Table of contents

An icosahedral phase is obtained in rapidly cooled Al_8.8Fe_3.7Ce alloy. The authors do not find the decagonal phase found in samples of this alloy prepared by conventional metallurgical methods. Adding the rare earth Ce to the Al-Fe system appears to stabilise the icosahedral phase, which is not usually present in this system.

The authors report on a neutron powder diffraction study of the magnetic structure of double hexagonal close-packed Ce_0.75Y_0.25 alloy. An antiferromagnetic structure with wavevector 0.5 tau ₁₀₀ is observed below 7 K. The diffraction pattern is well fitted by a model in which ordered moments lie in the basal plane with an average value of 0.58 mu _B on the hexagonal sites and 0.33 mu _B on the cubic sites at 1.8 K. Their results cannot be fitted by the magnetic structure for DHCP cerium suggested by Wilkinson et al. (1961) and they give reasons for supposing that the magnetic structure of their alloy is essentially that adopted by cerium itself.

The authors have studied a series of compounds of the type Y_1-xBa_xCuO_y (y undetermined) with the aim of identifying the superconducting phase in the multiphase system Y_1.2Ba_0.8CuO₄ reported recently. The compounds with x=0.65 and 0.7 exhibit zero electrical resistance at 91 K and the onset of diamagnetic susceptibility at 92 K. These compounds are identified to crystallise in the La₃Ba₃Cu₆O₁₄ type tetragonal (or distorted form) structure.

An expression for the total scattering from a disordered binary alloy is developed through the use of a cumulant expansion method. The technique permits the authors to formulate the scattered intensity exactly without limiting approximations for the static and dynamic displacements. The intensity is naturally decomposed into a Bragg term (including the attendant static and dynamic diffuse scattering), a symmetric short-range order term and a size-effect modulation term. In a null-matrix alloy, for which the Bragg term vanishes, only the symmetric short-range scattering survives.

In order to gain some insight into the nature of multi-level systems the authors investigate a large number of computer generated amorphous arrangements of atoms. Using a special method they generate multiplets of amorphous equilibrium systems which have nearly the same energy and differ little in their atomic positions. Their systems, consisting of up to 500 atoms of one or two types interacting with each other by Lennard-Jones-like potentials in a pressure-free cube with periodic boundary conditions, typically have total energies differing by only 10^-4 to 10^-2 times the average binding energy per atom. In a few cases the atomic positions of two such systems were almost identical (shifted by less than 10^-2 d where d is the average atomic distance) except for a few neighbouring atoms which exhibit larger shifts ('local rearrangement'). In most cases, however, the displacement field is 'diffuse', i.e. when the system switches to the new equilibrium state many atoms exhibit small shifts of comparable size ('diffuse rearrangement' of atoms).

The authors present microscopic calculations of the chemical short-range order (CSRO) in amorphous Cu_xZr_1-x alloys and Ni_xZr_1-x alloys. To simulate transition-metal amorphous alloys, they have used the Bethe lattice in which every atom is located in a FCC environment. A tight-binding Hamiltonian including sd hybridisation is used to describe the concentration dependence of electronic properties of these alloys. They correlate the CSRO with high negative heats of formation. Moreover, they present a discussion of the stability of these alloys in the glass-forming range.

The heat of H absorption by beta and delta Zr hydrides has been measured by isoperibol calorimetry over the range of H concentration 0.1-1.6 H/Zr at temperatures 873-1273 K. In the beta hydride the heat per H atom (differential heat) increases and then decreases as the H concentration increases. In the delta hydride only a decrease at large H concentrations is clearly observed. The increase in the beta hydride is related by self-consistent calculations to a pair indirect interaction between H atoms; the decreases in the beta and delta hydrides are due to a pair direct interaction, which is of the form of a screened Coulomb potential. The differential heat is estimated from the pair indirect and direct interactions by the use of Monte Carlo simulations and compared with the measured differential heat.

