Table of contents

Pressure-composition isotherms of PdH_x films of 100-400 nm thickness have been measured between 300 and 500K at pressures up to 10 bar by means of an oscillating quartz microbalance. For all the films investigated the two-phase region is considerably narrower than that of bulk PdH_x. This narrowing of the miscibility gap, together with the observed lowering of the critical temperature, is due to a weakening of the hydrogen-hydrogen interaction which probably originates from the elastic boundary conditions at the film-substrate interface.

The mathematical relation between Wohlfarth's model and Mookerjee and Chowdhury's percolation model of spin glasses and its physical meaning are discussed.

Reports results of a systematic experimental investigation of Cs 5p, Rb 4p and K 3p core excitation spectra in alkali-alkali alloys containing these atomic species. Various excitation profiles at threshold are observed. These apparently belong to a universal sequence of lineshapes. It is possible to predict which lineshape occurs in any particular alloy using a parameter xi which is a ratio of potential to kinetic energy in the excited configuration. An 'X-ray edge anomaly' occurs only in one limit of the sequence, namely xi large.

Constant-energy surveys through the spin-wave dispersion function of a nickel crystal (containing 0.8 at.% molybdenum as impurity) have been taken at 8, 12, 16 and 20 meV by means of the scattering of polarised neutrons. The experimental configuration was so arranged that at low temperatures, with the crystal in its magnetic ground state (all spins parallel or antiparallel to the magnetisation direction) a neutron beam polarised oppositely to the magnetisation could give no spin-wave scattering. By observing the onset of spin-wave scattering in this configuration as the temperature was raised the authors were able to detect the onset of spin deviation in the crystal with approach to the Curie temperature. The authors' results are analysed in terms of two theories, one due to Sokoloff (1978) and to Korenman, Murray and Prange (1977), and one to Liu (1980). The former essentially attributes the magnetic disorder at elevated temperatures to the continuous directional fluctuation of the magnetisation vector across a magnetic continuum. Locally, a Stoner band theory of ferromagnetism would be valid in this theory. If the conditions for the theory be held to apply, the authors' data show that the band splitting in nickel declines only by about a quarter or a third as the Curie temperature is approached, while a local moment appears.

Molecular dynamics calculations for liquid sodium at 373K are reported. Five different potential functions computed from a local pseudopotential and a variety of dielectric functions have been considered. Results for the specific heat at constant volume, the radial distribution function, the mean-square displacement and the spectral density of the velocity autocorrelation function are given and discussed. The potential functions computed from the pseudopotential of Ashcroft and the dielectric function of Geldart and Vosko are found to give properties that fit very closely to the available experimental results.

The electric field gradient at ¹⁰⁰Rh probe nuclei caused by charge perturbation around fifth-row impurities (Cd, In, Sn, Sb) in silver, has been investigated employing the perturbed angular correlation technique. Due to an attractive interaction between probes and impurities a high fraction become nearest neighbours. The binding energies obtained as well as the resulting non-axial electric field gradients are discussed considering valence and size effect contributions.

Reports on the structural phase stability of certain binary alloys of less simple metals. The results show that the one-electron contribution arising from the virtual crystal approximation dominates the structural prediction for some of the phases observed in these systems. The neutral object contribution describing the residual interaction of the pseudo-atoms simply reflects the concentration behaviour of the one-electron contribution. The calculations re-emphasise that the first non-vanishing reciprocal lattice vectors g and the diameter of the free electron Fermi sphere 2k_F play a crucially important role in the structural energetics of the systems they consider.

The electronic structures of 15-atom clusters (Fe₁₅, CoFe₈Co₆, FeCo₈Fe₆ and Co₁₅) in the geometry appropriate to the bulk BCC metals and alloys have been determined from spin-polarised self-consistent field X alpha scattered-wave calculations. The general features of the results are quite similar to those obtained from band calculations for the bulk alloys. Examination of the local density of states on each atom and in each spin manifold has yielded a consistent and simple picture of the important interplay in these alloys between magnetic and chemical effects, the key factors being the greater electronegativity of Co and the larger magnetic moment and exchange splitting of Fe. The calculations lead to a qualitative explanation for the concentration dependence of the magnetisation, the volume and the thermal expansion coefficient, which show similar (though less dramatic) behaviour to that observed for the FCC Fe-Ni (Invar) alloys. Calculations of the contact hyperfine fields have led to a model in which the core contribution is proportional to the magnetic moment and the valence contribution depends on the moments of neighbouring atoms. The calculations have allowed some comments to be made on the effects of the order-disorder transition of these alloys on the volume, magnetisation and hyperfine fields.

