Table of contents

The small-angle (magnetic) scattering of neutrons from an ensemble of randomly oriented, identical, homogeneous, ferromagnetic fine particles is considered. It is found that the usual Guinier expression derived for the small-angle scattering of X-rays is invalid if there is a net correlation between particle moment and particle shape. In this case the coefficient R² which occurs in the Guinier law is given, at low temperatures, by R²=³/₅(R_G²+R_zz²) where R_G² is the squared radius of gyration and R_zz² is the moment of inertia of the particle about the axis of stable magnetisation (normalised to unit mass). This is in contrast to the classical Guinier result R²=R_G².

The crystal symmetry of the reaction product of hydrogen with the intermetallic compound Mg₂Ni has been determined to be tetragonal with unit cell dimensions: a=6.533 AA and c=7.499 AA, by in situ X-ray diffractometry at about 623K and under 4.6 MPa hydrogen pressure. The resultant hydride phase transforms at 503K to an orthorhombic structure with lattice dimensions: a=11.394 AA, b=11.196 AA and c=9.165 AA. This phase transition is of polymorphic type as is evident from the well defined exothermic (endothermic) peak in the pressure-differential scanning calorimetry curves obtained during cooling (heating) after the formation of the nearest stoichiometric hydride Mg₂NiH₄. The results are briefly compared with previously reported data.

The phonon theory of Percus and Yevick (1958) is used to calculate the entropies of liquid metals. The zeroth-order picture is of independent density fluctuations accommodated within an extended Debye sphere of radius k₀=3^1/3 k_D; most of the entropy arises harmonically from the extended region (k_D, k₀). The anharmonic contribution amounts to about 5% of the total and comes mainly from the interaction of phonon pairs with large (approaching 2k₀) wavenumber differences. Observed structure factors are used as basic input data in the calculations: they must therefore be known with good accuracy especially in the longer wavelength region (k_D, K₀). The density-fluctuation model outlined above has been corroborated directly (by neutron diffraction experiments) only for Rb. Nevertheless, accurate calculated entropies are obtained for a variety of metals.

The derivatives of non-linear coupling coefficients in the Taylor expansion of thermodynamic potentials should play a major role in solids driven off equilibrium. A measurement is described of the derivatives of the Gruneisen thermomechanical state function gamma with respect to the imposed tensile deformation epsilon *. For alpha -Ti, these derivatives are found to be delta gamma / delta epsilon *=-73+or-0.9 and delta ² gamma / delta epsilon *²=-(4.13+or-0.02)*10⁵.

The field dependence of the hyperfine interaction of ¹⁶¹Dy in Au has been studied in external magnetic fields up to 3.21 T by means of Mossbauer spectroscopy. ¹⁶¹Dy was introduced into a single crystal of Au by means of low-temperature ion implantation. From the measurements the authors determine the parameters of the cubic crystalline electric field acting on the Dy nuclei: A₄(r⁴)=-25.4_-2.5^+1.2K and A₆(r⁶)=+1.34_-1.22^+0.50K, in agreement with EPR results. The ground state is a Gamma ₈⁽¹⁾ quartet separated from a Gamma ₈⁽²⁾ excited quartet by Delta =10.8_-0.6^+0.9K.

A resistor-network model has been devised which sheds light on many of the resistive properties of inhomogeneous, anisotropic conductors. The model is applied to explain the changes of resistance with temperature (1.5-300K) and pressure (0-140 bar) for samples of (SN)_x subjected to small amounts of mechanical damage. The well-known minimum of resistance with respect to temperature frequently seen in (SN)_x is due to inhomogeneous current flow, which is greatly enhanced by mechanical damage, while the inhomogeneity which gives rise to the slight minimum in the starting material (apparent resistance ratio of about 40) can be allowed for, giving a corrected resistance ratio of about 120, and also giving corrected values of rho /sub /// (T) over the range 2-300K. The value of d ln rho /sub ////d ln T is approximately two above about 100K, but falls to almost unity near 40K before rising to values of about four below 10K in a way characteristic of phonon scattering with theta _R approximately=43K. The quasi-T² form of rho /sub ///(T) at high temperatures results from thermal expansion, and no evidence is found for any intrinsic 'T²' behaviour.

Electrical resistivity measurements have been made on dilute and concentrated binary magnesium alloys containing Ag, Zn, Al, Nd, Pb, Sn and Th over the temperature range 77-320K. Small deviations occur from Matthiessen's rule for alloys that have been solution treated and quenched to retain the solid solution. Much larger deviations occur when the alloys are heat treated to precipitate the solute atoms from the solid solution. Possible causes of this effect are discussed.

Measurements of the electrical resistivity as a function of temperature are described for the temperature interval from 1.1 to 4.2K for potassium samples with resistance ratios varying between 390 and 8200. The measurements show the existence of phonon drag in the resistivity and also a weakly temperature-dependent term T^m (with m close to 2), with a sample-dependent magnitude. The temperature-dependent resistivity rho (T) in this temperature interval can be represented by rho (T)=AT²+BT exp (-20 K/T). The possibility of ascribing the AT² term to electron-electron scattering is discussed. The possible influence of the dislocation density on the linear magnetoresistance and on the deviations from Matthiessen's rule is discussed briefly.

