Table of contents

The authors show, on the basis of a semi-self-consistent density functional calculation of the equilibrium position of the first lattice plane at the three low index faces of aluminium, that the screening of the ionic relaxation dipole by the electrons is essentially perfect. As a result the surface energy and work function are nearly independent of small displacements of the first lattice plane. They also demonstrate that the 'frozen profile' approximation for the electron density grossly exaggerates the curvature of the potential in which the first lattice plane sits and so cannot be relied on for geometrical predictions. The results for the first interplanar spacing of the close-packed (111) surface agree with the 2% expansion (relative to bulk) found in the recent LEED analysis of Jona et al. (1980) and do not favour the 8% contraction suggested by the surface EXAFS study of Bianconi and Bachrach (1979).

The compositional dependence of the ⁵⁷Fe Mossbauer effect spectra for the metallic glasses Y₆₆(Fe_xMn_1-x)₃₄ has been studied. For high concentrations of Fe the spectra consist of two symmetric peaks with the spectra becoming progressively more complex as the Fe content decreases. The smooth variation of the parameters derived from the spectra indicates that Fe and Mn perform the same functions in the glass. The author finds evidence for chemical ordering in these glasses and demonstrates the existence of two distinct atomic sites for Fe. Site 1 is attributed to Fe atoms having predominantly Y near neighbours and site 2 is identified as Fe atoms having a larger number of Fe (Mn) near neighbours.

Magnons in an antiferromagnetic gamma -MnCu alloy (94 at.% Mn) have been studied at room temperature by inelastic neutron scattering up to an energy of 12 THz. The magnons can be represented by a dispersion relation of the form omega _q²=C²q²+ omega ₀² with damping Gamma = Gamma ₀ omega _q. Interpretation of the data on this basis yields values for the spin-wave velocity of C approximately 40 THz AA( approximately 25 km s^-1), in general agreement with theory, and energy gap omega ₀=1.1+or-0.1 THz.

The magnetic region in erbium was investigated by measuring the electrical resistivity between 4.2 and 100K. The introduction of hydrogen in solid solution lowers the Neel temperature, the helical temperature and the spin-disorder resistivity, while it strongly enhances the manifestation of the Curie temperature. A characteristic point is observed near 30K and attributed to a higher-order harmonic due to the squaring up of the oscillating antiferromagnetic structure.

The authors present results of a Mossbauer investigation of Au-Fe alloys (15 and 20 at.% Fe) which provide evidence for marked precipitation of alpha -Fe in alloys annealed for short times between 200 and 450 degrees C. While the results are consistent with the published Au-Fe phase diagram they cast doubt on the conclusions of other investigations.

The excitation energies of d band holes in noble metals and their dilute alloys are calculated accurately using methods that provide a clear insight into the electron liquid response to the d hole. Quantitative descriptions of the d-hole-phonon interaction and the hole-hole interaction are also given.

Anharmonic contributions to free energy, pressure, specific heat, C_p^an(T), and thermal expansion, ( Delta v/v)_an, have been calculated for five alkali metals. A pseudopotential model that gives a rather accurate description of the atomic properties of these metals is used. In the range of T where vacancy concentration is negligible the calculated contributions C_v^an amount to 0.3-0.5 of the difference C_p-C_v and most of them are in good agreement with experiment. The volume dependence of C_v^an is very weak and anharmonic pressure P_an is quite small. The contribution ( Delta v/v)_an is determined mainly by the bulk modulus quasiharmonic variation due to thermal expansion. The lattice defect heat capacities C^d (T) of Na, K, Rb, and Cs have been determined from the experimental specific heat C_p and the calculated C_v^an and C_p-C_v. The vacancy concentrations n_v and formation energies E_1v^F have been estimated from the C^d(T). The values of n_v obtained are larger, and those of E_1v^F smaller, than most previous estimates.