A new, versatile method of calculating positron states in solids is described. The main purpose is to develop a practical scheme accurate enough to reliably predict annihilation characteristics for positrons trapped at defect clusters of varying size, geometry and impurity environment, in order to help analyse experimental data. The positron potential is constructed from the full lattice electrostatic potential, and a local-density approximation for the correlation potential. The three-dimensional positron Schrodinger equation is solved by finite-element techniques. Enhancement effects in the annihilation rate are parametrised in a simple way, both for simple and transition metals. The applications discussed include vacancies, vacancy clusters and impurity-vacancy complexes in a number of metals. Good agreement with experiment in cases where comparison is possible suggests a good predictive power for the scheme.

Reports 10 and 35 GHz ESR measurements upon single crystals of ScGd doped with 300 and 2400 PPM. The deduced crystal field parameter is D=-330+or-30 G, the Korringa rate b=200+or-40 G K^-1, the g value 2.13+or-5 and the conduction electron lattice rate due to the Gd³⁺ impurity delta _eL^Gd=3*10¹¹ s^-s¹ at.%^-1. The authors' results represent a prime illustration of the Barnes-Plefka exchange narrowing theory both for the bottleneck and non-bottleneck regimes. In particular, they show for the first time the increased narrowing as a consequence of the bottleneck. Also the authors have shown unambiguously that there is a dynamic effect in the line position for this system, and illustrate this by its angular dependence.

The variation of DHVA amplitude with measurement frequency reveals the existence of inhomogeneities in single crystals of potassium. Such inhomogeneities may be associated with dislocated or locally strained regions of the crystal and account for the orientational dependence of DHVA amplitude in this metal.

Mookerjee's theory (1980) of magnetoresistance of spin glasses has been applied to Au-6.6 at.% Fe and Cu-0.7 at.% Mn. The strength of the s-d interaction in these two spin glasses has been estimated from comparison of the theoretical predictions with the experimental data of Nigam and Majumdar (1982).

For pt.I see ibid., vol.12, p.3039 (1982). A general theory given previously for the calculation of the galvanomagnetic properties using an expansion of the vector mean free path in sets of Fermi surface harmonics is applied to copper single crystals containing 3d transition metal impurities. Both the anisotropic Fermi surface of Cu according to an H NFE TB band structure and the anisotropy of the impurity scattering in terms of scattering phase shifts are taken into account. The Kohler diagrams of magnetoresistivity are calculated for the (001) direction of the magnetic field. Furthermore the authors discuss the influence of 3d transition metal impurities on the galvanomagnetic properties in low, intermediate and high magnetic fields.

The room-temperature Hall coefficient of pure Pd is calculated. A moderate anisotropy of the scattering rate changes the result by a factor of two and is necessary for good agreement with experiment.

The influence on the optical properties of the tails of the bulk wavefunctions near the surface is investigated. It is shown that this surface contribution to the effective dielectric function is strongly anisotropic (in the author's model it is uni-axial with the optical axis perpendicular to the surface) and of the same order of magnitude as the intraband contribution observed experimentally. For the imaginary part of the surface contribution to the effective dielectric function in the intraband region and in the limit omega <<k_F² the authors obtain epsilon ₂^eff=(16k_F⁴ omega _p/ pi c omega ²) cos² theta , where theta is the angle between the polarisation vector and the surface normal. If, in addition 1/ tau << omega , this leads to a surface contribution to the intraband relaxation time of 1/ tau y_s=(3k_F omega _p/c) cos² theta .

