Showing papers in "Journal of Physics F: Metal Physics in 1987"
TL;DR: In this paper, the electron-electron and electron-ion structure factors in a liquid metal were extracted from X-ray and neutron scattering experiments using the static and dynamic direct correlation functions.
Abstract: The X-ray scattered intensity from a liquid metal as an electron-ion mixture is described using the structure factors, which are exactly expressed in terms of the static and dynamic direct correlation functions. This intensity for a metal is shown to differ from the usual scattered intensity from a non-metal in two points: the atomic form factor and the incoherent (Compton) scattering factor. It is shown that the valence electron form factor, which constitutes the atomic form factor in a liquid metal, leads to a determination of the electron-electron and electron-ion structure factors by combining the ionic structure factor. The authors clarify some confusion which appeared in the proposal by Egelstaff et al. (1974) for extracting the electron-electron correlation function in a metal from X-ray and neutron scattering experiments. A procedure to extract the electron-electron and electron-ion structure factors in a liquid metal is proposed on the basis of the formula for the scattered intensity derived.
181 citations
TL;DR: In this article, the Curie temperature of Fe, Co and Ni as well as of their compounds with Y (including Y2Fe14B) are calculated by including the effects of spin fluctuations in the long wavelength limit via a renormalisation of Landau coefficients.
Abstract: The Curie temperature of Fe, Co and Ni as well as of their compounds with Y (including Y2Fe14B) are calculated by including the effects of spin fluctuations in the long wavelength limit via a renormalisation of Landau coefficients. The numerical values depend on the use of recent calculations of correlation effects and of the band structure. As a result, good agreement is found with the experimental values of TC of the materials covered.
154 citations
TL;DR: In this paper, the Miedema's semi-empirical function fBA, which defines the degree by which an A atom is surrounded by neighbours of B atoms, is modified for amorphous alloys and is used for the calculation of the formation enthalpy of crystalline alloys.
Abstract: The function fBA-which defines the degree by which an A atom is surrounded by neighbours of B atoms in Miedema's semi-empirical model and is used for the calculation of the formation enthalpy of crystalline alloys-is modified for amorphous alloys. For the five alloy systems considered, the fBA function turns out to be well described by fBA=cBs(1+5(cAscBs)2). This fBA also reflects the existence of chemical short-range order. In addition, Buschow's prediction for the crystallisation temperature of amorphous alloys is modified using this fBA, resulting in the relation Tx=5 Delta Hhs+275, where Delta Hhs represents the formation enthalpy of a hole of the same size as the smaller atom.
118 citations
TL;DR: In this article, the optical constants of a thin gold film over primarily the visible region of the spectrum were measured using the surface plasmon-polariton resonance condition in the attenuated total-reflection configuration.
Abstract: Measurements are presented of the optical constants of a thin gold film over primarily the visible region of the spectrum using the surface plasmon-polariton resonance condition in the attenuated total-reflection configuration. Using this technique precise results are obtained and these are compared with those found using other experimental methods and also with the classical free-electron model.
112 citations
TL;DR: In this article, the authors reported the experimental study by Mossbauer spectroscopy, at room temperature (RT), of Fe-Al alloys in the disordered phase, with an Al concentration q in the range 0
Abstract: The authors report the experimental study by Mossbauer spectroscopy, at room temperature (RT), of Fe-Al alloys in the disordered phase, with an Al concentration q in the range 0
101 citations
TL;DR: In this article, the authors present results obtained by 57Fe Mossbauer spectroscopy on R2Fe14B, where R represents any of the rare earth elements except Pm, Eu and Yb.
