Table of contents

The Fermi surface of the cubic Laves compound YAl₂ is studied by means of the de Haas-van Alphen (dHvA) effect. The results are compared with Fermi surface studies of LaAl₂, which is isoelectronic with YAl₂. While high dHvA frequencies (A approximately 0.19 au) are comparable with those observed in LaAl₂, lower dHvA frequencies (0.021<or approximately=A<or approximately=0.0012 au) differ from those in LaAl₂. Following the APW energy band calculations by Switendick (1973) the author ascribes this difference to f band admixture in LaAl₂.

Experimental observations of the audio phase shift of the radiofrequency size effect resonance caused by the nonuniform penetration of the audiofrequency modulation field in a potassium specimen are examined in the light of several theoretical treatments.

It is shown that the local magnetic excitations density of states in rare earth transition metal amorphous materials is independent of energy down to very low energies. It causes linear temperature dependence of the specific heat and other anomalous properties of these substances at low temperatures.

The distribution of hyperfine fields at iron atoms in FeCo and FeNi alloys containing ¹/₂% of added impurity has been measured as a function of hydrostatic pressure up to 16 kbar. The frequency ( nu ) of the main and satellite lines of FeCo was found to change such that ( delta ln nu / delta P)_T=-1.75*10^-7 kg^-1 cm². In FeNi the value for the main line was -1.64*10^-7 kg^-1 cm² and -2.02*10^-7 kg cm^-2 for the satellite line. The temperature dependence of the difference between the normalised main and satellite lines was found to be proportional to T^3/2 at temperature T, even after the measurements had been reduced to constant volume, which is in disagreement with the prediction of spin wave theory.

The author corrects and comments on the calculated cohesive properties of Th, given in an earlier publication (see ibid., vol.8, p.L163 (1978)). Here the outermost p core states (5p for La and Ce, 6p for Th) were calculated self-consistently as band states, using the linear muffin tin orbitals (LMTO) method within the atomic sphere approximation (ASA).

The internal friction spectra of preferentially orientated polycrystalline and single-crystal samples of a dilute Al-Fe alloy have been measured after electron irradiation at 4.7K and after annealing at various temperatures. Several low-temperature internal friction peaks were found that are attributed to the stress-induced reorientation of self-interstitial-Fe complexes. At least three different defect configurations with large binding and migration energies co-exist in the sample after annealing at 80K. All four major peaks anneal simultaneously, providing strong evidence of vacancy migration in stage III. Defects with tetragonal or (100)-orthorhombic symmetry are not consistent with the observed orientation dependence of the major peaks; trigonal defects ((111) symmetry axis) possess the highest symmetry in agreement with the three largest peaks. Previous interpretations of channelling and Mossbauer measurements based on the formation of isolated, (100)-tetragonal, mixed-dumbbell interstitials in Al-Fe alloys are not consistent with the present internal friction results.

The Doppler-broadened lineshape for electron-irradiated nickel shows a small positive temperature dependence between 100 and 300K. No such dependence has been observed in similar work by others.

The phenomenon of segregation of major alloy constituents under irradiation is considered. The available evidence on segregation of nickel to sinks in Fe-Cr-Ni alloys is reviewed, and some new data presented. The theory of the Kirkendall effect is extended to include the effect of superimposed vacancy gradients in binary alloys and applied to the case of an irradiated metal. It is found that if the effect of the interstitial flux is neutral, then solute segregation can occur at sinks because of differences in the vacancy diffusion coefficients of the alloy constituents. The theory predicts that slow-diffusing solutes will be enriched at sinks, as observed in Fe-Cr-Ni alloys. It is pointed out that one consequence of segregation is that an extra vacancy flux (the 'Kirkendall flux') is induced, which opposes the radiation induced flux to the sink. The consequences of this for void nucleation and growth are discussed.

A lattice dynamical model for the face-centred-cubic (FCC) metals has been proposed by incorporating the ion-ion central and angular and the electron-ion interactions through well known theories. The model satisfies the translational invariance and the condition for the internal force equilibrium of the lattice. The application of this model has revealed fairly well the phonon dispersion curves and the temperature variation of Debye-Waller factor and electrical resistivity of copper.

In the present work, the author considers the nonlocality of the transition metal model potential (TMMP) of AOE Animalu and deduce expressions for the coefficients of linear terms in the expansion of the TMMP form factor in the nonlocal formalism of Eschrig and Wonn. A complete expression for the energy-wavenumber characteristic G(q) of this theory is used to compute the phonon dispersion curves of Sc, Y and Zr along the Gamma A, Gamma M and Gamma KM symmetry directions. Reasonably good agreement is found between the computed and experimental results for Sc and Y, but relatively poor for Zr.

Summarises the present knowledge about electronic structure and order in carbon-transition metal (Fe, Co, Ni) interstitial alloys. The electronic structure of carbon atoms in a ferromagnetic transition metal is computed in a Hartree-Fock scheme taking full account of the Friedel sum rule. Taking into account an extended perturbation up to second-nearest neighbours, reasonable agreement with saturation magnetisation and hyperfine fields can be found. The electronic binding energy of a carbon atom with another point defect is estimated. A strong anisotropy of the electronic binding energy of a carbon pair in alpha iron is found.

