Table of contents

Presents evidence that the magnetic ordering in the 'magnetic superconductors' Gd_xTh_1-xRu₂ and Gd_xCe_1-xRu₂ is of the 'spin glass' type.

The complete expression for the hyperfine field at a magnetic ion shows that the previously assumed correlation with paramagnetic resonance g shift is not valid. It is suggested that much of the complexity in the hyperfine field can be removed if NMR measurements are made in diluted samples.

The structure of thin, vapour quenched films of some Gd-Co and Ho-Co alloys has been investigated by electron diffraction. The atomic arrangement at short range in these films is discussed in terms of the radial distribution function obtained by transforming the experimental interference function. The findings are compared with published data from X-ray diffraction studies of similar alloys. At the same time attempts to explain the structure in terms of some common microcrystalline structures are shown to be fruitless. It is concluded that the films are amorphous and that in further attempts to explain their structure other models of atomic packing such as the dense random packing of hard spheres should be considered.

An atomistic model, completely devoid of empirical parameters, of a copper crystal is presented. The crystal energy, composed by an electrostatic term and electron statistical approximations for the exchange, correlation and kinetic energies, is expressed as a function of the lattice parameter, and then minimized. The equilibrium compressibility is deduced. Resulting values of energy, lattice parameter and compressibility agree with experiment. Furthermore, the model is found to satisfy the virial theorem. It is concluded that the cohesive properties of Cu are well described by electron statistics.

The authors measured the Debye-Waller factor, the isomer shift and the second-order Doppler effect in a quenched aluminium-based alloy containing 200 PPM of ⁵⁷Fe, after an annealing treatment up to temperatures ranging from 78 to 623K. A discontinuity in the smooth variations of the Mossbauer parameters appeared for samples annealed at temperatures around 260K, which depends on the quenching rate and the thermal history of the specimen. This discontinuity was interpreted both in terms of vacancies trapped at ⁵⁷Fe impurities and the subsequent agglomeration process and of annealing of the dislocation loops up to the initial stage of the precipitation of iron. Supplementary investigations of the overall variations of the Mossbauer parameters led to information on the 3d to s electron transfer process related to the isomer shift temperature dependence.

Computes the electronic contribution to the energy of formation of vacancies in disordered transitional alloys in a tight-binding model. The authors show that this contribution is smaller than the value obtained from a linear interpolation between the values of the alloy constituents. This behaviour is compared with that of the cohesive energy which is known to be often larger in the alloy than the value obtained in the ideal solution approximation (i.e. the energy of formation of the alloy is negative). These two quantities have the same value in a pair approximation and the difference in their behaviour is therefore due to band structure effects, the cohesive energy being related to the average properties of the system while the energy of formation of a vacancy is related to the range of a perturbation of the system.

A description is given of Mossbauer studies of interstitial atom trapping at ⁵⁷Co impurities in Al after fast neutron irradiation at 4.6K with doses from 0.2*10¹⁸ n cm^-2. Trapping and detrapping during irradiation and during subsequent isochronal annealing was detected directly by the appearance and development of an additional Mossbauer line due to ⁵⁷Co atoms with trapped interstitials. Simultaneous electrical resistivity measurements on the Mossbauer samples allowed the novel information from the Mossbauer studies to be correlated with the well known annealing stages of the resistivity. The isomer shift of the additional Mossbauer line was +0.42 mm s^-1 relative to ⁵⁷Co on substitutional sites, its small quadrupole splitting was 0.17 mm s^-1. From irradiation dose and annealing temperature dependence it was concluded that trapping starts during irradiation at 4.6K and continues when interstitials are freely mobile (stage I_E). Detrapping occurs by vacancy migration to the trapped interstitials (stage III).