Abstract: The authors present results obtained by 57Fe Mossbauer spectroscopy on R2Fe14B, where R represents any of the rare earth elements except Pm, Eu and Yb. Additional measurements have been performed on oriented powder for R=Y, Pr and Ho. For the case of Y the measurements at 4.2 K were performed in applied magnetic fields up to 4 T. From these measurements it is deduced that the only compounds with an easy c axis at room temperature which show a spin reorientation of the iron sublattice at a lower temperature are the Nd and Ho alloys. Iron hyperfine fields are anisotropic and the anisotropy has different signs on different sites. An analysis of the non-4f contribution to the electric field gradient at the 161Dy or 166Er nuclei in Dy2Fe14B and Er2Fe14B is consistent with published results on 155Gd in Gd2Fe14B. In order to explain that a spin rotation is found only for R=Nd, Ho the authors invoke the relative small quadrupole moment of the 4f shell for these two ions and crystal field terms of higher than second order.
92 citations
TL;DR: In this article, a relativistically extended version of the one-particle formalism is used to calculate the electronic structure of nickel, iron, and palladium metal, and the band structure of iron is discussed in detail.
Abstract: The authors use a relativistically extended version of the one-particle formalism. The electron-electron interaction is assumed to be non-relativistic Coulombic as before. As a consequence, the one-particle Dirac-type equations which now stand in place of the formerly employed Kohn-Sham type equations, contain a diagonal 4*4 matrix whose elements represent the various potential contributions including spin-dependent local potentials describing exchange and correlation as in the non-relativistic case. If asphericity effects of the resulting potential are negligible, these Dirac equations can exactly be reduced to relativistic two-component Pauli-type equations with a diagonal exchange-correlation matrix and a spin-orbit coupling matrix. The latter equations are used to self-consistently calculate the electronic structure of nickel, iron and palladium metal. In addition, the calculation provides magnetic anisotropy energies. The band structure of iron is discussed in detail and compared with other theoretical studies based on different band theoretical methods. The calculations on Pd metal lead without spin-orbit coupling to ferromagnetic spin alignment when the metal is expanded by approximately 5% of the lattice constant. If spin-orbit coupling is included in the self-consistent calculations, the alignment disappears. This effect can only be counteracted by further expanding the lattice up to approximately 10%.
88 citations
TL;DR: The elastic properties of YBa2Cu3O7- delta ceramic have been determined from ultrasonic wave velocity measurements between 5 K and 260 K as discussed by the authors, showing that the elastic stiffness of this material is substantially smaller than that of ideal perovskite materials, a feature which can be abscribed to the oxygen defect.
Abstract: The elastic properties of YBa2Cu3O7- delta ceramic have been determined from ultrasonic wave velocity measurements between 5 K and 260 K. In the vicinity of the superconducting transition temperature both longitudinal and shear mode velocities show decreases with temperature, an indication of long-wavelength acoustic mode softening. Below Tc the mode velocities increase quite steeply. The elastic stiffness of this material is substantially smaller than that of ideal perovskite materials, a feature which can be abscribed to the oxygen defect, layer-like structure.
63 citations
TL;DR: In this article, it was shown that the resistivity increment per unit concentration of Cr in Fe depends strongly on the solute concentration, which can only be explained by assuming that the Frenkel pair specific resistivity, rho F, is an increasing function of the Cr concentration.
Abstract: FeCr alloys with concentrations ranging from 50 p.p.m. to 3% Cr have been irradiated with electrons of 1.6 MeV at low temperature (30 K) and studied by resistivity measurements. Pre-irradiation measurements show important deviations from Matthiessen's rule in the annealed alloys: the resistivity increment per unit concentration of Cr in Fe depends strongly on the solute concentration. It increases from 180 mu Omega cm for a few per cent of chromium up to 400 or 600 mu Omega cm when the chromium concentration is lowered down to 50 p.p.m. This increase is explainable in the two-current model if one considers the dilute alloys with the residual impurities as ternary alloys. The radiation-induced resistivity, measured as 4 K, is also found to depend strongly on the Cr concentration. This can only be explained by assuming that the Frenkel pair specific resistivity, rho F, is an increasing function of the Cr concentration. This variation of rho F in the alloys can be interpreted, in the two-current model, as a deviation from Matthiessen's rule in the irradiated alloys considered as ternary alloys. Mixed Fe-Cr interstitials seem to be slightly more mobile than self-interstitials.