Quenching and annealing experiments were performed on silver-zinc alloys with 8.14 and 30 at.%Zn. From the changes of the electrical resistivity due to an increase of the degree of short-range order, the activation energy of self-diffusion was determined to be Q_SD=1.60 and Q_SD=1.38 eV for both alloys, respectively. For the migration energy of vacancies, a value E_V^M=0.64 eV was found for the alloy with 8.14 at.%Zn. Evidence is given that the vacancy migration energy E_V^M of the alloys with 30 at.%Zn is smaller than 0.60 eV in agreement with data given by Berry and Orehotsky (1968). The results of measurements of radiation enhanced diffusion obtained by a Russian and a French group, are reinterpreted. It follows that the increase of the degree of order during irradiation is obtained only by vacancy enhancement of diffusion and that the migration activation energy of self-interstitials is E_I^M approximately=0.46 eV and E_I^M approximately=0.41 eV for the alloys with 8.14 and 30 at.%Zn, respectively.

The molecular dynamics technique has been used, in conjunction with the interionic potentials of Dagens et al. (1975), to study the stability, configuration, binding, and induced migration of mixed dumbbells in an irradiated Al-Zn alloy. For the purpose of comparisons, self-interstitials in pure Al were also investigated. The Al-Al and Al-Zn interactions were described by pair potentials which extended to ninth-neighbour distances. Both the self-interstitial dumbbell and the mixed dumbbell were found to be stable in the (100) configuration. The formation energy of the self-interstitial is 2.89 eV and the mixed-dumbbell binding energy is 0.38 eV. As a result of this strong binding, the threshold energy required to induce the migration of the mixed dumbbell is about 1.2 eV, which is significantly larger than the minimum energy of about 0.15 eV transferred to a self-interstitial to induce its jumps in pure Al. Caging motions of the mixed dumbbell were observed. The present computer-simulation results are compared with experimental measurements.

The lattice distortions and elastic relaxation energies due to interstitial hydrogen defects in metals are calculated from the method of lattice statics. Experimental values of the change in volume, Delta V, due to the defect are used to determine the hydrogen-metal forces. Calculations are performed for tetrahedral and octahedral interstitials in Pd, Ni, Nb, V and Ta for a model in which only nearest-neighbour hydrogen-metal forces are considered. Estimates for a number of other metals in which Delta V is not known are made by assuming Delta V=2.9 AA³. Nearest-neighbour distortions of metal atoms are found to be approximately 1% of the metal lattice parameter and relaxation energies are calculated to be around -0.1 eV except for the alkali metals for which the estimated relaxation energies are of the order of -0.001 eV.

A previously proposed six-parameter phenomenological screened shell model is applied to the calculation of the phonon dispersion curves of palladium. The resulting fit, for all symmetric branches except the (111)_T, is as good as or better than a seven-parameter calculation using Krebs' model (1965) extended to include two-body forces to third neighbours.

Harmonic phonons and phonon-limited resistivities have been calculated for Rb and Cs using pseudopotentials obtained from first principles. These pseudopotentials are highly non-local and the role of non-locality in influencing physical properties has been emphasised. Both phonons and phonon-limited resistivities are in excellent agreement with available experimental results.

A new statistical thermodynamic approach based on free-volume theory has been applied to the study of the properties of binary cadmium-bearing solutions. The calculated values using expressions developed in this approach are in very good agreement with the experimental data over the whole range of concentration of such solutions using infinite dilution parameters and they also explain the asymmetric behaviour of some of these systems.

Discussing a liquid binary alloy for definiteness, the author investigates a model of electromigration in which the ions are treated as classical particles subject to velocity-independent driving forces varying in general with configuration and which are assumed to be derivable from a many-body potential. If the driving forces are independent of isotopic mass there is no Haeffner effect in this model. The result obtained for the equilibrium concentration gradient is the same as if the driving forces were steady at the averaged values of the configuration-dependent forces. However, the elementary formula for the gradient of the logarithm of the concentration is shown to be modified in an exact analysis by the factor f to fF/k_BT where F is the driving force and f=S_cc/c(1-c), c being the concentration and S_cc the long-wavelength limit of the concentration-concentration structure factor. There is a similar correction to the usual expression for the drift velocity before a significant gradient is set up. The remarks are illustrated by reference to experimental and theoretical work on Na-K.