The effect of long-range atomic order on the electronic density of states has been recalculated for the A15-type structure within the linear-chain model. The author finds that a defect concentration c reduces the density of states at the Fermi level by a factor (1+c/c₀)(c/c₀)^-3(ln(1+c/c₀))³. This result is in qualitative agreement with experimental data on the specific heat, magnetic susceptibility and superconducting transition temperature of V₃Au.

A quantum kinetic equation for electrons in metal in a uniform electric field is derived by means of the Keldysh diagrammatic technique. The principle of detailed balance for the quantum collision integral is formulated. Using this concept the analogue of the single-particle distribution function, which depends on the electron 4-momentum, is defined. The linear response of an electron system to the electric field is not completely determined by the nonequilibrium part of this function. To give a complete description of the nonequilibrium state it is necessary also to know the nonequilibrium correction to the electron spectral function. An inhomogeneous integral equation for this correction is obtained, in which the inhomogeneous term is defined via the solution of the kinetic equation. This equation, supplementary to the kinetic one, is solved in the general case by the methods of the Landau theory of the Fermi liquid. While calculating the resistivity, account is taken of the spectral response results in the change of the bare electron mass by the 'Coulomb' effective mass. The electron-phonon contribution to this effective mass is as small as theta epsilon _F.

Thin films of iron were prepared by condensation from the vapour phase onto a liquid helium-cooled substrate. The 'as deposited' films possessed large resistivities similar to that of liquid iron and were ferromagnetic. The extraordinary and ordinary Hall coefficients of the films were measured and were found to be of the order of fifty times greater than their crystalline counterparts, although the films possessed a magnetic moment of up to 50% smaller. On annealing these quantities, at a specific temperature, irreversibly changed to values similar to those of the crystalline state. Results are presented for the Hall coefficients of the annealed polycrystalline films from 4.2-500K.

The electrical resistivity has been measured below 40K for annealed specimens of Al, Pd and Ag and for strained specimens with varying densities of dislocations. The temperature-dependent part of the resistivity decreases with increasing dislocation density in Al and Pd and remains substantially constant in Ag. The authors argue that only theories that include the anisotropy of electron scattering on the Fermi surface can account for these results. The anisotropy of the dislocation scattering must be similar to that of low-temperature phonons to explain the results. Most of the dislocation scattering is therefore small-angle, as would be generally expected from the strain field.

The Ettingshausen cooling in single-crystal bismuth was investigated using pulsed currents far in excess of the equilibrium value. At 80K, transient cooling of nearly five times the equilibrium value was observed using rectangular current pulses. By using ramp-shaped pulses even larger transient cooling was observed. The authors suggest that this phenomenon may be due to the localised nature of the generation and recombination of the carriers at the sample surface.

The isotropic and anisotropic magnetoresistance have been measured for dilute alloys of PdFe, PdCo, PdNi at low temperatures. Physically different effects dominate in each case. In PdFe there is a temperature-dependent negative magnetoresistance from the suppression of electron-magnon scattering. In PdNi there is a positive isotropic magnetoresistance due to the effect of the magnetic field on the magnitude of the Ni moments, plus an anisotropic magnetoresistance arising from local orbital moments in Ni sites. In PdCo there is a negative isotropic magnetoresistance associated with magnetising the sample. The authors suggest that this is due to non-ferromagnetic interactions between Co moments.

Measurements are reported of the thermopowers and resistivities of some dilute Cu-Au, Cu-Zn, Cu-Ga and Cu-Ge alloys in the temperature range 195-666K. The results have been used in a careful examination of the validity of Kohler's relation in the guise of the more familiar Nordheim-Gorter rule (NGR) for the diffusion thermopower of the alloys. It is found that very substantial deviations from the NGR occur and that for dilute alloys of each system the deviation can be expressed in terms of a concentration-independent but temperature-dependent parameter Omega . The temperature dependence of Omega appears to be correlated with the deviation from Matthiessen's rule, as was hypothesised in an earlier paper. On the basis that this correlation exists in the low-temperature regime also, the phonon-drag thermopower has been extracted in the system Ag-Au and the results compared with separation using the original NGR.

A recent fundamental description of the lattice dynamics and electron-phonon interaction in potassium (L Dagens et al., see Phys. Rev., vol.11, p.2726 (1975)) is shown to lead to very good agreement with experiment for both the high-temperature electrical resistivity and thermoelectric power. When the authors allow for the possibility of mixed electron-phonon and electron-electron scattering, the calculated result for the volume derivative of the thermopower supports the suggestion that the effect of normal electron-electron scattering is seen in the measured high-temperature transport properties of potassium.