Measurements have been made of the superconducting transition temperature, together with the bulk magnetisation as a function of applied field, for superconducting PdH_x alloys over the concentration range 0.82(x=H/Pd<or=0.91. The observed magnetisation behaviour is consistent with the presence of two superconducting forms of PdH_x in the concentration range 0.84<or approximately=x<or approximately=0.91 which can be regarded as a mixed phase region giving superconducting behaviour similar to that observed for mixed phase regions in other alloy systems. In the PdH_x case, for the above concentration range, disordered beta phase and ordered beta phase are the most likely constituents of this mixed phase region.

Thermal propagation and stable hot spots (normal domains) are studied in various high T_c superconducting films (Nb₃Sn, Nb, NbN and Nb₃Ge). The prediction of the thermal propagation velocity of the long-standing model of Broom and Rhoderick (1960) is verified quantitatively in the regime of its validity. A new energy balance model is shown to give reasonable quantitative agreement of the dependence of the propagation velocity on the length of short normal domains. The steady state (zero velocity) measurements indicate the existence of two distinct situations for films on high thermal conductivity (sapphire) substrates. For low power per unit area the film and substrate have the same temperature, and the thermal properties of the substrate dominate. However, for higher power densities in short hot spots, the coupling is relatively weak and the thermal properties of the film alone are important. Here a connection is made between the critical current stability of superconducting films and a critical hot spot size for thermal propagation. As a result efficient heat removal is shown to dominate the stabilisation of superconducting films. These models are used to explain quantitatively self-healing of hot spots and a curious switching phenomena in a superconducting film in which a hot spot grows and collapses repeatedly over a very narrow range of current. Finally there is a discussion of the potential use of the thermal propagation model in applications of superconductors, especially switches.

A theory of substitutional as well as amorphous RKKY-type spin glasses has been developed following a qualitative idea first suggested by Smith and more recently by Mydosh. The theory has been developed to a stage where quantitative calculations become possible on realistic systems. The paramagnetic to spin-glass transition has been described as a percolation of temperature dependent clusters of spins in a continuum. The glass transition temperature has been obtained as a function of the magnetic impurity concentration, after taking the effects of frustration and finite mean free path into account. Different 'critical' temperatures for different experimental probes with characteristic time and length scales have also been studied.

The dynamic magnetic response of an antiferromagnetic Cr-10 at.%Fe alloy was determined by neutron time-of-flight spectroscopy over the temperature range 10-250K. Quasielastic magnetic scattering was observed both above and below the Neel temperature at small scattering vectors ( kappa approximately 0.4 AA^-1). This scattering was attributed to the relaxation of ferromagnetically coupled clusters of Fe moments 5-10 AA in size which coexist with the antiferromagnetic spin density wave. The low-field magnetisation of several antiferromagnetic CrFe alloys was measured over the concentration range 10-15.5 at.%Fe and showed anomalies consistent with the freezing of such clusters at low temperatures.

For pt.I see ibid., vol.13, p.441 (1983). The magnetic phase diagram for CrFe alloys was determined by neutron scattering and low-field magnetisation. Two critical concentrations were found as the state of magnetic order evolved from itinerant antiferromagnetism for Fe concentrations less than c_A=16.0+or-0.5% to ferromagnetism for Fe concentrations greater than c_F=19.0+or-0.5%. Spin-glass behaviour was observed between the two critical concentrations. The onset of ferromagnetism was studied by neutron small-angle scattering. Analysis of the concentration and temperature dependence of the magnetic correlation range below c_F suggested that a geometrical element is involved in the development of ferromagnetic long-range order; these results were in semi-quantitative agreement with models for the percolation multi-critical point. The bulk magnetic properties of alloys between the two critical concentrations were consistent with a simple fine magnetic particle model of percolation clusters of Fe moments fluctuating against barriers arising from magnetostatic shape anisotropy.

For pt.II see ibid., vol.13, p.451 (1983). The results of a series of neutron small-angle scattering measurements on ferromagnetic CrFe alloys close to the critical concentration for ferromagnetism (c_F) have been analysed to yield the concentration and temperature dependence of the magnetic inverse correlation range, kappa ₁(c,T). It is found that kappa ₁ does not go to zero at T_c, but varies as (c-c_F)((T-T_C)/T_C)^nu + kappa _R where nu =1.2+or-0.1. At low temperatures the alloys showed significant long-range inhomogeneity in the ferromagnetic correlations; this became more pronounced as the concentration approached c_F. These features are discussed in relation to the other complex magnetic properties of these highly disordered ferromagnets.

