added as a ternary element.
 [Show abstract] [Hide abstract] ABSTRACT: A combination of density functional theory (DFT), kinetic Monte Carlo and meanfield rate theory is applied to analyze point defect migration and its effect on the observed growth of hexagonal closepacked (hcp) Zr under 1 MeV electron irradiation. DFT is used to study stability of various configurations of vacancies and selfinterstitial atoms (SIAs) and migration barriers. The data are used in kinetic Monte Carlo modeling of defect diffusion at different temperatures. It is found that both defects exhibit anisotropic diffusion, predominantly parallel to the basal planes. The ratio of diffusion coefficients parallel and perpendicular to the basal planes is found to be higher for vacancies as compared to SIAs at temperatures below ∼600 K. This raises doubts that the observed radiation growth in Zr irradiated with 1 MeV electrons, namely positive strains in prismatic and negative strains in basal directions, and void alignment along basal planes, can be accounted for by the anisotropy of point defect diffusion, which predicts opposite strain signs. It is speculated that formation of small SIA clusters with higher diffusion anisotropy may be responsible for the experimental observations.

Dataset: Paper 171 2013

Article: Magnetic states of the fiveorbital Hubbard model for onedimensional ironbased superconductors
[Show abstract] [Hide abstract] ABSTRACT: The magnetic phase diagrams of models for quasi onedimensional compounds belonging to the ironbased superconductors family are presented. The fiveorbital Hubbard model and the realspace HartreeFock approximation are employed, supplemented by density functional theory to obtain the hopping amplitudes. Phase diagrams are constructed varying the Hubbard $U$ and Hund $J$ couplings and at zero temperature. The study is carried out at electronic density (electrons per iron) $n = 5.0$, which is of relevance for the already known material TlFeSe$_2$, and also at $n = 6.0$, where representative compounds still need to be synthesized. At $n = 5.0$ there is a clear dominance of staggered spin order along the chain direction. At $n = 6.0$ and the realistic Hund coupling $J/U = 0.25$, the phase diagram is far richer including a variety of ``block'' states involving ferromagnetic clusters that are antiferromagnetically coupled, in qualitative agreement with recent Density Matrix Renormalization Group calculations for the threeorbital Hubbard model in a different context. These block states arise from the competition between ferromagnetic order (induced by double exchange, and prevailing at large $J/U$) and antiferromagnetic order (dominating at small $J/U$). The density of states and orbital compositions of the many phases are also provided. 
Conference Paper: First Principles Studies of the Magnetic Properties of Alnico Permanent Magnet Materials
[Show abstract] [Hide abstract] ABSTRACT: Until the advent of rare earth based magnets Alnico was one of the highest energy product hard magnets available. Recently, interest in this system has been rekindled as system whose properties and utility may be further enhanced but does not contain rare earth elements. Recent experiments on Alnico alloy suggest that there is no sharp interface between the disordered bcc FeCo magnetic phase and the ordered B2 NiAl nonmagnetic phase; thereby undermining our understanding of the large coercivity of this material. By utilizing several electronic structure methods we first study the issue of the effect of substitutions of additional elements into B2 NiAl phase. We also calculate the magnetic moment distribution across the interface and examine the magnetic ground state. These calculations suggest that the magnetic structure of the B2phase as well as the interface in much more complex than previously thought.  [Show abstract] [Hide abstract] ABSTRACT: Mobility of screw dislocations controls low temperature plasticity in bcc metals including ferritic alloys. Density functional theory (DFT) is an effective tool in providing parameterfree information on the energetic and magnetic properties of defects including screw dislocations. We summarize DFT calculations on atomic properties of 1/2<111> screw dislocations in FeCr system. The periodic quadrupole approach was applied to model the core dislocation structure, core interaction with Cr solute atoms and to estimate their effect on Peierls stress and barrier. The binding energy of Cr impurity atoms with a screw dislocation and its effect on the dislocation core structure are discussed and the importance of magnetism in the effects of Cr on screw dislocation mobility is demonstrated. This work was supported by the Center for Defect Physics, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences.
 [Show abstract] [Hide abstract] ABSTRACT: Classical Molecular Dynamic (MD) simulations characterizing extended defects typically require millions of atoms. First principles calculations employed to understand these defect systems at an electronic level cannot, and should not deal with such large numbers of atoms. We present an efficient coarse graining (CG) approach to calculate local electronic properties of large MDgenerated structures from the first principles. We used the Locally Selfconsistent Multiple Scattering (LSMS) method for two types of iron defect structures 1) screwdislocation dipoles and 2) radiation cascades. The multiple scattering equations are solved at fewer sites using the CG. The atomic positions were determined by MD with an embedded atom force field. The local moments in the neighborhood of the defect cores are calculated with firstprinciples based on full local structure information, while atoms in the rest of the system are modeled by representative atoms with approximated properties. This CG approach reduces computational costs significantly and makes largescale structures amenable to first principles study. Work is sponsored by the USDoE, Office of Basic Energy Sciences, ``Center for Defect Physics,'' an Energy Frontier Research Center. This research used resources of the Oak Ridge Leadership Computing Facility at the ORNL, which is supported by the Office of Science of the USDoE under Contract No. DEAC0500OR22725.
 [Show abstract] [Hide abstract] ABSTRACT: Great interest in magnetic refrigeration techniques based on the magnetocaloric effect (MCE) has grown recently due to its high efficiency and environmental friendliness. Although the thin film form of the materials is very important in both application and fundamental research, as the properties of films can be tailored by parameters like epitaxial strain, studies on MCE in single crystal films are limited by the difficulty of the growth. In this work, LaFe2Si2 thin films are successfully tuned from Pauli paramagnetic to ferromagnetic, and MCEs are observed around 50K. The ferromagnetic transition is a first order transition, and the magnetic entropy δS 8.5 J/Kg K is obtained under a magnetic field of 7T. The magnetocaloric effect is characterized by a 14 K hysteresis in the field cooling and field warming process. Our temperature dependent Xray measurements exclude the correlation between the striking MCE of the thin film and structural transition. Density functional theory (DFT) calculations indicate that the strain induced distance variations of SiFe bonds control the magnitude of the magnetic moment and MCE.
Publication Stats
2k  Citations  
239.64  Total Impact Points  
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Institutions

20082014

Oak Ridge National Laboratory
 Materials Science and Technology Division
Oak Ridge, Florida, United States


20032012

Iowa State University
 • Department of Physics and Astronomy
 • Ames Laboratory
Ames, IA, United States 
isu
Sŏul, Seoul, South Korea


19881998

Kurchatov Institute
Moskva, Moscow, Russia
