Table of contents

Low-temperature magnetisation measurements for various fields (10-50 kOe) on solid solutions of alpha TmH_x (x taking values from 0 to 0.1) have revealed a decrease of T_N and T_C with increasing x; the deduced values of the paramagnetic Curie temperature, theta _p, and of the effective paramagnetic moment, mu _eff^p, also decrease with increasing x. The saturation magnetic moment, mu _sat(0), increases with increasing field, the addition of hydrogen accelerating this increase. The results are explained in the framework of the RKKY model, assuming an H-H pair structure for the hydrogen configuration, which favours the magnetisation due to the c axis alignment of both pair axis and spins.

Transmission spectra of thin terbium films are reported. The measurements in the photon energy range 1.6-3.1 eV give evidence of direct optical transitions at 1.80, 2.02, 2.17, 2.43, 2.67, 2.85 and 3.02 eV for ferromagnetic Tb in good agreement with electron bandstructure calculations. The manifestation of the magnetic ordering effects in the transmission spectra of Tb is also discussed.

The applicability of the kink-pair model of solid-solution hardening, initially developed for concentrated alloys with low Peierls potentials by Feltham (1968) to polycrystalline hexagonal alloys is examined with special reference to Ti-Al containing 1-10 at.% of aluminium. The available data on the yield-stress between 77 and 300K are well encompassed by the model; the agreement suggests that the effect of alloying on the resistance to plastic deformation is akin to increasing the Peierls force.

A dense random packing model of amorphous iron was analysed in terms of the Voronoi polyhedral statistics and the recently proposed local structure parameters, in particular stresses and site symmetry coefficients (Egami et al., 1980). The purpose of this study is to clarify the relationship between these two types of methods of describing the local structure of amorphous solids. It has been shown that the topology of the local structure represented by the Voronoi polyhedra is related to the local stresses and symmetry only through the coordination number. On the other hand, the degree of distortion of the environment, which is of primary importance to many physical properties and is well characterised by the local parameters, is practically independent of the polyhedral topology. It is concluded, therefore, that the topological description based upon the polyhedral statistics does not provide an unambiguous microscopic basis for relating the local structure of the dense random packing model to its physical properties and could even be misleading.

The monovacancy formation enthalpy, H_1v^F, was determined for Ni using the Doppler broadening technique. The ⁵⁸Co positron source was ion implanted in Ni of 99.995 wt.% purity. A ¹⁰³Ru gamma -ray line (497 keV) was recorded simultaneously with the positron annihilation 511 keV line, and was used in stabilising the data. Measurements were performed on three similar Ni samples, which were annealed in an ultra-high vacuum chamber (10^-8-10^-9 Torr) at approximately 30 degrees C below their melting point for at least two hours prior to the in situ measurements. For one of the samples, a comparison was made of H_1v^F values deduced prior to and after in situ melting. A value of H_1v^F=1.8+or-0.1 eV was determined from the present data. In addition, a value for the monovacancy migration enthalpy, H_1v^M=1.1+or-0.1 eV, has been deduced from a comparison of the present H_1v^F measurement with previous self-diffusion results.

For pt.I see ibid., vol.10, p.2359 (1980). The stable configurations and binding energies of interstitial-solute complexes in Al-Be, Al-Ca, Al-K, Al-Li and Al-Mg alloys were calculated using the molecular dynamics technique in conjunction with interatomic potentials derived from first principles. The most stable complexes in these alloys do not have the same symmetry. In Al-Be and Al-Li alloys, the solutes are close to the octahedral sites, but displaced in the (100) and (111) directions respectively; the resulting complexes have binding energies of 0.89 and 1.75 eV. The type b first-neighbor complexes are the most stable in Al-Ca and Al-K, with respective binding energies of 0.65 and 0.62 eV. In Al-Mg, interstitial-solute interactions are insignificant.

Discusses the mobility parameters for both vacancies and self-interstitials, in concentrated alpha -AgZn solid solutions. Previous work based on the determination of the radiation enhancement of the ordering rate is reviewed. Some of the available data are reanalysed and interpreted in the light of recent quenching results. Convincing evidence is provided that the self-interstitials are less mobile than the vacancies and that they do contribute to the ordering process under irradiation, for either composition considered, 9 at.% and 30 at.% Zn, contrary to the scheme suggested recently by Schule (1980), namely that ordering occurs entirely by a vacancy mechanism, with the self-interstitials migrating faster than the vacancies in these alloys.

For original paper see M. Halbwachs and J. Hillairet, ibid., vol.11, no.11, p.2247 (1981). Some recent developments in the radiation damage field are pointed out. It has been found that the recombination volume between vacancies and self-interstitials is much greater than was generally assumed. In addition, due to an interaction of the high energy particles with lattice atoms during irradiation, the migration energies of the defects decrease. The effect of these factors on the analysis of Halbwachs and Hillairet is discussed.

