Table of contents

Magnetic excitations in a Co-Fe alloy were investigated by inelastic neutron scattering in a wide temperature range. The observed peaks remained fairly intense and well defined close to and above T_c even in the low momentum transfer region where no spin wave peaks could be observed in pure nickel and iron.

A face centred cubic phase occurs when thin films are vacuum condensed from erbium vapour. The weight increase which occurs when this phase is oxidized to Er₂O₃ shows that the phase is a new form of metallic erbium and is not ErO as has been suggested by other workers.

Cluster effects are dealt with self-consistently in disordered alloys within the multiple scattering framework. Pairs are handled explicitly. The results are also shown to be compatible with the findings of the diagrammatic analysis and the mean field approach.

Photoemission measurements from clean liquid mercury are reported for photon energies up to 21 eV. The results are consistent with a dip in the density of states close to the Fermi level. In addition, a new value for the binding energy of the 5d core levels has been derived from the data.

De Haas-van Alphen oscillations of magnetic susceptibility have been observed in a lithium dispersion in paraffin wax. Analysis of the results provides an upper bound for the distortion of the Fermi surface, which is inconsistent with contact with the Brillouin zone faces. An estimate has been obtained for the mean value of the cyclotron mass.

Caplin, Napoli and Sherrington (1973) have stated that the normally accepted condition for the independence of electron-phonon and electron-impurity scattering, ql>>1, is wrong and a much stronger condition is needed. It is shown that the normally accepted condition is in fact correct, and that the condition derived by Caplin et al. does not apply for their discussion of electrical resistivity at low temperatures.

Measurements on ZnAl and ZnAg alloys shows a T⁵ phonon resistivity at the lowest temperatures with the coefficient varying logarithmically with residual resistivity.

The electronic part, eta , of the electron-phonon mass enhancement is calculated at normal and reduced volumes for Nb and V. It is found that eta increases rapidly with decreasing volume and the coefficients dln eta /dlnV are in good agreement with empirical analyses of the pressure dependence of T_c in these transition metals.

Behaviour previously detected by Walmsley (1972) in samples of a Pb-Tl alloy is again observed in a Pb-In alloy under significantly different experimental conditions. It is suggested that this behaviour is typical of all nearly reversible superconductors when currents and magnetic fields are applied in parallel geometry.

Mossbauer effect measurements on Ni₃Al and Ni₃Ga sources and absorbers demonstrate the preference of Co for the Ni site and Fe for the Al/Ga site. Quadrupole splitting at the Ni site is about 0.50 mm s^-1. The magnetic hyperfine fields in Ni_74.75Al₂₅Fe_0.25 at the Ni and Al site are 227 and 238 kOe respectively. The role of inhomogeneities is discussed and it is shown that these can explain the different results recently reported in the literature.

NMR spin echo spectra of ⁸⁹Y were studied at 4.2 K for YFe₂, YFe₃ and Y₂Fe₁₇. The sign and magnitude of the hyperfine field at the Y sites was determined and has been interpreted in terms of long range conduction electron spin polarization.

The creation of Frenkel pairs in fcc copper caused by primary knock-on atoms of energies between 10 eV and 125 eV has been studied by computer simulation of the many body dynamics of the displacement process. The interstitial-vacancy separation has been obtained as a function of the primary energy and initial direction of motion, and the interatomic potential. Maximum separation occurs along the (110) direction and minimum along the (111) direction for all energies. The qualitative trends are not very sensitive to potential although the quantitative results are. For directions close to (110) the higher energy sequences are less well focused and the resulting pairs have a smaller separation than those for the lower energy primaries. Implications of this result for the concept of a displacement threshold are discussed.

The results of ultrasonic measurements of wave velocities in polycrystalline samples of pure copper and four copper-aluminium alloys, containing up to 16.6 at.% Al, show that the three elastic moduli, K, G and E, decrease with increasing aluminium content. The relative effect of the alloying, calculated for pure copper from the average slope of values of the measured moduli between 4.2 and 16.6 at.% Al, is 0.338 for K, 0.384 for G and 0.379 for E. In all three cases the contribution to the total change from the increase in interatomic distances is negative, while that from the increase in the electron concentration is positive. The measured values of the moduli agree fairly well with the values corresponding to recent precision measurements on single crystals.

Single crystals were examined at approximately 100 kHz over the temperature range 4.2-300 K. The specimens were oriented near the centre of the stereographic triangle and were deformed in tension at 156 K and 450 K. Increasing the magnitude of the applied strain amplitude was found to increase the background damping in all specimens and to produce an alpha peak in some instances. The occurrence of the alpha relaxation peak was found to depend on the presence of an optimum dislocation configuration, internal stresses, and applied strain amplitude. The optimum dislocation configuration appears to be one of edge type segments present after small amounts of deformation and under the influence of locally high internal stresses. Other segments apparently achieve this or other optimum configurations after much larger amounts of plastic deformation when the overall internal stress level is higher. The results are interpreted in terms of the depinning of kinks trapped by point defects as the likely mechanism of the alpha peak.