52 citations
TL;DR: In this article, it is suggested that the heavily suppressed pre-superconducting transition, which provides the lattice instability and the soft phonon modes, gives rise to the high Tc.
Abstract: Ultrasonic measurements have indicated that there are possible structural changes near 250 K, 160 K and just above Tc in the high-Tc Y-Ba-Cu-O and Y-Ba-Cu-Nb-O systems, which were confirmed to be first-order-like phase transitions by thermal analysis (DSC) results. It is suggested that the heavily suppressed pre-superconducting transition, which provides the lattice instability and the soft phonon modes, gives rise to the high Tc.
50 citations
TL;DR: In this paper, the electronic structure of La2CuO4 and YBa2Cu3O7 has been evaluated within the local density approximation, using the LMTO-ASA method.
Abstract: The electronic structure of La2CuO4 and YBa2Cu3O7 has been evaluated within the local density approximation, using the LMTO-ASA method. The calculated lattice parameter for the La-based oxide is within 1% of the experimental value. Theoretical photoemission and inverse photoemission spectra (XPA and BIS) are presented. The spectra are dominated by the emission from the Cu sites and show a Cu d band strongly affected by the bonding with the O p levels.
TL;DR: In this article, self-consistent spin-polarized calculations are used to study the electronic structure and magnetic behaviour of transition-metal impurities of the 3d and 4d series in Ni.
Abstract: Self-consistent spin-polarised calculations are used to study the electronic structure and magnetic behaviour of transition-metal impurities of the 3d and 4d series in Ni. Electronic exchange and correlation are treated in the local spin-density approximation of density functional theory. The single-particle equations are solved for a muffin-tin model of the electronic potential. The potentials in the impurity muffin-tin sphere and in the surrounding muffin-tin spheres of the first-shell neighbour host atoms are assumed to be perturbed and calculated self-consistently. A correct embedding of these potentials in the infinite array of unperturbed host potentials is achieved by a Green function technique. The calculations yield local impurity moments which are antiparallel to the Ni host moments for the early elements Sc to Cr, Y to Mo, parallel for the late elements Mn to Co, Tc to Pd and nearly zero for Cu, Zn, Ag and Cd. For most impurities the results agree rather well with the information from neutron scattering and magnetisation measurements; however, there are unexplained discrepancies for Cr and Rh impurities.
TL;DR: In this paper, the authors show that the small reversible property changes seen during annealing of metallic glasses happen more quickly than the irreversible changes simultaneously observed, by modeling isothermal property changes as the sum of contributions from many two-level systems in thermal contact.
Abstract: The small reversible property changes seen during annealing of metallic glasses happen more quickly than the irreversible changes simultaneously observed. By modelling isothermal property changes as the sum of contributions from many two-level systems in thermal contact, the authors attribute this experimental result to a correlation, in metallic glasses, between the barrier heights and the energy splittings of the two-level systems. A previously published (activation energy spectrum) treatment is the starting point in the authors' derivation; they remove an important limitation and show that (i) the previously published treatment is restricted to the analysis of irreversible changes; and (ii) glasses which exhibit relatively large reversible structural changes must contain relatively many two-level systems, with small energy splittings.
TL;DR: In this article, the thermal expansion and magnetic susceptibility have been measured for Y(Mn 1-xAlx)2 pseudobinary Laves phase intermetallic compounds and the amplitude of local spin fluctuations at the Mn site has been estimated on the basis of the phenomenological theory of magnetovolume effects.
Abstract: The thermal expansion and magnetic susceptibility have been measured for Y(Mn1-xAlx)2 pseudobinary Laves phase intermetallic compounds. A large volume change at TN and a remarkable enhancement of the thermal expansion coefficient above TN were observed in YMn2. The substitution of Al for Mn makes these anomalies less clear. The temperature dependence of the susceptibility, being almost temperature independent for YMn2, becomes of the Curie-Weiss type as the Al content increases. Such behaviour is explained as the characteristic features of spin fluctuations. The amplitude of local spin fluctuations at the Mn site has been estimated on the basis of the phenomenological theory of magnetovolume effects. It has been revealed that the magnetic ground state of Al-substituted compounds is a spin glass.