Longitudinal attenuation measurements along the c axis of single crystal erbium in its magnetic ordered region ( alpha , beta , gamma phases) and at the phase transitions, are reported. The critical attenuation data at 15-105 MHz for the paramagnetic side of the sinusoidal ( alpha )-paramagnetic transition scaled to the reduced temperature epsilon =(T-T_N)/T_N as Delta alpha =A omega ⁿ epsilon ^-x=Bqⁿ epsilon ^-x with x=1.27+or-0.04, n=1.91 and B=2.65*10^-9 dB cm. Both B and x agree with the estimates of Laramore and Kadanoff (1969). Measurements at 45 MHz in the ordered region confirm the occurrence of the main magnetic phase transitions ( gamma - beta at approximately 20K and beta - alpha at approximately 53K). Additional attenuation peaks occur at 26K (upon heating) and 22K (upon cooling).

It is shown that Lloyd's expression (see Proc. Phys. Soc., vol.90, p.207 (1967) and Adv. Phys., vol.21, p.69 (1972)) for the density of states of a muffin-tin Hamiltonian can be manipulated into a form more physically transparent and suitable for numerical calculations.

The time-differential perturbed angular correlation of gamma rays from ¹¹¹Cd located in similar non-cubic Hume-Rothery phases of AgAl, AgZn and AgCd alloys has been studied. The power spectrum of the observed precession pattern is approximately gaussian and the magnitude of the major frequency component, omega _Qmax, has been found to vary linearly with the conduction electron per atom ratio. The changing lattice constants appear to be of little importance for these systems.

A new interpretation based on collective excitations of the electron gas (plasma oscillation) has been given for the emission of the K beta " satellite for metallic titanium and chromium. Agreement between theory and experiment supports the conclusion that the satellite K beta " is a low-energy plasmon satellite of the diagram line K beta ₅ (KM_IV,V) in the aforesaid metals.

Systematic experiments are reported on residual resistivities of Al-based dilute alloys of 4d and 5d transition elements. For all the dilute alloys investigated, there is no temperature dependence of the residual resistivities over the range 1.5-4.2K. It is found that the residual resistivities of 4d impurities agree with those of 3d and that the residual resistivities of 5d impurities are smaller than those of 3d. The experimental results are explained by incorporating the spin-orbit coupling on the impurity atom into Friedel's resonance model.

A microscopic theory of Fermi liquids is used to study the temperature dependence of the susceptibility chi (T) of normal paramagnetic metals. A T² ln T variation is proved to exist in the case of spin-independent forces. This variation agrees with Misawa's calculations based on phenomenological Landau theory. Previous theories by Stoner (1938), Beal-Monod et al. (1968) and Kawabata (1974) are critically discussed. The T² ln T term provides an explanation for the maximum in chi (T) observed in many metals as well as for the overall temperature dependence at low temperatures. Comparison with experiments on Pd, U₂C₃, YCo₂, LuCo₂, He³ and Ni₃Ga is given. The coefficient of the T² ln T term is calculated within the 'paramagnon model'. The effects of impurities and imperfect order are studied and are found to suppress the maximum in chi (T). This provides an explanation to the observed behaviour in different palladium alloys as well as to the lack of a maximum in Ni₃Ga.

In an attempt to explain the anomalous low-temperature magnetic properties of some amorphous rare earth-iron alloys, the authors have generalized the random anisotropy model of Harris, Plischke and Zuckermann. The results of the investigation for spin one and the classical case of infinite spin suggest the possibility of a 'spin glass-like' ground state, whose existence is confirmed by a Monte Carlo simulation. In addition, the magnetization curves show a well defined 'spin glass-like' ground state, whose existence is confirmed by a Monte Carlo simulation. In addition, the magnetization curves show a well defined 'spin glass-like' transition temperature and a sizeable coercive field, in qualitative agreement with experiments on amorphous REFe₂ compounds.

Ultraviolet photoelectron spectra taken with 40.81 eV photons are presented for a series of AgSn alloys. The results are interpreted collectively with similar results previously obtained for a wide variety of Ag alloys and indicate that the width and splitting of the Ag 4d band are (i) independent of the second metal and (ii) strongly dependent on the mean number of nearest Ag neighbours.

The calculation of the singularity index which describes the lineshape for core electron photoemission has been extended to the case of an arbitrary band structure. Using phase shifts derived from de Haas-van Alphen and resistivity data for dilute noble metal alloys, the calculated singularity index is in reasonable accord with direct experimental determinations.