Table of contents

Volume 17

Number 1, January 1987

Previous issue Next issue

LETTERS TO THE EDITOR

L1

and

A high-pressure Cs IV phase of alkali metals is equivalent to the beta -Sn(II) structure predicted previously. At higher pressures, the beta -Sn(II) phase tends to become more stable than beta -Sn(i), which corresponds to the well known beta -Sn-type structure as a high-pressure phase of diamond-type structure. Numerical calculations of the relative stability of diamond or zinc-blende, beta -Sn(I) and beta -Sn(II) structures are performed for Si and GaSb, and of BCC, FCC, beta -Sn(I) and beta -Sn(II) structures for Rb and Cs using a perturbational treatment based on the pseudopotential formalism. Numerical results for the transition pressure, relative volume, volume discontinuity and transition heat are then discussed together with the observed data.

L7

and

The technique of electron energy loss spectroscopy is well adapted to a search for charge density waves (CDW) in alkali metals since it couples preferentially to dipole oscillators perpendicular to the surface. Measurements on clean K films in the energy range h(cross) omega <1eV reveal no CDW features. On exposure to H2O or O2, losses are observed that may be readily attributed to either adsorbate or phonon vibrations.

L13

, and

It has been shown that the renormalisation of the Landau parameters of an itinerant system leads to a simple formalism for the influence of spin fluctuations on their properties, in terms of an effective magnetic pressure. This formalism is now shown to lead in a very simple way to a pressure formula similar to those derived by other authors, giving the magnetic change of the volume of the system. The pressure dependence of the Curie temperature is also derived.

L19

, and

Measurements of 165Ho NMR in Ho2Fe14B in zero and applied fields of up to 8 T at 4.2 and 1.3 K in the frequency range 6000-7500 MHz are presented. The generally accepted spin-canted structure of Ho2Fe14B results in four magnetically inequivalent rare-earth sites. Although the authors have identified only two sites, one of which has an almost fully polarised magnetic moment and the other is quenched, nuclear quadrupole measurements are shown to be consistent with a spin reorientation. The Ho and Fe sublattices are found to be coupled ferrimagnetically.

L25

, , , , and

Two new uranium-based alloys UAuCu4 and UPdCu4 have been prepared and their magnetic properties studied. The NMR of the isotope 63Cu in these alloys suggests that they are well ordered ternary materials. There is a strong correlation between the occupancy of the (4c) sites in the structure and the relative size of the two non-uranium atoms in these alloys.

L29

, and

The authors report np5(n+1)s2 to np6(n+1)s outer-core emission spectra for the heavy alkali metals. The assembled data provide the first comprehensive view of many-body effects in emission and of spin-orbit partner behaviour for this series of simple metals.

PAPERS

1

and

The neutron diffraction structure factors for three isotopic compositions of Fe40Ni40B20 have been determined. These structure factors seemed to be affected by the presence of small amounts of hydrogen in the specimens. From the three total structure factors three partial structure factors and three partial reduced radial distribution functions were obtained. The metal-metal partials were found to be identical. The metal-metalloid partials showed a pronounced splitting of the first RDF peak and distinct differences between GFeB(r) and GNiB(r). The results are compared with the literature data for the binary glasses Fe80B20 and Ni81B19.

19

, , and

Shear and longitudinal wave velocities, as functions of isothermal annealing time, have been studied for a Pd40Ni40P20 metallic glass prepared by slow cooling. There is no evidence for reversible structural changes, in sharp contrast with the behaviour of rapidly quenched metallic glasses.

27

Displacement correlation functions have been calculated as a function of temperature for the A15 materials Nb3Sb, Nb3Ge and Nb3Sn. For the computations, lattice dynamical models have been used that reproduce the experimental phonon dispersion curves extremely accurately. The results are in very good agreement with EXAFS data and with crystallographic data. A very pronounced correlated motion of neighbouring atoms on a chain is observed, due to the strong bond-stretching force constants along the chains.

37

, and

A study of ordering in (Ni3Fe)1-xCrx alloys (0<x<17%) is described. The critical temperature for ordering (Tc) has been determined using electron diffraction to detect long-range order (LRO). Low-temperature (<450 degrees C) anneals were performed with a 1 MeV electron microscope, using electron irradiation to increase the ordering rate. Mossbauer spectroscopy was used to characterise the short-range order (SRO) in annealed unirradiated samples. Tc was found to decrease rapidly with increasing Cr content, falling from 502 degrees C in near-Ni3Fe material, to about 450 degrees C at a Cr concentration of 6%. No LRO was detected in samples with >or=12% Cr. The Mossbauer spectra of the ordered alloys contain a contribution attributed to Fe atoms each with a Cr neighbour. The magnitude of this component is smaller than it would be in a random alloy, implying that Cr atoms preferentially occupy Fe sites in Ni3Fe. In the alloys with the higher Cr contents little evidence of SRO was detected. A very slight increase in the mean hyperfine field on aging these alloys may indicate the formation of Fe-rich regions, as previously proposed by Binnatov and co-workers (1975).