TL;DR: In thin polycrystalline gold films of four different thicknesses, the average grain diameter is varied by different heat treatments as mentioned in this paper, and the resistivity as a function of temperature and grain diameter can be analyzed satisfactorily in terms of grain-boundary scattering alone.
Abstract: In thin polycrystalline gold films of four different thicknesses the average grain diameter is varied by different heat treatments The resistivity as a function of temperature and grain diameter can be analysed satisfactorily in terms of grain-boundary scattering alone The result is identical with experiments in which the resistivity is measured as a function of the film thickness Therefore grain-boundary scattering must be the dominant contribution to the excess thin-film resistivity in polycrystalline thin films Only a rough estimate of the small amount of surface scattering can be given
TL;DR: In this paper, the spin structure of gamma-Fe precipitates was examined using a neutron diffraction technique and it was shown that the spins are within a c plane and modulated in space with a wavevector Q which is the same as that of the displacement wave of the lattice.
Abstract: The recent discovery of a periodic distortion of the lattice structure of gamma -Fe precipitates in Cu motivated the authors to re-examine the magnetic structure of these precipitates using a neutron diffraction technique. They find that the spin structure is that of a longitudinal type-1 antiferromagnet as is well known but the spins are within a c plane and modulated in space with a wavevector Q which is the same as that of the displacement wave of the lattice. Several features of the spin structure are discussed.
TL;DR: In this article, a new local potential for exchange and correlation in ferromagnetic materials is introduced, based on a recent paper by Fritsche (1986) concerned with the alternative foundation of a one-particle theory of many-electron systems.
Abstract: The authors introduce a new local potential for exchange and correlation in ferromagnetic materials. Its derivation rests on a recent paper by Fritsche (1986) concerned with the alternative foundation of a one-particle theory of many-electron systems. The one-particle equations arrived at in that paper contain a 'non-local' exchange-correlation potential which in the present work is approximated by a local expression. On self-consistently solving the associated one-particle equations for nickel metal, they obtain a band structure whose exchange splitting gap of 0.39 eV is considerably closer to the experimental value (namely 0.31 eV) than those which have so far been obtained by using the familiar potential of von Barth and Hedin (1972). The magnetic moment per atom is found to be 0.53 Bohr magnetons and is in fair agreement with the experimental value. Extension of the authors' calculations to finite temperatures leads to a lowering of the magnetic moment. However, due to the neglect of spatially varying excitations they arrive at a hypothetical Curie temperature of approximately 4500 K.
TL;DR: In this article, the authors present a justification of this idea and show how this is directly related to the 'anomalous dispersion' model of Howson and Morgan (1985).
Abstract: The fact that there is a strong correlation between the derivative of the density of states and the Hall coefficient (RH) in amorphous metals has been noted by a number of authors. The authors present a justification of this idea and show how this is directly related to the 'anomalous dispersion' model of Howson and Morgan (1985). They also present calculations of RH for amorphous CuxZr1-x alloys using the theory of Howson and Morgan and obtain very reasonable agreement with experiment.
TL;DR: In this article, a new approach based on a Thomas-Fermi type approximation was proposed to improve 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.
Abstract: 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.
TL;DR: In this paper, the authors used a single-crystal sample of gamma-Fe-Mn in magnetic fields up to 9 T. The results were not consistent with the (001) model of a collinear antiferromagnet.
Abstract: Mossbauer transmission spectra taken, at a temperature of 4.2 K, through a thin single-crystal sample gamma -Fe-Mn, in magnetic fields up to 9 T are presented. These spectra have been fitted to the three energetically favoured magnetic spin models of gamma -Mn. The authors' results are not consistent with the (001) model of a collinear antiferromagnet, whereas fits to the (111) and (110) models are good but indistinguishable.
