Quasicrystals are metal alloys whose noncrystallographic symmetry and lack of structural periodicity challenge methods of experimental structure determination. Here we employ quantum-based total-energy calculations to predict the structure of a decagonal quasicrystal from first principles considerations.We employ Monte Carlo simulations, taking as input the knowledge that a decagonal phase occurs in Al-Ni-Co near a given composition, and using a few features of the experimental Patterson function. The resulting structure obeys a nearly deterministic decoration of tiles on a hierarchy of length scales related by powers of τ , the golden mean.

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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, thermally driven interdiffusion is concluded to have a strong effect on the Ta(seed)/ Ni81Fe19 as-deposited interface, based on the doubling of the magnetically dead layer to 1.2±0.2 nm. The Ni81Fe19/Ta(cap) as-deposited interface slightly increases its equivalent magnetically dead layer upon annealing to 1.2±0.2 nm. As-deposited interfaces of Ta(seed)/permalloy and permalloy/Ta(cap) are not chemically equivalent and result in different magnetically dead layers, whereas after annealing to 600 K both interfaces attain comparable intermixing and magnetically dead layers. It was also shown that a half-hour anneal at the lower 525 K annealing temperature, which is closer to the actual processing temperature, results in only slight increase of the magnetically dead layer at both interfaces.

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 over-binding 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 AA. 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 VASP (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.Comment: 18 pages, 5 figures, submitted to PR

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 Green's 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.