Reports the first ultra-low temperature thermopower measurements on bismuth samples of various orientations, including a preliminary study in a small transverse magnetic field. Cooling is provided by a dilution refrigerator, and the thermoelectric voltage is detected using a SQUID null detector. Phonon drag makes a dominant contribution to the thermopower throughout the whole temperature interval investigated. Negative minima are observed on all samples around 0.3 to 0.4K. The position of the positive maxima above 1K seems to correlate with the limiting wavevector of those phonons which can interact with carriers for each particular sample orientation. A large negative phonon drag thermopower of about -10 mu V K^-1 is observed in the bisectrix direction near 4K. The present data and those of J Boxus and J-P Issi indicate that the phonon drag thermopower near 4K is intrinsically negative in the basal plane of bismuth.

A model of planar spin rotators, introduced by Edwards and Anderson, is extended to include magnetised states. The average equilibrium energy (2^N degenerate) is shown to contain a term dependent on the square of the magnetisation per spin m. From this the zero-temperature limit of the susceptibility can be derived. It is further shown that the relaxation rate towards decrease of magnetisation is exponential in m, thus leading to a logarithmic relaxation in time.

The authors have measured the magnetisation of ferromagnetic pseudobinary Fe_xCo_1-xTi alloys in fields up to 150 kOe, at temperatures between 1.4 and 300K and under high pressures up to 7 kbar. They have analysed the temperature dependence of the spontaneous magnetisation. Their experimental results suggest that the alloys, weak itinerant ferromagnets for x<0.5, become more localised weak ferromagnets for x>0.5. The results are analysed in terms of indirect coupling models.

The elastic scattering of the polarised thermal neutron has been calculated reducing the number of a priori hypotheses on the system being investigated. The authors use a general formulation that can be applied to ferromagnetically aligned systems and is particularly useful in the case of ferromagnetic alloys. The general formulation for the cross section has been employed to derive the incoherent neutron scattering cross section within the coherent potential approximation (CPA). It has been shown that the CPA predicts nonzero fluctuation of the magnetic moment of a given atom of a random alloy, so that the incoherent scattering of neutrons is not simply that due to a random arrangement of two magnetic moments.

Derives the RKKY indirect exchange interaction by applying Green's functions and a multiple scattering formalism to a model system corresponding with a ferromagnetic rare earth crystal. Except for the neglecting of spin flip, localised spin dynamics are taken into consideration. The conduction electron spin polarisation is calculated in terms of an effective definition arising from the formalism. In contrast with existing RKKY theory for periodic spin systems, physically realistic occupation of states arises from multiple scattering of conduction electrons, leading to a net spin polarisation enhancing the average spin per atom. By extending the formalism to apply to HCP structures and choosing a cut off on momentum transfer to afford qualitative agreement between the calculated and measured spatial dependence of the spin polarisation of gadolinium, the indirect interaction of a localised spin with all other localised spins is calculated. The self polarisation, usually incorrectly neglected, is also automatically contained in the interaction. A spin enhancement factor of 1.10 is predicted, in comparison with the experimental value of 1.09.

Knight shifts have been calculated for liquid nontransition metals using a nonlocal pseudopotential method within the framework of the nearly-free electron model. The contact density of the conduction electrons at the nucleus is evaluated by explicitly orthogonalising their pseudowavefunction to the ionic core states, and is then combined with the best available value of the spin susceptibility to obtain the direct contact contribution to the shift. The results are compared with experiment for 18 metals. In general the agreement between theory and experiment is reasonably good and substantially better than that obtained using a local pseudopotential. It is therefore concluded that it is essential to use a nonlocal pseudopotential in calculations of this type.

A close connection between the Mossbauer effect in ¹⁶¹Dy and magnetic anisotropy in dysprosium metal is used to probe the limits of the Callen and Callen theory (1965) of magnetic anisotropy. It is shown that the behaviour of the ¹⁶¹Dy reduced quadrupole parameter P_4f(T)/P_4f(T=0K), arising from the partly filled 4f shell, is not in good agreement with the predictions of the Callen and Callen model, in marked contrast to the magnetic anisotropy results of Ferron (1970). A modified version of the Callen and Callen model is presented which is in better agreement with quadrupole data, and it is suggested that the difference between the magnetic anisotropy and the Mossbauer results may be due to a temperature-dependent B₀² crystal field parameter.

Distribution of hydrogen and its interaction with iron in hydrogenated ferromagnetic Fe-45 at.% Ni alloy were studied by means of ⁵⁷Fe Mossbauer spectroscopy. As a result of hydrogenation two phases (FCC gamma ' and FCC beta ) appeared, and the Mossbauer spectrum was separated into two components. One corresponding to gamma ' exhibits very small changes in the Mossbauer parameters compared with the original gamma phase, while the other corresponding to beta has a large positive isomer shift change, line broadening, and a remarkable internal field reduction. In the gamma ' phase presumably hydrogen atoms prefer the interstitial sites close to nickel atoms producing inhomogeneous distribution, large lattice distortions and no direct interaction with iron atoms. In the hydride-like beta phase, hydrogen atoms will have an ordered structure and may interact strongly with iron atoms, by which the 3d state of iron will be considerably changed and the magnetic moment of iron reduced. The lattice expansion seems to contribute only one-third of the isomer shift change.