TL;DR: In this article, the electronic structure of superconducting and non-superconducting YBa2Cu3O7 and YBa3F2O6 compounds was calculated and the effect of fluorine on the Cu-O5 pyramids and the O-Cu-O chains was investigated.
Abstract: The authors have calculated the electronic structure of the superconducting YBa2Cu3O7 and the non-superconducting YBa2Cu3O6 compounds. They find that on reducing the oxygen content the Cu in the O-Cu-O linear chains becomes insulating. They have also studied some hypothetical YBa2Cu3F2O6 systems to examine the effect of fluorine on the Cu-O5 pyramids and the O-Cu-O chains.
TL;DR: In this article, low-temperature specific heat and electrical resistivities were measured on 12 samples in two series of Ca0.7Mg0.3-xAlx and Ca 0.6Mg 0.4-alx metallic glasses over the temperature ranges 1.5-6 and 2-300 K respectively.
Abstract: Low-temperature specific heats and electrical resistivities were measured on 12 samples in two series of Ca0.7Mg0.3-xAlx and Ca0.6Mg0.4-xAlx metallic glasses over the temperature ranges 1.5-6 and 2-300 K respectively. The Hall coefficient was also measured for all Ca0.7Mg0.3-xAlx metallic glasses in the range 77-300 K. A substantial departure from the free electron behaviour is deduced from the composition dependence of the electronic specific heat coefficient. The electrical resistivity is found to vary widely from 50 to 400 mu Omega cm as Mg is replaced by Al. The temperature dependence of resistivity in the low resistivity regime ( rho <150 mu Omega cm) can be explained in the generalized Faber-Ziman theory. The electron transport in the high-resistivity regime (200-400 mu Omega cm) is found to be still strongly composition- and temperature-dependent. These unique features are, the authors believe, far beyond the scope of the generalised Faber-Ziman theory.
TL;DR: In this article, the Faber-Ziman partial structure factors as well as the Bhatia-Thornton (1970) structure factors of the Ni33Y67 and Cu33y67 metallic glasses have been evaluated by neutron diffraction using the isotopic substitution method and X-ray diffraction.
Abstract: The three Faber-Ziman partial structure factors as well as the Bhatia-Thornton (1970) structure factors of the Ni33Y67 and Cu33Y67 metallic glasses have been evaluated by neutron diffraction using the isotopic substitution method and X-ray diffraction. The chemical ordering is described by a function alpha (R) related to the partial coordination numbers zij(R). The relevant CSRO parameter alpha 1 in the first coordination shell is chosen equal to the value of alpha (R) at the position of the first minimum of GNN(R). Thus, the parameters alpha 1 show a strong chemical ordering in Ni33Y67 and, in contrast, a tendency towards random mixing of both constituents in Cu33Y67. The evaluation of alpha 1 by the same method in several NixMt1-x glasses (x approximately=0.33, Mt=Y, Ti, Zr, Dy) gives evidence of a relation between the strength of the bonding and the nature of the d transition element alloyed with Ni.
TL;DR: The rapid loss of straightforward ferromagnetism for small substitutions of Al on Fe sites in CeFe2 has been studied by AC susceptibility measurements with and without small superposed steady fields as discussed by the authors.
Abstract: The rapid loss of straightforward ferromagnetism for small substitutions of Al on Fe sites in CeFe2 has been studied by AC susceptibility measurements with and without small superposed steady fields. A fall in magnetic response at low temperatures like that of re-entrant spin glasses is already present at 1% substitution, by 3.5% the ferromagnetic divergence is greatly reduced and by 8% it has disappeared to leave only spin-glass behaviour. Possible origins of this behaviour, which is related to that of alloys with other elements, and the possible relevance of recent work on other RFe2 phases are discussed.
TL;DR: In this article, the authors present self-consistent band-structure calculations of HCP Be made by means of the full-potential linearised-augmented-plane-wave (LAPW) method and on the basis of the local density approximation.