The phonon dispersion curves of C-centred monoclinic beta gallium at its melting temperature, 256.8K, have been determined by the neutron inelastic scattering technique, using three-axis spectrometers at the Institut Laue-Langevin high-flux reactor in Grenoble. Measurements were made in the Xi _x (and S_x), Delta , Xi _z, S_z and R directions. In view of the metastable character of beta Ga, the sample crystal was grown and oriented in situ. The most unusual feature of the dispersion curve occurs in the Xi _x (S_x) or ( xi 00) direction, where the acoustic mode polarised along the propagation direction peaks in frequency at xi approximately=0.5, and then falls to a very low frequency at xi =1. This behaviour suggests a desire to double the unit cell by suppression of the C-centred lattice points, but no such doubling has actually been observed.

Using the modified axially symmetric model with volume forces, the phonon frequencies of indium have been investigated. The lattice structure of indium has been treated as both BCT and FCT in order to avoid the confusion created in the past in understanding the crystallographic equivalence of these two lattices. The umklapp processes have been taken into account to fulfil the symmetry requirements of the lattice. The calculated phonon dispersion curves are hardly influenced by the choice of different zone boundary frequencies, different screening approximations or by the choice of the two C₄₄ expressions obtained. The results are compared with the other available models.

A numerically tractable theory of electron-phonon coupling in the frame of the local density functional approach is developed describing the self-consistent screening of an ion field to first order in its displacement. The equations are transformed such that one has to screen an effective perturbation weaker than the original dipole field and non-singular in the whole space. In this way one essentially calculates corrections to the rigid muffin tin approximation. In addition the connection to force constants is pointed out. Applying the formulation to aluminium, the Eliashberg function and force constants are evaluated. The theory is shown to account for the discrepancy between measured mass enhancement values and results based on the RMTA.

The concentration behaviour of the various contributions to the Helmholtz free energy and certain other thermodynamic properties in the liquid CuZn, CuAl and CuSn systems are investigated using a formalism based on the pseudopotential perturbation theory and the Gibbs-Bogoliubov inequality.

The behaviour of electron-hole systems such as Bi and Bi_1-xSb_x in a strong magnetic field has been investigated using simple models. The ground-state energy is numerically calculated using the Hartree-Fock, random-phase and Hubbard approximations. The correlation energy of the system is shown to have the properties inherent in a quasi-one-dimensional and one-component spin system. The effect of electron correlation on ultrasonic attenuation is discussed.

The extension of the resonant model potential (RMP) perturbation method to the true transitional metals (with the Fermi level inside the d band) requires a new perturbative scheme. The small parameter is the ratio S_d=W_d/ epsilon _s ( epsilon _s is a typical s band width) which, in RMP theory, controls both the O(S_d) d band width and the O( square root S_d) hybridisation gap width. Physical properties are generally not holomorphic functions of S_d at S_d=0. This unwanted behaviour is related to the S_d resonant coupling which is large for any S_d in the so-called 'hybridisation shell' and is responsible for the Heine hybridisation gap. A method is described where this non-holomorphic contribution is separated out and approximately calculated first for the resonant bandstructure and then for the model Green functions. The method is applied to the calculation of the hybridisation contribution to the metal bandstructure energy and to the calculation of the unscreened model density response function; detailed numerical results show that the hybridisation contribution to the response function is large for the three transitional metal series.

To obtain information on the electronic structure of isolated 4d and 5d transition metal atoms dissolved in noble metals, the authors have used the de Haas-van Alphen effect to study the Fermi surface changes and electron-impurity scattering in alloys of copper with a few hundred PPM of Pd, Pt, Rh and Ir. In Cu(Ir) alloys the results indicated metallurgical problems, but in other systems changes of Fermi surface area and Dingle temperatures have been measured, and inverted using Fourier expansions and phase shift parametrisations. Sets of phase shifts, characteristic of each impurity, have been found that reproduce, within experimental error, the Fermi surface changes, the Dingle temperatures and the impurity resistivity.

A set of Zr-based amorphous transition metal alloys is investigated. It is found that the electron-phonon interaction lambda is proportional to the temperature derivative of the electrical resistivity d rho /dT, i.e. lambda =kd rho /dT. The experimental results for lambda and d rho /dT are consistent with the value of k calculated from the generalised Ziman model and show that k of the order -20K ( mu Omega cm)^-1 for these alloys.

The relation of deviations from Matthiessen's rule (DMR) at high temperatures to electron-phonon interactions is analysed. It is shown that the Fogelholm-Rapp relation (1977) for DMR at high temperatures implies that changes in the electron-phonon interaction parameter, lambda , due to dilute impurities are caused predominantly by interference between electron-phonon and electron-impurity scattering.

