Table of contents

The scattering structure factor S(Q) describing distortions induced by a substitutional defect in a simple metal has been calculated from lattice statics and Shaw model potentials; interactions of arbitrary range can be included via reciprocal lattice sums. Diffuse elastic neutron-scattering measurements on Al single crystals with a few at.% Mg confirm the dependence on the momentum transfer hQ predicted for a Mg impurity in Al. Phenomenological fits assuming forces acting upon the next-nearest neighbours of a solute atom cannot explain that dependence.

Usual polycrystalline particles measuring <or=1 mu m were observed in and near FCC Co, the binder phase of hard metals, bearing a strong resemblance to certain spinels occurring in nature.

Recent Brillouin scattering data for the (111) surface of Cr (Sandercock 1978) are compared with the theoretical cross section computed for a semi-infinite anisotropic elastic medium undergoing free-surface boundary conditions. The surface ripple mechanism is found to be responsible for the continuum of phonon states localised between the transverse and longitudinal thresholds.

The magnetoresistivity is studied in the framework of exchange scattering by a system of magnetic ions distributed over an amorphous structure. The resistivity depends on the magnetisation and the nearest-neighbour spin correlation, which are calculated in a self-consistent two-spin cluster approximation within the random anisotropy model. Theoretical curves are presented for both ferromagnetic and antiferromagnetic interaction between the rare earth ions; the former account for experimental data on DyNi₃ and HoNi₃ alloys, while the latter account for those on the TbAg alloy.

For bulk alloys of Au+8.5 at.% Fe and Au+12.3 at.% Fe the temperature T_f shifts by 0.9K and 1.1K to higher values when the frequency increases from 110 Hz to 5500 Hz. T_f of Au+4.5 at.% Fe is independent of the frequency. The reduction of the maximum of the initial susceptibility due to applied DC fields decreases with rising frequency. These results can be interpreted in terms of superparamagnetic Fe clusters freezing out near T_f.

Using nuclear orientation at 15 mK the magnetic hyperfine field at ⁵⁴Mn nuclei in dilute solution in HCP Co is measured as +97+or-7 kG. This is in dramatic contrast to the field on Mn in FCC Co of -358 kG (confirmed in this work). The result is discussed in terms of a change of sign of the local moment which, for Mn, halfway through the transition series, is particularly sensitive to the exact electronic structure of the host.

The antiferromagnetic phase boundary in the Cr-Fe system has been determined by neutron diffraction. Antiferromagnetism disappears at a concentration of 16.0+or-0.5 at.% Fe. Resistivity measurements indicate that T_N is marked by the maximum in the temperature coefficient of resistivity rather than the minimum in the resistivity. On the basis of these measurements it is apparent that no long-range order exists in Cr-Fe alloys between 16 and 19 at.% Fe.

Reports on an absolute determination of the lattice parameter of potassium at 20 degrees C. Data were obtained with the aid of a time-of-flight neutron spectrometer. In addition, the variation of the lattice parameter from 6 degrees C to 60 degrees C has been measured using a neutron double crystal backscattering spectrometer. Some information on the crystal quality is also given.

Magnetisation measurements have been carried out in Fe_xSn_1-x amorphous alloys obtained by atomic deposition. A critical composition, x_cr approximately=0.4, for the onset of ferromagnetism has been found along with a rapid increase of the magnetisation when x>x_cr. Comparison with data previously obtained for Fe_xSi_1-x and Fe_xGe_1-x amorphous alloys suggests that the onset of magnetic ordering may result from localised interactions; details of the short-range order and chemical correlations might be quite different for Si, Ge and Sn compounds. Temperature dependence of the magnetisation has shown that there is not a well defined Curie temperature in amorphous Fe_xSn_1-x which is consistent with magnetic inhomogeneities.