Ultrasonic propagation in dilute magnetic alloys at low temperatures is theoretically investigated based on the s-d interaction model. It is found that the ultrasonic attenuation coefficient in these systems behaves in a qualitatively different way with temperature from that in pure metals in the low temperature region. Also the attenuation coefficient is quantitatively very small compared to that in pure metals below the Kondo temperature. The extent of decrease is greater the larger the impurity concentration, the higher the Kondo temperature. The associated frequency shift turns out to be very small.

An empirical model is presented in which transition metals in a binary alloy are assumed to contribute individually to the total electronic heat capacity gamma (or rather to the density of states). From an analysis of experimental data it is demonstrated that these contributions are approximately characterized by the individual number of electrons per atom, Z, that is, gamma as a function of Z is determined by atomic rather than (crystal) structure effects. For each of the metals in a binary alloy the value of Z is determined both from its position in the periodic table, Z⁰, and from the transfer of electrons which takes place upon alloying. The latter correction, Delta Z, is calculated from the electronegativity, phi *, scale presented in a previous paper. Using Delta Z=1.2 Delta phi * (1-c) and characteristic curves for gamma as function of Z, a description of all available heat capacity data for Pauli paramagnetic solid solutions of the transition metals is obtained.

The thermoelectric powers, S, of the pure metals Mg, Ca, Sr, and Ba have been measured across their melting points. The values obtained for the liquids at the melting point are: Mg, 1.6 mu V degrees C^-1; Ca, 9.9 mu V degrees C^-1; Sr, -0.4 mu V degrees C^-1; Ba, -0.2 mu V degrees C^-1. S shows little change with melting for Ca, Sr, and Ba; for Mg, S changes sign upon melting. The measured liquid thermopowers, in light of recent resistivity data for these liquids, indicate a progressively increasing energy dependence of the pseudopotentials with increasing atomic number (Mg to Ba). The temperature dependence of the thermopower, dS/dT, for these liquids is discussed in terms of a simple theory.

Hayman and Carbotte (1972) have calculated the electron-phonon scattering time tau ₀(k,T) for an electron in the state mod k> at temperature T in the alkali metals. They found that the tau ₀(k,T)'s can be very anisotropic as k varies over the Fermi surface even when it is spherical. Strictly speaking their results apply only to pure metals. In practice impurities will be present giving rise to an additional scattering mechanism. This will modify the scattering times. The authors find that small concentrations of defects can effectively wash out the anisotropy in the scattering times. This gives rise to a nonlinear behaviour for the resistivity as a function of impurity content. The effect is found to saturate rapidly with concentration and it is also temperature dependent, leading to deviations from Matthiessen's rule.

In the presence of a steady magnetic field B₀ and a pulsed field B₁ sin omega t, echo effects were observed in powder samples of the metals Cu, Al, Zn, Cd, V, Ag and in the alloys CuCo (0.23% and 0.05%). No such effects were observed in Pb, Pt and Nb powder samples. Various experiments on the echoes are described and some mechanisms for their appearance discussed. The mechanism which most easily describes the experimental data is one involving collective oscillations of the whole sample, that is, many granule oscillations.

An analysis is made of the AC electrical conductivity of a thin metal film of relatively short electronic mean free path deposited on a pure metal substrate, assuming such good contact that a proportion of the electrons approaching the interface cross over to the other side without being scattered. An approximate but selfconsistent method is developed for combining Maxwell's equations with the solution to the Boltzmann transport equation. The results show how the effective conductivity of the film and the surface impedance of the film-substrate combination vary with the film thickness, the electric mean free path in each medium, the frequency of the applied field and the probability of electron cross over. The effect analysed is the classical analogue of the proximity effect between superconducting and normal metals.

A method of calculating the long wavelength spin wave frequencies in a multiple band itinerant antiferromagnet is described. No special features of the band structure, such as nesting, are assumed. The frequencies are derived by examining the poles of the RPA generalized susceptibility for the d electrons. A natural consequence of the treatment is the existence of a spin wave damping proportional to the wavevector. This is attributed to the decay of the spin wave into an electron-hole pair. The formulae are applied numerically to gamma manganese. Rough agreement with experiment is found.

The excess magnetic susceptibility of a dilute magnetic alloy is calculated for the case of equal g factors from the zero magnetic field partition function of an s-d model with one localized impurity spin. The partition function is obtained from an expression for the excess specific heat derived in a previous publication. The calculated susceptibility is finite at temperature T=0 due to the linear variation of the specific heat with T at T approximately=0, and rapidly approaches a Curie-Weiss form with rising temperature.

It is shown that a simply averaged value of (g-1)²J(J+1) can be used to explain the variation with composition of the magnetic disorder resistivity of Gd-Dy alloys observed by Bozorth and Suits (1964).