The electronic structure of antiferromagnetic chromium (100) thin films with bulk interlayer distance and surface relaxation is calculated by the spin-polarised self-consistent-charge discrete variational X alpha method. The electronic bandstructures, the local density of states, atomic charges and spin polarisations of each of the atoms, including the spatial distributions and their dependence on the surface relaxation, are presented. The magnetic moments on the surface, second and third layers are 2.57, 0.96 and 0.64 mu _B respectively. The surface spin polarisation decreases with surface contraction and increases with surface extension.

Using a Green function approach, the spatial dependence of the pair amplitude F is calculated for a normal-superconducting (N-S) system, assuming a finite thickness and mean free path of the normal layer. The model developed is in good agreement with the DC Josephson current (which probes F) measured in Nb-Nb_xO_y-Cu/Pb proximity junctions with varying Cu thicknesses.

The Anderson model is studied in the limit U to infinity using a Green function decoupling procedure. It is shown that the solution gives the correct results in the intermediate valence case, and in the Kondo limit, at low and high temperatures: for the intermediate valence case, the position of the virtual d level is obtained as a function of temperature; for the Kondo case it is shown that the density of states has a peak of width T_K at the Fermi level, which disappears above the Kondo temperature.

The tetragonality of BCT gamma manganese is investigated by calculating the volume conserving axial pressure. This pressure is obtained using the energy bands and wavefunctions of a bandstructure calculation for gamma manganese. The bandstructure and the resulting pressure indicate that, for this structure, gamma manganese is tetragonal with c/a<1. This result is compared with experiment and a new spin structure is proposed for the observed c/a>1 tetragonal phase of manganese-nickel alloys with high nickel content.

The dielectric matrix for d and f band metals is derived using the non-local model potential approach. It is pointed out that the dielectric matrix obtained by Oli and Animalu (1976) underestimates the depletion hole by approximately a factor of a half. Using the Heine-Abarenkov non-local model potential (1964), the depletion hole contribution for vanadium is calculated to be 45.8%. It is found that the major contribution to the depletion hole comes from the strong energy dependence of the l=2 resonance term in the non-local model potential.

The empirical evidence for a systematic dependence of the laws regarding the pressure dependence of the Curie temperature of alloys on their structural and magnetic heterogeneity is presented. The materials covered include both crystalline and amorphous alloys. Previous interpretations of the results are given, followed by a discussion on the basis of the Landau-Ginzburg formalism which is concerned with concentration fluctuations and which reduces the magnetovolume problem to a purely magnetic one. A reasonable approach to the problem gives results in semi-quantitative agreement with the empirical evidence.

Magnetic susceptibility and Mossbauer measurements are reported for amorphous (Fe_1-xNi_x)₇₇Si₁₀B₁₃ in a Ni concentration range 0.85<or=x<or=1, where the breakdown of magnetism occurs. The authors propose a magnetic phase diagram in which, for x<0.91, long-range magnetic order undergoes a transition to a cluster-glass state. The pronounced influence of magnetic clusters upon the temperature and field dependence of the magnetisation is reflected in strongly curved Arrott plots and gives rise to spin-wave contributions even for alloys with low Curie temperatures. The fairly high ratio of the paramagnetic moment to the spontaneous moment is discussed in terms of a recent theory by Wagner and Wohlfarth (1980).

The optical absorption spectrum of Cu₂MnAl is calculated within the RPA using energy values and wavefunctions determined by the tight-binding orthogonalised plane wave (TB OPW) method and is compared with experiment. It is found that the transitions between OPW in the hybridised states dominantly contribute to the main structure of the spectrum. The correlation effect due to the electron-hole interaction is evaluated taking into account the G(q, omega ) function which describes correlation and exchange effects. The result suggests that the correlated electron-hole pair has a broadening effect on the optical spectrum, particularly on the prominent peak.

Doppler broadened spectra of positron annihilation in an aluminium crystal were measured for 59 different temperatures from 298 to 897K. The instrumental response function was unfolded from observed annihilation spectra and intrinsic distributions were obtained. Two lineshape parameters S and W were defined for the central and the tail portions of the spectrum, respectively. It was shown that the fractional change of the W parameter is about one order of magnitude larger than that of the S parameter. The temperature dependence of S and W parameters in the prevacancy region does not correspond to that of the lattice volume. In addition, the S parameter shows strong temperature dependence in the high temperature region, in contrast to the W parameter which saturates in this region. Using the trapping model the vacancy formation enthalpy was obtained from the result. The value strongly depends on the fitting function for the lineshape parameters.

A theory of angle-resolved photoemission from the surface of a random alloy is described. It is argued that this theory contains a proper treatment of the disorder (via the Korringa-Kohn-Rostoker coherent potential approximation, KKR CPA), the surface, the LEED-type final state, and the electron-photon matrix elements. The results are illustrated by comparing calculations for pure Cu and Cu-Ni alloys.