Clustering of vacancies in cold-rolled binary nickel alloys containing 1 at.% of Al, Ti, Ge, Zn, Sb, Pb and In has been investigated by positron annihilation technique (lifetime, Doppler broadening and angular correlation peak counting-rate measurements). The most intensive clustering of vacancies was observed for alloys containing oversized, polyvalent solutes: Sb, As, Pb and In. The effect of Zn and Ge admixtures is much weaker. Ti, Al and W admixtures do not induce the clustering of vacancies. The tendency for clustering of the vacancies correlates well with the values of the solute-vacancy binding energy estimated on the basis of the vacancy model of melting of metallic materials.

The interaction between two soluble impurities in metal hosts leads to a non-random distribution of the impurity atoms among the lattice sites. It was found from the investigation of the Mossbauer effect on ¹¹⁹Sn nuclei in AgMe(Sn) and CuMe(Sn) alloys (Me=Rh, Pd, Pt) that there is an attractive interaction between the Sn and Me impurities in the silver and copper hosts. The binding energies of the Sn-Me pairs are deduced.

The potential around impurities in metals has been investigated, using a model of an inhomogeneous electron gas. The inhomogeneity of the system is taken into account by the use of the non-local density approximation (NLDA). The present calculation goes beyond the usual Thomas-Fermi (TF) model not only because it includes exchange-correlation energy in determining the Thomas-Fermi-Dirac (TFD) equation but also because of its nonlocal treatment of the kinetic energy. Results obtained in the case of copper, compared with those from the existing literature, show that non-locality and exchange-correlation effects combine to give a smaller resistivity due to the impurity atoms.

Curves for 2D ACPAR have been computed on the basis of LAPW band structure calculations and compared with experimental data. A new approach to enhancement was proposed based on a Thomas-Fermi type approximation. This treatment resulted in improving the agreement between theory and experiment by a factor of about three (as concerns the relative contribution of d- and s-electrons to positron annihilation) in comparison with the independent particle model. In order to extract information about the physical effects responsible for the residual differences between theory and experiment the isotropic average of the electron-positron density rho ₀(p) was computed from the experimental data using the Cormack method and compared with theoretical predictions.

The low-temperature properties of non-magnetic metallic glasses and alloys are described. It is shown that the appearance of a minimum of electron density of states at the Fermi level as well as a minimum in the temperature dependence of resistivity and the non-linear dependence of the electron specific heat is a consequence of the interference of the Coulomb electron interaction and elastic electron scattering on the dynamical concentration excitations (short-range order type) of disordered systems. The theory accounting for concentration excitations in particular cases leads the authors to consider the electron scattering on impurities. The results are compared with the experimental data.

The electronic structure of the system Fe_xNi_100-x (x=0, 11, 31, 41, 61, 66 and 100) has been investigated using angle-resolved photoemission from FCC single crystals in the (111) orientation, except in the case of pure Fe, where polycrystalline material was used. The authors observe that in the results k-localised direct transitions gradually change into density-of-states transitions with increasing Fe concentration. A comparison is made with calculated densities of states and other experimental (electronic and magnetic) data. The anomalies in the Invar region (x approximately=65) do not show up in the observed electronic structure.

Amorphous alloys of the composition Ni₃₃Ti_67-xV_x (0<or=x<or=20) were prepared by melt-spinning and were studied with X-ray diffraction, differential thermal analysis and electrical resistivity techniques. The crystallisation products were NiTi₂ for the x=0 composition, and NiTi₂, NiTi and V for the x not=0 compositions. X-ray diffraction for the x=14 composition suggests the presence of a metastable icosahedral phase. The activation energy for the crystallisation process was found to be proportional to the crystallisation temperature and to increase with vanadium concentration. The electrical resistivity was found to have a negative coefficient in the temperature range 4.2-300 K. The results on stability and transport properties are discussed in terms of the applicable theoretical models.