For pt.I see ibid., vol.10, p.651 (1980). The electronic surface densities of states of CuNi alloys have been computed in the tight-binding approximation using the moments method. The results are given for (001), (110) and (111) surfaces. An electronic model of the surface segregation is then proposed. In this model, the segregation internal energy is deduced from the densities of states. In a first step, the model is analysed within a two-moment theory in which the electronic correlation effects are shown. The theory is in good agreement with existing data on surface segregation in dilute transition metal alloys. Finally, the surface composition of CuNi alloys is computed and is compared with experimental results.

A non-interacting local Fermi liquid theory applicable to Ce, Sm, Eu and Yb metallic intermediate valence materials is presented. The induced density of states of a given rare earth ion is taken as the Lorentzian appropriate for a rare earth ion impurity, for which the justification is tentatively discussed. The degeneracy of the resonance is that of the magnetic configuration of the ion. The results are applied to the f occupation, susceptibility and specific heat. A relationship existing in the literature between the zero temperature susceptibility and linear coefficient of specific heat follows exactly from the present very simple theory. The results are compared with a range of experimental data.

A formalism, previously developed with the aim of calculating the surface impedance of a metal in the presence of an arbitrary magnetic field parallel to the sample surface, is considered in the limit of a vanishingly small magnetic field. It is shown that the correct limiting value are obtained in the normal and anomalous skin effect situations.

Superconducting La₃S₄ single crystals containing a definite number of cation vacancies are analysed by magnetic susceptibility, specific heat, upper critical magnetic field and low temperature X-ray diffraction measurements. The authors find experimental proof that the cubic to tetragonal distortion of the unit cell found in crystals close to stoichiometric La₃S₄ is of Jahn-Teller band type. The strongest electron-phonon coupling and the maximum superconducting transition temperature is found when the cubic to tetragonal phase transformation is just suppressed by doping with vacancies.

Accompanying papers calculate the degeneracy of the ground state of a spin glass. These ground states may be accessible or inaccessible to one another. It is argued that the procedure offered by Edwards and Anderson (1975, 1976) implicitly assumes inaccessibility, and a case can be made for the correctness of this assumption. It implies that the entropy calculated by these authors differs from that based on the accessibility hypothesis, and at absolute zero temperature this is given by kappa lnN where N is the number of ground states.

The hydrogen storage compound LaNi₅ is an exchange-enhanced Pauli paramagnet with susceptibility chi (LaNi₅)=4.6*10^-6 EMU g^-1. Hydrogen absorption leads to a decrease of the susceptibility of chi (LaNi₅H_6.0)=1.3*10^-6 EMU g^-1. This decrease is usually covered by a strong irreversible increase of surface magnetism, where, through surface segregation and decomposition, superparamagnetic Ni precipitations are formed. The two effects explain the controversial results about the magnetic properties of the LaNi₅ hydride. The implication of this decomposition on the cyclic lifetime is discussed.

The author has investigated the influence of the state of remanent magnetisation on the transverse and longitudinal magnetoresistances of AuFe spin-glass alloys. Large hysteresis effects have been observed for the first time. Whatever the magnetic history of the sample, the magnetoresistance depends only on the macroscopic magnetisation m= chi H+m_r where chi H and m_r are the reversible and the irreversible magnetisations. This dependence is very simple and quadratic in m: Delta rho / rho ₀=- alpha ( chi H+m_r)². The coefficient alpha is found to be almost independent of the concentration when this concentration is varied from 200 PPM to 70000 PPM (at least). These data contradict a cluster model for AuFe spin-glass alloys but are consistent with a homogeneous picture for both the reversible and the irreversible magnetisations. The anisotropy field concept is not found to be useful in the case of AuFe spin-glass alloys.

