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Atom probe and small-angle neutron scattering (SANS) measurements were carried out in order to investigate the chemical composition of (bcc) Cu precipitates in a (bcc) Fe matrix. The ratio of nuclear and magnetic integrated intensities ( R-value) found by SANS is compared to a theoretical model which is based on the chemical composition obtained from the atom probe data. Furthermore, density functional theory calculations were performed to derive the magnetic moment per atom and the lattice parameters of the Cu alloys modeled in terms of supercells. The calculated and measured R-values agree rather well suggesting that Fe atoms are incorporated in the Cu precipitates up to concentrations of about 50 at.%. [ABSTRACT FROM AUTHOR]

Spin transition has attracted the interest of researchers in various fields since the early 1930s, with thousands of examples now recognized, including those in minerals and biomolecules. However, so far the metal centres in which it has been found to occur are almost always octahedral six-coordinate 3d4 to 3d7 metals, such as Fe(II). A five-coordinate centre is only rarely seen. Here we report that under pressure SrFe(II)O2, which features a four-fold square-planar coordination, exhibits a transition from high spin (S = 2) to intermediate spin (S = 1). This is accompanied by a transition from an antiferromagnetic insulating state to a ferromagnetic so-called half-metallic state: only half of the spin-down (dxz,dyz) states are filled. These results highlight the square-planar coordinated iron oxides as a new class of magnetic and electric materials. [ABSTRACT FROM AUTHOR]

Information on the atom jump processes during the establishment and destruction of long-range order can be gained from measurement of residual electrical resistivity (REST). By its nature this very sensitive method yields an integral signal, so theoretical efforts are mandatory to properly understand how the single atom jumps work together in generating it. As a valuable approach we report on Monte Carlo simulations of atom kinetics which process the individual jumps into macroscopic configurational changes. In order to create a more reliable quantitative basis for individual jump frequencies we propose ab initio calculations of defect energies and migration profiles, a method which has become increasingly accessible in recent years. When experimental investigation is enlightened by combining these two theoretical methods, we expect deep insight into the hierarchy of atomic movement during changes of the ordered state. First steps towards such an ideal program are discussed. An outlook is given how the theoretical methods described offer structural and kinetic information on various scales and in different systems. [ABSTRACT FROM AUTHOR]

Complex surface reconstructions and surface oxides, in particular, often exhibit complicated atomic arrangements, which are difficult to resolve with traditional experimental methods, such as low energy electron diffraction (LEED), surface X-ray diffraction (SXRD) or scanning tunnelling microscopy (STM) alone. Therefore, ab initio density functional calculations are used as a supplement to the experimental techniques, but even then the structural determination usually relies on a simple trial and error procedure, in which conceivable models are first constructed and then tested for their stability in ab initio calculations. An exhaustive search of the configuration space is usually difficult and requires a significant human effort. Solutions to this problem, such as simulated annealing, have long been known, but are usually considered to be too time-consuming in combination with first principles methods. In this work, we show that ab initio density functional codes are now sufficiently fast to perform extensive finite temperature molecular dynamics. The merits of this approach are exemplified for two cases, for a complex two-dimensional surface oxide on Pd(111), and for the oxygen induced c(6 x 2) reconstruction of V(110). [ABSTRACT FROM AUTHOR]

Bonding between unlike metals is the base of compounds with particular properties: good mechanical resistance even at high temperatures, magnets, superconductors, and so forth. A somewhat exceptional property can interest art, watchmaking, and jewelry; some of these new metals are colored. The origin of the effect is the photon- electron interaction subject to strict rules of quantum states and of crystal structure. Brittleness of these materials can be a nuisance, but it is not an intrinsic property as experiments with cobalt- disilicide show. [ABSTRACT FROM AUTHOR]