The general behaviour of a T=0K metal phonon spectrum near a Fermi surface (FS) topology transition is studied in the framework of the quasiharmonic adiabatic approximation. The transition parameter is taken as the separation Delta =E_F-E_c between the Fermi level and the energy of the electronic Van Hove singularity responsible for the transition. The dominant anomaly of the static electron density response function is first derived within the RPA and substituted in a perturbation formula (derived from the general formulation of Pick, Cohen and Martin (1970)) which relates a phonon energy change to a perturbative variation of the irreducible electronic polarisability. The overall variation Delta omega ² with Delta has been estimated from a crude model involving a cut-off parameter related to the 'critical states' bandwidth ( Delta omega ² includes the cut-off independent dominant anomaly Delta omega ²); phonons dips, as observed in Nb or Pb for instance, are expected quite generally when a Van Hove singularity is near E_F. These results have been used to study the Delta dependence of a phonon spectrum average of a frequency-dependent function (such as ( omega ⁿ)); a C_+or- mod Delta mod ^3/2 anomaly is found, with different coefficients C_+or- when Delta to +or-0. The theory is applied in a semiphenomenological way to the Tl-Bi-Pb alloys phonon softening at the X point.

Previous studies of dilute ZnFe alloys have indicated that, although the iron atom does not possess a well defined localised magnetic moment, it is on the verge of magnetism. In this work alloy samples containing up to 0.4 at.% Fe in Zn, prepared by a rapid quenching technique to overcome the limited solubility, have been studied using the Mossbauer effect in ⁵⁷Fe. The zinc-rich intermetallic compound Zn₁₃Fe has also been similarly studied. Measurements at temperatures down to 4.2K and in applied magnetic fields up to 60 kG show that the mean square displacements of the Fe atoms in Zn are markedly anisotropic, as expected from elasticity measurements and lattice dynamic calculations. In the alloys and in Zn₁₃Fe the electric quadrupole splitting is shown to be virtually temperature-independent. The high-field-low-temperature spectra, although complex, are shown to indicate a very low limit to the local susceptibility consistent with the conclusion that the Fe solute atoms in Zn carry no appreciable local moment.

Thermal expansion measurements have been made between 1.5K and 30K of samples of Al₁₀V and Al₁₀V+0.8% Ga which reveal anomalous behaviour similar to that reported by Caplin and Nicholson in the specific heat of the same samples. The anomalous expansion, which is ascribed to the volume dependence of local mode excitations is two orders of magnitude greater than the thermal expansion of other metals at these temperatures. This allows the energy of the first excited state to be obtained with greater accuracy than from the specific heat, the values being 22+or-0.3K and 8+or-0.4K when Al and Ga respectively occupy the local mode site. The expansion was found to fit the Schottky function for four equispaced non-degenerate energy levels rather better than the Einstein function assumed previously. Re-analysis of the specific heat indicates a large temperature dependence of the Debye temperature which is thought to result from hybridisation of the phonons with local modes. The Gruneisen parameters were found to be 81 for the Al mode and 85 for the Ga mode.

Volume plasmon dispersion and damping of polycrystalline Al and In films has been measured in the wave vector region k<k_c by means of electron energy loss spectroscopy. The dispersion in both materials shows a change in the slope of the dispersion curve at wave vectors k approximately 0.5 AA^-1. This probably can be attributed to bandstructure effects. Values of the dispersion and damping coefficients are given for Al and In.

The Fermi surface touching the Brillouin zone boundary not only stabilises the BCC crystal structure, but also promotes ferromagnetism in Heusler alloys.

A recently developed formalism is applied to the evaluation of the surface impedance for a metal in a magnetic field. A spherical Fermi surface is assumed for the metal and the field configuration considered is the Azbel-Kaner geometry. The formalism makes use of the dispersion relation for extraordinary cyclotron waves in an essential way and the authors therefore give a detailed treatment of this dispersion relation for a finite value of omega tau and for frequencies of the order of 25-50 GHz. The fundamental cyclotron resonance and the surface impedance anomalies, occurring above the fundamental resonance, observed experimentally in potassium by Dunifer and co-workers are calculated. The correlation between the surface impedance and the dispersion relation for the cyclotron waves is pointed out and the significance of the polarisation properties of the waves is emphasised.