Mossbauer hyperfine and magnetometer measurements have been made on different compositions of the spin-glass system Pd_1-yFe_yH_x. It is found that in the spin-glass phase the hyperfine field H_HF below the ordering temperature T₀ shows a distribution P(H_HF) which is zero for H_HF less than a lower critical limit H_HF⁰. Satisfactory numerical agreement is found with the calculations of Palmer and Pond (1979) and of Sommers (1978, 1979) for the equivalent limiting molecular field. The effect of a magnetic field on the spin orientation (T<T₀) is shown to be described in terms of an effective superparamagnetic cluster. For T>T₀, strong relaxation effects have been discovered in the Mossbauer spectra in the presence of an external magnetic field. Macroscopic susceptibility and magnetisation measurements have been used to verify the presence of ferromagnetic clusters. One result is that for T>T₀, the effective Curie-Weiss temperature value strongly depends on external field, caused by the very wide spectrum of interactions in a spin glass.

It has been pointed out that the onset of ferromagnetism, as a function of concentration, in atomically disordered transition metal alloys is a cooperative magnetic phase transition which is unavoidably magnetically inhomogeneous. Here it is argued that the inhomogeneity of this phase transition should not invalidate the applicability of Landau's theory. Within the framework of this theory the concentration dependence of, and the effect of pressure on, the spontaneous magnetisation, initial susceptibility and the ferromagnetic Curie temperature are discussed.

High-accuracy measurements of the temperature derivative of the electrical resistivity (d rho /dT) and magnetic susceptibility ( chi ) are used to study the complex antiferromagnetic behaviour of dilute Cr-Al alloys (c<or approximately=7 at.%Al). Striking anomalies appear in both properties at intermediate compositions (1.1<or approximately=c<or approximately=3 at.%Al), suggesting the onset of a new inhomogeneous structure with some form of localised magnetic moments. Possible implications of recent models on point-defect-induced magnetism are examined. The authors' results provide very detailed information on the complex dependence of the Neel temperature on Al content (T_N(c)). They also explain the reason for the apparently strong discrepancies among T_N values quoted in the literature for samples with intermediate Al concentrations.

Following a suggestion by Egami (1979), the author's earlier Monte-Carlo simulation calculations (Harris and Sung, 1978), for the random anisotropy model are shown to be incomplete. Using a modified algorithm, it is shown explicitly that the ferromagnetic (asperomagnetic) state is unstable against the reversal of a small number of spins, and that the energy of the resultant new ground state is significantly less than that of the asperomagnetic state, particularly for large anisotropy values. Thus, these new results appear to conform with the predictions of the renormalisation group calculations of Pelcovitz et al. (1978). Some implications for magnetisation processes and coercive field behaviour in real materials are also discussed.

The electrical resistivity and Young's modulus of several samples of chromium have been measured in the vicinity of the Neel transition. To within the accuracy obtainable, no thermal hysteresis was observable.

For pt.I see ibid., vol.10, p.2041 (1980). The generation and attenuation of ultrasonic waves due to incident radio frequency magnetic fields at the surface of a ferromagnetic metal are studied. Measurements are made of the attenuation and generation of longitudinal and shear waves as a function of the applied magnetic fields, frequencies and temperature (below 300K) for propagation along the (100) and (110) directions of nickel single crystals. Two processes are shown to be important: a magnetorestrictive process at low fields and a modified Lorentz force process at high fields. The field-dependent attenuation results are explained better by a damped magnetoelastic-spin-wave theory than by a domain-wall damping theory.

The optical properties of evaporated films of uranium have been studied for the first time in the energy range 0.6-25 eV. The data have been found similar to the corresponding data for Th. Some qualitative conclusions are drawn about the bandstructure of U.