The relative contributions to the order dependence of the electrical resistivity of FeCo from the relaxation time tau and the effective number of conduction electrons n_eff have been determined from the residual resistivity and temperature coefficient of resistivity at various states of order. It was found that increased atomic correlations at the critical order-disorder temperature T_c give rise to a decrease in tau and an increase in n_eff. Below T_c both n_eff and tau increase with increased long-range order.

The dependence of the electrical resistivity of Pb on hydrostatic pressure is determined over major portions of the temperature and pressure ranges 1-300K and 0-10 GPa (100 kbar), respectively, and for Sn at 300K. The results are compared with a simple Bloch-Gruneisen law including volume changes due to thermal contraction. It is demonstrated that both Pb and Sn are ideally suited for use as accurate resistive manometers, enabling a reliable continuous determination of pressure over a wide temperature range. The agreement between the pressure at low temperatures indicated by both resistive and superconducting (T_c) Pb manometers is excellent. T_c(P) calibration curves are given to 22 GPa for Pb and to 5 GPa for Sn and In. A general method is presented which shows how pressure can be used to test for the presence of electron-electron scattering; in the temperature range studied, T>or=7.2K, electron-phonon scattering constitutes the dominant scattering mechanism in Pb, as expected.

The electron density of states has been calculated by means of the recursion method and the continued-fraction technique for the case of a Cu overlayer on a Ni surface for different values of the coverage fraction. The calculations have been performed in the tight-binding scheme taking into account only the d bands. The results are discussed in the light of recent results from ultraviolet photoemission spectroscopy.

The effect of macroscopic surface roughness upon the resistivity of thin metal films has been calculated by computer modelling of the form of these films. It is shown that, as a consequence of the tendency for the films to be islandised, the resistivity as a function of decreasing mean film thickness rises well above the predictions of conventional surface scattering theories. Theoretical results are compared with existing experimental evidence and good agreement is obtained for films of potassium, gold, aluminium, copper and palladium.

The superconducting transition in conjunction with structural changes of amorphous bismuth thin films is investigated by way of electrical resistance measurements and low-temperature electron diffraction. In the process of annealing, bismuth films show two distinct stages of superconducting transition. One, at higher temperature, is associated with a sharp drop in the film resistance and its critical temperature gradually decreases by annealing to higher temperatures. The other, at lower temperature, exhibits a gradual decrease in resistance due to a widespread transition over a few degrees. Using low-temperature electron diffraction, a mixture of the amorphous phase and the crystalline phase with the normal rhombohedral structure is observed and no intermediate phase is observed during the crystallisation process. The two-step superconducting transition is interpreted by the size distribution of residual amorphous regions in the film.

The problem of the low-lying excitations in a model of commensurate chromium alloys is examined. Using a density matrix formalism the authors find all possible excitations and show that, in addition to the well known transverse spin waves, there also exist longitudinal spin waves coupled to charge density waves with an acoustical dispersion relation. The authors discuss the physical meaning of these results.

Reports a study of crystal-field (CF) excitations by neutron spectroscopy and a study of magnetic susceptibility and electrical resistivity of some cubic cerium intermetallics having the CsCl or a related structure; namely the compounds CeZn, CeMg₃ and CeInAg₂, CeZn undergoes a first-order transition to antiferromagnetism with a strong magnetostriction at T_N=30K, whereas for CeMg₃ and CeInAg₂ (T_N=3.4 and 2.3K respectively) the transition is of second order. The magnetic resistivities of the cerium compounds obtained by subtracting the resistivities of homologous lanthanum compounds were found to decrease with increasing temperature over certain temperature ranges, indicating an incipient Kondo effect. CeMg₃ has a CF splitting Delta of 195K, the ground state being the Gamma ₇ doublet in contrast to the case of the CeMg compound with its Gamma ₈ ground state. For CeZn and CeInAg₂, the CF splitting is rather small ( Delta approximately 65 and 18K respectively).

AC susceptibility and electrical resistivity measurements have been carried out on a series of dilute PdMn alloys containing between 0.5 and 2.5 at.% Mn. The AC susceptibility taken in various collinear biasing fields exhibits a maximum in the vicinity of the Curie temperature as predicted by the scaling law equation of state. Further, the field dependence of these maxima is uniquely governed by the critical index delta , and estimates for this latter index have been made along the index gamma and the Curie temperature T_c. The electrical resistivity data, taken on the same samples, are in good overall agreement with the results obtained from the susceptibility study.

The effect of an anisotropic conduction electron g factor on transmission electron spin resonance (TESR) through metal foils of finite (non-zero) thickness is analysed theoretically. The role of electron-electron exchange interaction is considered and the results of Monod's experimental observation of TESR in Pd are discussed.

The Doppler-broadened line shape of 511 keV gamma rays emitted from annihilating positron-electron pairs, the positron lifetime and electrical resistivity of the amorphous Fe₄₀Ni₄₀P₁₄B₆ alloy have been measured in the 0-500 degrees C temperature range using isochronal annealing. The W parameter of the Doppler-broadened line shape, related to core-electron annihilation, increased in two stages by annealing below the crystallisation temperature. These increases appear to be due to the loss of excess free volume in the as-quenched amorphous alloy.