The Hall coefficients of several TaS_2-xSe_x layer compounds have been measured as a function of temperature between 4K and 200K. Anomalous changes in R_H, which are associated with the occurrence of charge density wave periodic lattice distortion formation, have been observed and the variation of the onset temperature T₀ with composition has been obtained. Values of T_c and dT_c/dP have been obtained for single-crystal TaSe₂ which are higher than those reported for polycrystal material. A strong inverse correlation is found between T₀ and the superconducting transition temperature T_c. A comparison between T₀ and dT_c/dP indicates that the pressure-induced increase of T_c for these compounds is not entirely due to the suppression of the charge density wave, as previously suggested.

The response to an AC magnetic field of a type II superconducting single-crystal Pb-Na cylindrical rod has been observed as a function of longitudinal DC fields. The results show two different regimes of flux pinning within the specimen. At low fields in the mixed state the specimen is largely reversible in the bulk and all pinning appears to be at the surface. In a field close to H_c2, there is a peak region where bulk pinning rapidly dominates. Evidence for these regimes is discussed.

It is shown that there are two distinct contributions to D in disordered itinerant electron systems: the 'average exchange' contribution and the 'magnon scattering' contribution. Only the former has been considered in existing quantitative work on transition metal alloys and only this contribution is retained in deriving a new formula for D within the framework of the KKR-CPA method. Both contributions to D are discussed further using a tight-binding model, and the 'magnon scattering' contribution is related to previous work by Morkowski (1976-77). The success of existing quantitative work on D in transition metal alloys suggests that in most of these systems the 'average exchange' contribution is dominant. However, it is shown for a simple model of an amorphous system that the two contributions are equally important in determining the difference between D in the amorphous and crystalline states.

The low-temperature magnetic structure of the hexagonal modification of FeGe has been determined by neutron diffraction methods. Below a transition temperature of 30.5+or-0.5K, the iron moments cant away from the (001) direction to form a double-coned structure in which alternate antiferromagnetically aligned (001) sheets of iron atom moments make an angle of about 16 degrees to (001). The repeat period of the basal plane component of the moment is 0.009+or-0.001 AA^-1. The iron moment remains sensibly constant across the transition with a value somewhere in the range 1.60 and 1.74 mu _B, dependent on the particular form factor used to interpret the data. This moment value is consistent with that derived from neutron powder diffraction measurements made at 4.2K.

The basal plane anisotropy coefficient K₆⁶ has been measured for alloys of composition Tb_xSc_1-x where x=0.89 and 0.825, at temperatures above 77K. The results have been compared with those of Bly (1967) for pure terbium and used in estimating the driving energies F_d to ferromagnetism at the Curie temperatures of the alloys. These are about -0.7K/atom, at least four times as great as for pure terbium, and indicate the extent to which a small scandium content makes the helical phase energetically favourable. The magnitude of F_d at 0K falls below 0.75K/atom when the scandium content reaches 20 at.%, in agreement with the absence of spontaneous ferromagnetic ordering in alloys with greater scandium content.

The authors use plane polarised light at normal incidence in the spectral range up to 21 eV to measure the angular resolved energy distribution functions of photoelectrons emitted from Cu (001) and (111) surfaces. With the plane of detection parallel to a mirror plane of the crystal, the authors observe drastically different spectra for the planes of polarisation parallel and perpendicular to this mirror plane. Using this polarisation dependence, which is caused by optical selection rules, they are able to discriminate the various initial states of the 3d and the sp bands according to their parity with respect to the mirror reflection. The strong intensity variation of the observed peaks is caused by a correspondingly strong k dependence of the dipole matrix element. In addition to the intense zero-order structure, weaker features caused by surface umklapp are also identified.