Nassrin Y. Moghadam

Total-energy-based prediction of a quasicrystal structure

Quasicrystals are metal alloys whose noncrystallographic symmetries challenge traditional methods of structure determination. We employ quantum-based total-energy calculations to predict the structure of a decagonal quasicrystal from first-principles considerations. Our Monte Carlo simulations take as input the knowledge that a decagonal phase occurs in Al-Ni-Co near a given composition and use a limited amount of experimental structural data. The resulting structure obeys a nearly deterministic decoration of tiles on a hierarchy of length scales related by powers of t, the golden mean.

The effect of Ta on the magnetic thickness of permalloy (Ni81Fe19) films

The effect of Ta and Ta/Cu seed layers, and Ta and Cu cap layers on the effective magnetic thickness of ultrathin permalloy (Ni81Fe19) was investigated for MRAM applications. The films were deposited by Ion Beam Deposition. The magnetic moment of each as-deposited permalloy film was measured using a B-H looper and a SQUID magnetometer. The films were further annealed at either 525 K for 1/2 h or 600 K for 1 h to study the effect of thermally driven interdiffusion on the magnetic moment of the permalloy film. Our theoretical calculations showed that the presence of 12% intermixing at the interface reduces the Ni moments to zero. Experimentally, it was shown that the tantalum rather than the copper interfaces are primarily responsible for the magnetically dead layers. The Ta seed layer interface produces a loss of moment equivalent to a magnetically dead layer of thickness 0.6±0.2 nm. The Ta metal in the cap layer results in a loss of moment equivalent to a dead layer of thickness 1.0±0.2 nm. Upon annealing...

Transition-metal interactions in aluminum-rich intermetallics

The extension of the first-principles generalized pseudopotential theory ~GPT! to transition-metal ~TM! aluminides produces pair and many-body interactions that allow efficient calculations of total energies. In aluminum-rich systems treated at the pair-potential level, one practical limitation is a transition-metal overbinding that creates an unrealistic TM-TM attraction at short separations in the absence of balancing many-body contributions. Even with this limitation, the GPT pair potentials have been used effectively in total-energy calculations for Al-TM systems with TM atoms at separations greater than 4 A. An additional potential term may be added for systems with shorter TM atom separations, formally folding repulsive contributions of the three- and higher-body interactions into the pair potentials, resulting in structure-dependent TM-TM potentials. Towards this end, we have performed numerical ab initio total-energy calculations using the Vienna ab initio simulation package for an Al-Co-Ni compound in a particular quasicrystalline approximant structure. The results allow us to fit a short-ranged, many-body correction of the form a(r 0 /r) b to the GPT pair potentials for Co-Co, Co-Ni, and Ni-Ni interactions.

Effects of Ta on the magnetic structure of permalloy

In permalloy (Py) based heterostructures Ta has deleterious effects on the magnetic properties resulting in magnetic dead layers. Using the Korringa–Kohn–Rostoker coherent potential approximation method, it is shown that the average moment of Py (Ni0.8Fe0.2) is rapidly quenched by Ta substitutional additions. Locally self-consistent multiple-scattering method calculations for large supercell models of Py0.9Ta0.1 show that Ta additions also result in substantial fluctuations in the size of the local moments. The configuration dependent reductions in the local moments are larger for Ni than for Fe sites and are the largest for Ni sites closest to Ta.

Energetics of alloys

Calculations on one-dimensional differential equations and simplified threedimensional tight-binding models have proved helpful in the development of a theory for the electronic states of macroscopic solids that do not have translational symmetry.1 Using modern order-N methods, it is now possible to calculate the electronic states of large clusters f atoms with realistic self-consistent density functional theory local density approximation (DFT-LDA) potentials. It has been proposed by Anderson,2 quite correctly, that certain aspects of the electronic structure of macroscopic solids must be qualitatively different from those of finite clusters of atoms, and this argument holds even if the clusters are periodically reproduced to fill all space. When coupled with other theoretical insights, however, results for large finite systems can be extrapolated to give a reliable picture of the electronic states of macroscopic disordered systems.

The mathematics of the polymorphous coherent potential approximation

The original coherent potential approximation (CPA) used for calculating the electronic states in substitutional solid-solution alloys contains the implicit assumption that the alloy is isomorphous. That is, all of the atoms of a given chemical type are assumed to be identical. The extension of the CPA philosophy to treat an alloy model in which all of the atoms are allowed to have distinct charges and potentials is called the polymorphous CPA (PCPA). This extension requires some interesting changes in the mathematical formalism that is used to develop the CPA equations. Aspects of the mathematical formalism of the PCPA will be discussed. In particular, the ergodic theorem from measure theory will be invoked to justify the new equations for the average Greens function.

Angular momentum convergence of Korringa-Kohn-Rostoker Green's function methods

The convergence of multiple-scattering-theory-based electronic structure methods (e.g. the Korringa-Kohn-Rostoker (KKR) band theory method), is determined by lmax, the maximum value of the angular momentum quantum number l. It has been generally assumed that lmax = 3 or 4 is sufficient to ensure a converged ground state and other properties. Using the locally self-consistent multiple-scattering method, which facilitates the use of very high values of lmax, it is shown that the convergence of KKR Greens function methods is much slower than previously supposed, even when spherical approximations to the crystal potential are used. Calculations for Cu using 3≤lmax≤16 indicate that the total energy is converged to within ~0.04 mRyd at lmax = 12. For both face-centred cubic and body-centred cubic structures, the largest error in the total energy occurs at lmax = 4; lmax = 8 gives total energies, bulk moduli, and lattice constants that are converged to accuracies of 0.1 mRyd, 0.1 Mbar, and 0.002 Bohr respectively.

Tests of the Polymorphous Coherent Potential Approximation

The coherent potential approximation (CPA) is a powerful mathematical technique for approximating the electronic structure of substitutional solid solution alloys. Most applications of the CPA to date have assumed an isomorphous model of the alloy in which all of the A atoms are assumed to be the same, as are all of the B atoms. The derivation of self-consistent potentials for the alloys within the framework of the CPA and the isomorphous model leads inevitably to the conclusion that the Madelung potential at each site must be zero. The approximate theory resulting from this derivation is called the KKR-CPA. The polymorphous CPA (PCPA) makes use of supercells that contain many atoms, and the Madelung potentials at all of the sites are calculated exactly. PCPA calculations produce a polymorphous alloy model in which every atom in the supercell is different. Tests will be shown that demonstrate the advantages of the PCPA over the KKR-CPA in explaining experiments that depend critically on the charge transfer in an alloy.

Application of the screened Korringa-Kohn-Rostoker method

In the familiar Korringa-Kohn-Rostoker (KKR) or Green function method, wave propagation between scattering sites is described by the so-called structure constants of the lattice (KKR structure constants), where these quantities are treated as having infinite spatial extent. In a recent development, it has been shown that the KKR method can be formulated in terms of screened structure constants that are of finite range (the screened KKR method). Here, we present an alternative formulation of the screened KKR method that is derived from simple manipulations of the multiple-scattering equations. We carry out density-of-states and total-energy calculations for spin-polarized and non-spin-polarized materials using the method. We point out possible unphysical features of the method.