Calculations are presented for the longitudinal magnetoresistance of thin wires using Chambers kinetic formulation. Results are obtained for both angularly independent and angularly dependent specularities. Unlike many other circumstances in which angularly dependent surface scattering produces results that are very different from those predicted from angularly independent theories, the results here are remarkably similar. Furthermore, the initial increase in magnetoresistance in low fields found for thin foils is not present for either surface scattering model applied to circular wires. It is clear from this last result that wires of rectangular geometry cannot be approximated to circular wires as is often done.

X-ray measurements of the structure factors of the paired reflections (333)-(511), (442)-(600) and (551)-(711) have been performed on the FCC alloy Fe₆₆Mn₃₄. The ratios of the structure factors show that the charge density of this alloy is very nearly spherical with only a small deviation toward T_2g symmetry. The results of this experiment have been compared with similar X-ray measurements on Fe₇₀Ni₃₀ and with spin-density measurements on the Fe-Ni system. From an analysis of the experimental charge and spin-density anisotropies it is deduced that while the charge density of the high-spin state of FCC Fe and Mn has a preference for T_2g symmetry, the low-spin state of FCC Fe preferentially has E_g symmetry.

For pt.I see ibid., vol.17, p.1419 (1987). Spin-density measurements for a single crystal of the FCC alloy Fe₆₆Mn₃₄ have been performed using elastic and inelastic neutron scattering. Combining the results from elastic scattering with the experimentally determined symmetry of the spin wave spectrum it has been unequivocally deduced that this alloy has a collinear magnetic structure. The magnetic form factor of the alloy, the average magnetic moment per atom and the spin-density asphericity have been obtained from a Fourier inversion of the neutron diffraction data. Combining these results with a measurement of the magnetic scattering amplitude of a powdered sample of the alloy Fe₈₂Mn₁₃C₅ allows the magnetic moments of Fe and Mn in the iron-rich region of the Fe-Mn system to be discussed. Comparison of the experimental charge- and spin-density asphericities of Fe₆₆Mn₃₄ with available calculations of the band structure of FCC Fe shows relatively poor agreement. The possibility of modifying the band structure of FCC Fe by introducing band splittings corresponding to a state-dependent potential is discussed and shown to lead to better agreement with experimental data.

Accurate measurements of the temperature and field dependence of the static magnetisation of the amorphous alloy Fe₈₄B₁₆ have been performed. The saturation magnetisation and the saturation susceptibility have been determined over the temperature range 4.2-150 K. The temperature dependence of the saturation susceptibility exhibits a minimum at low temperature. The temperature dependence of the saturation magnetisation is analysed in terms of spin wave and Stoner excitations and the mechanism of the thermal demagnetisation is discussed. The influence of the magnetovolume effects is discussed qualitatively.

It is demonstrated that published magnetic susceptibility and magnetisation data on Pt_1-cNi_c alloys for 0<c<1 are consistent with a localised model of ferromagnetically interacting Kondo moments in which the Ni atoms are magnetic at all concentrations and have a Kondo temperature of around 300 K. The Ni effective moment at low concentrations is around 2.4 mu _B with a giant moment enhancement factor of approximately 1.46, the latter value being close to that for dilute PtFe. This interpretation is also shown to provide a better explanation for the concentration dependence of the electronic specific heat coefficient than current models of weak itinerant ferromagnetism.

The electronic structure of the disordered substitutional alloy system Cu_xRh_1-x has been studied by means of charge self-consistent Korringa-Kohn-Rostoker coherent potential approximation (KKR-CPA) calculations. The results of these investigations are used to discuss recent photoemission, susceptibility and NMR data on Cu_xRh_1-x in detail.

In transverse electron focusing experiments in Ag an asymmetry is observed in the effect of a positive or negative emitter current on the position and height of the focusing peak. A simplified calculation shows that both effects may be explained qualitatively by the influence of the magnetic field of the emitter current on the orbits of the injected electrons.