47

By application of the Jahn-Teller theorem to degenerate excited states of hydrogen in metals it is shown that Jahn-Teller effects are of considerable importance in these systems. Consequences for inelastic neutron scattering spectra are discussed and compared with available data. The Jahn-Teller effect appears to provide a natural explanation for several details in the spectra. Furthermore, the anomalous temperature dependence of the modulus C' in TaH0.19 can be explained for the first time by taking into account a Snoek effect due to degenerate excited states.

59

and

Low-frequency internal friction has been measured for palladium containing 0-4 at.% hydrogen over a temperature range from 80 to 530 K. A lambda -shaped peak is observed as a function of temperature, the location of which is different for heating and cooling measurements. The peak exhibits the features characteristic of the 'precipitation peak' observed for group V (a) metals: V, Nb, and Ta; the peak height is proportional to the heating or cooling rates and inversely proportional to the measuring frequency. This is the first report of the precipitation peak in the Pd-H alloys.

75

The nonstoichiometric PdHx is approximated by two analytic model descriptions. The first, which incorporates only diagonal disorder, makes use of the doubly periodic Bethe lattice. The second, which is capable of incorporating both diagonal and off-diagonal disorder, employs the cluster Bethe lattice method. Both of these model descriptions are incorporated within the self-consistent scheme of the coherent potential approximation. The electron densities of states of PdH/sub /x obtained with these models are presented and compared with the results of other methods.

87

, , and

The full recursion method of V Heine and co-workers (1980) has been modified with a simple approximation for calculating the local density of states in the tight-binding model. The efficiency of this approximation has been tested on atoms in a dislocated molybdenum lattice. The computation time has been reduced by a factor of 200 without substantial sacrifice of the accuracy of the force.

99

, and

The influence of variations in the tight-binding parameters on lattice stability and defect formation is investigated. An effective attractive pair potential is derived which is used to replace the noncentral attractive interactions due to the tight-binding contributions. This replacement (using the same repulsive component) significantly alters the resistance of the lattice to both localised and long-range defect formation.

109

, and

A modified tight-binding recursion method is used to calculate the d electron contribution to force components on various segments of an unrelaxed screw dislocation in the presence of a solute atom. The model parameters correspond to an iron solute atom embedded in a molybdenum lattice. The solute atom is introduced at various sites in the core region of the dislocation. This quantum mechanical calculation of the solute-atom-dislocation interaction force is contrasted with a more conventional elastic continuum model as well as with a semi-empirical (Johnson) pair potential atomistic model. Significant differences are obtained in both the magnitude and the orientation dependence of the force, as computed with the atomistic model as opposed to the continuum model. Somewhat fewer differences were obtained for the quantum mechanical as opposed to the pair potential atomistic model.

117

, and

A thorough investigation of the spin-splitting factor for the Gamma 6 Fermi surface sheet of iridium has been performed. Several spin-splitting zero contours have been mapped out together with the amplitude variation in the symmetry planes, giving a detailed picture of the anisotropy of the cyclotron orbit g-factor.

123

The author presents an analysis of the electrical resistivity data for Cu that reveals the effects of strain on the temperature-dependent component as a function of temperature from 2 to 35 K. In addition, magnetic impurities are utilised as 'tracers' to show that dislocations do not screen out the effects of magnetic impurities.

131

, and

The temperature dependence of the electrical resistivity has been measured over a wide temperature range, 2-300 K, for a number of Y- and L-based metallic glasses. Low-temperature specific heats have also been measured to determine the density of states at the Fermi level and the Debye temperature. All of the resistivity data in the range from about 40 to 300 K can be well described in terms of the equation rho / rho 0=A+B exp(-T/ Delta ). According to Mizutani's classification of metallic glasses, the present alloy systems are to be classified in group (4), together with systems such as Cu-Zr and Cu-Ti. The authors revealed that: (i) the above equation is valid for all group (4) metallic glasses studied so far; (ii) the characteristic temperature Delta increases with increasing Debye temperature theta D; and (iii) the resistivity increases with decreasing density of d states at the Fermi level. Using the generalised Faber-Ziman theory; it is difficult to interpret all these findings in a consistent manner. It is suggested that the d electrons at the Fermi level play a key role in the electron transport in group (4) metallic glasses.

143

, , and

The temperature dependence of the resistivity and Hall effect of Y9Co7 has been measured from room temperature to 1.6 K. The saturation of the resistivity at high temperature is similar to that of A15 compounds and can be interpreted in terms of a localised phonon mode formation as shown by Yu and Anderson (1984) for A15 compounds. A T2-dependence of the resistivity is observed for temperatures below approximately=25 K. A similar T2-dependence at low temperatures is always observed in A15 compounds as well and is not fully understood. However, a T2-dependence below 10 K does not seem to be due to itinerant ferromagnetism in Y9Co7 as concluded recently by Kolodziejczyk and Spalek (1984). The observation of a peak at approximately=25 K in the Hall coefficient suggests a spin glass type of freezing at low temperatures which possibly can account for the T2-dependence of the resistivity below approximately=10 K.