Abstract: The authors present self-consistent band-structure calculations of HCP Be made by means of the full-potential linearised-augmented-plane-wave (LAPW) method and on the basis of the local density approximation. In contrast to earlier calculations no shape approximations for the charge density or the potential are made and all electrons are taken into account. From the full charge density, X-ray form factors are calculated that agree very well with recent experimental data. This indicates that the significant deviations between experiment and earlier calculations are due to strong muffin-tin- effects. The electric field gradient (EFG) is very sensitive to the quality of the theoretical calculation. In earlier theoretical work not even the sign of the EFG could be determined unambiguously. Within the authors formalism the EFG is determined directly from a given charge density without any approximations. Starting with a muffin-tin potential the EFG is found to be 15 times larger than experiment. The present full-potential LAPW calculation, however, yields an EFG in good agreement with experiment. From calculations at different lattice constants it is found that the EFG depends strongly on the c/a ratio but only weakly on the volume. The equilibrium lattice parameters are determined by total-energy minimisation and agree with experiment within 2%.
TL;DR: In this paper, self-consistent PAW band-structure calculations were performed for B2 FeTi, CoTi and NiTi, and the resulting electron and difference electron densities together with partial local densities of states and band charges were shown to increase distinctly in the series NiTi-CoTi-FeTi.
Abstract: Self-consistent PAW band-structure calculations were performed for B2 FeTi, CoTi and NiTi. Using the resulting electron and difference electron densities together with partial local densities of states and band charges, the covalent bonding contribution is shown to increase distinctly in the series NiTi-CoTi-FeTi. Structure factors are extracted from the calculated electron densities and they can be compared with measured X-ray diffraction data as soon as the latter become available. X-ray emission and absorption spectra as well as angle-integrated photoelectron and bremsstrahlung isochromat spectra are calculated from the APW eigenvalues and wavefunctions and compared with measured spectra from the literature, if available. For the XPS of NiTi some discrepancies between theory and experiment can be eliminated by applying an estimated self-energy correction to the eigenvalues before the calculation of the spectrum.
TL;DR: In this paper, the authors carried out a transmission electron microscopy study of the quasicrystal structure of the alloy and found that the decagonal phase is similar but not identical to those found in Al6Mn and Al6Fe alloys.
Abstract: Dunlap and Dini (1986) have found a decagonal phase instead of the icosahedral quasicrystal in a rapidly solidified Al6Co alloy. The decagonal phase is similar but not identical to those found in Al6Mn and Al6Fe alloys. The crystalline phase coexisting with this decagonal phase is Al9Co2. The authors carry out a transmission electron microscopy study of the quasicrystal structure of the alloy.
TL;DR: The electronic structures of T2P (T=Mn, Fe, Ni) which have two types of T atoms TI and TII were calculated by the KKR and the LMTO methods in the framework of the LSD approximation as mentioned in this paper.
Abstract: The electronic structures of T2P (T=Mn, Fe, Ni) which have two types of T atoms TI and TII, were calculated by the KKR and the LMTO methods in the framework of the LSD approximation. From the position of the Fermi level in the paramagnetic DOS, one can roughly guess that Mn2P exhibits antiferromagnetism, Fe2P ferromagnetism and Ni2P paramagnetism and that the magnetic moment is smaller on TI than on TII in T2P. The existence of the very sharp and high peak of DOS at the Fermi level suggests that the magnetic properties of Fe2P are sensitive to the experimental conditions. Electrons transfer from the P and TII atoms to TI atoms.
TL;DR: In this paper, the positron annihilation technique was used to detect vacancies in cold-rolled binary nickel alloys containing 1 at.% of Al, Ti, Ge, Zn, Sb, As, Pb and In.
Abstract: 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.
TL;DR: In this article, two samples of the compound GdBa2Cu3O7-x have been studied by 155Gd Mossbauer spectroscopy in the temperature range 1.5 K
Abstract: Two samples of the compound GdBa2Cu3O7-x, one with x approximately 0 (Tc=93.5 K) and one with x>0.5 (Tc<1.8 K), have been studied by 155Gd Mossbauer spectroscopy in the temperature range 1.5 K