The magnetic and electrical properties of ferromagnetic Gd_1-xRE_xAl₂ alloys for RE=Tb, Dy, Ho and Er have been studied. The observed behaviour of magnetic susceptibility over the paramagnetic range and spin-disorder resistivity over the whole concentration range is anomalous and does not follow simple arguments. For x not=0 in the mixed alloys under investigation a curvature of 1/ chi versus temperature appears at about 700K due to the random Gd-RE ion substitution. The results suggest that lattice disordering occurs in this case.

The pressure variation of the Curie temperature T_C has been measured up to 3.6 kbar for GdNi, RNi₃ (R=Gd, Tb, Dy), R₂Ni₇ (R=Gd, Tb, Dy, Nd, Y), GdNi₅, R₂Ni₁₇ (R=Dy, Ho, Er, Y) and Gd₄Co₃ ordering at temperatures between 32 and 225K. The values of dT_C/dP are small ranging from -0.48 to 0.08K kbar^-1. Thermal expansion measurements on GdNi₃, Tb₂Ni₇, Ho₂Ni₁₇ and Y₂Ni₁₇ show no marked spontaneous volume magnetostriction. The quantity Gamma =d 1n T_C/d 1n V is observed to decrease with decreasing T_C. This contradicts the T_C^-2 behaviour which can be derived from the Stoner-Wohlfarth model of weak itinerant ferromagnetism and has been found to be valid for all R-Co and R-Fe compounds studied earlier but for those with T_C lower than about 400K. Since small or even negative values of gamma are observed for compounds where R=Y or La the reduction of Gamma cannot be attributed to R-3d and R-R interactions. The phenomenon is discussed also in terms of a compression-induced transfer of 3d electrons to other bands and of alloy heterogeneities, but a more plausible explanation is given by a combined model of localised and itinerant d electrons.

The authors examine the calculation of the direct contribution to the electron spin density at a nucleus in liquid binary alloys of simple metals by low-order pseudopotential perturbative methods. It is concluded that self-consistency requirements are sufficiently stringent so that it is simply impractical to use this approach and expect reasonable accuracy. Hence, in order to gain some physical insight into the problem, a simple model, using the concept of a charge transfer between ions, is proposed for the estimation of the total electron contact densities at a nucleus in the alloy. The results are applied to determine the concentration dependence of the corresponding Knight shifts and good agreement is found with observation.

The valence band K and L emissions of Co and Al in the intermetallic compound CoAl have been calculated by using the site- and symmetry-projected densities of states obtained from a self-consistent symmetrised APW band calculation. The results are compared with K emission spectra measured with a two-crystal X-ray spectrometer and L emission spectra measured previously. The measured K spectra are reproduced well by the calculations. On the other hand, the experimental Co L emission band is considerably broader than indicated by the calculation. Also in the Al L emission the agreement is only fair. The position of the d states is found experimentally in this case to deviate by 0.5 eV from the predicted position.

The K emission band spectra of 3d metals were measured with a two-crystal spectrometer. The total and partial densities of states and the K emission spectra were computed from the band structure calculation. Good overall agreement between theory and experiment is obtained. The longstanding discrepancy in the FWHM in Ni K and Fe K bands was removed by using a linear energy dependence suggested recently for Auger broadening. The systematic changes in the structures of the spectra can be explained by the present theory based on the one-electron description and dipole approximation. The results for V, Fe, Ni and Cu are presented here.

The electrical resistivity of various NbH_x samples has been measured in the temperature range 80-420K for hydrogen concentrations x varying between 0 and 1.1. These measurements were supplemented by a neutron diffraction study of a sample with x=0.825. The singularities in the temperature dependence of the resistivity and the superstructure reflections are used to identify phase transitions. The concentration dependence of the resistivity is discussed on the basis of a model of the Fermi surface which states that the sheet associated with the N symmetry points disappears on hydrogenation. Similarities between the behaviour of the resistivity and the intensity of the superstructure reflections of the hydride are pointed out and interpreted as pre-transitional effects.