153

and

The inter-relationship between structural transformation and superconductivity has been studied on B2 systems V54-xRu46Osx, V54Ru46-xOsx and V52Ru48-xOsx with 0<or=x<or=4. An increase of the parameter x in V54-xRu46Osx, where the electron-atom ratio e/a increases linearly with x, raises the transformation temperature TL sharply while lowering the superconducting transition temperature Tc rapidly. This is in contrast with the results for both systems V54Ru46-xOsx and V52Ru48-xOsx, where e/a is a constant independent of x. All samples in V54Ru46-xOsx with e/a=6.38 undergo no structural transformation, while showing superconductivity at nearly the same temperatures around 5 K. All samples in V52Ru48-xOsx with e/a=6.44, however, show no superconductivity, while undergoing transformation at nearly the same temperature around 200 K. Taken together, these facts confirm that both TL and Tc are completely dominated by the value of e/a; the lowering of TL, which is accompanied by a decrease of e/a, causes sharp enhancement of Tc. This provides decisive evidence for the electronic transition and its strong correlation with superconductivity.

165

Magnetic and one-electron properties of Fe(100) at finite temperatures are studied by means of the functional-integral method within the static and single-site approximations, but using the realistic tight-binding canonical d band model. Calculations of average magnetic moments on the first ten layers of the semi-infinite BCC lattice predict that the surface moment, which is enhanced at T=0 K compared with the bulk one, decreases more rapidly than the bulk upon heating. The temperature-induced change in the spectral density is shown to depend on the wavevector in the two-dimensional surface Brillouin zone. The exchange-split peaks in the Gamma point surface spectral densities approach each other as the Stoner bands when temperature is raised whereas the surface-state positions at Delta and Sigma are almost stationary. It is pointed out that the surface effect can play important roles in discussing angle-resolved photoemission experiments probing 'bulk' band structures of Fe.

185

and

The concept of free and frozen magnetisations is explored by defining a set of thermal and magnetic experimental conditions, and then relating the magnetisations measured under these conditions to the internal susceptibility and volume fraction of frozen material. Particular attention is paid to the frequently observed experimental result that the sum of the zero-field-cooled and thermoremanent magnetisations is equal to the field-cooled magnetisation. This approach suggests a simple explanation for certain unusual features observed in the thermoremanent magnetisation of a disordered Au80Mn20 alloy.

191

, and

The authors have made polarised neutron diffuse-scattering measurements on ferromagnetic alloys Fe1-cV/sub /c(0.01<c<0.2). The nuclear magnetic cross sections are analysed within Marshall's formalism, using the short-range order parameters previously measured. These new results show a negative constant moment of about -1 mu B on vanadium sites up to 10 at.% V. In particular they observe no steep enhancement of the vanadium moment at dilute concentrations. The iron and vanadium moments are in good agreement with theoretical predictions.

199

, , and

Nuclear diffuse neutron scattering measurements, are performed on PdMn. The Warren-Cowley atomic short-range order parameters alpha /sub /i are determined up to the third neighbour (i=1,2,3) for four different heat treatments of the specimen. Additional triple-axis neutron scattering measurements show a large inelastic contribution at room temperature. Susceptibility measurements are also carried out on these specimens, and these show a large dependence of the spin-glass freezing temperature Tf on the specimen treatments. The magnetic correlation function chi (T)T is determined from the high-temperature (T>Tf) susceptibility and analysed with a configuration group model which includes the measured Warren-Cowley parameters. From this analysis the exchange parameters J/sub /i are calculated up to the sixth neighbour.

221

, and

A number of BCC alloys have been studied theoretically by self-consistent LMTO calculations using supercells of different sizes. To account for disorder effects a density-of-states broadening is used. Electron-phonon coupling, Stoner factors and superconducting transition temperatures have been calculated, including the effect of spin fluctuations. It is found that spin fluctuations are important for a good description of superconductivity. Moreover they can explain many experimental trends; in particular they show the increase of the transition temperature with pressure in NbZr compounds.

233

, , , and

High-field magnetisation measurements have been performed on single-crystalline samples of hexagonal Ho2Co17, Ho2Fe17 and Ho2Co14Fe3 in fields up to 40 T. In this field interval the magnetisation curves of Ho2Co17 and Ho2Co14Fe3 exhibit pronounced magnetic transitions in the hexagonal plane. The experimental data are analysed in a two-sublattice model. Within this analysis, the anisotropy constants, the sublattice magnetic moments and the 4f-3d exchange parameter are derived for the different compounds. In particular the 4f-3d exchange parameter can be determined accurately from the observed values for the transition fields.

243

Measurements of the specific heats of ErAg and TbAg are reported; those for TbAg are complicated by the effects of Tb2O3 impurity, but it is shown to be possible to correct for this when the correction is not too large. The main conclusions are: (i) the hyperfine specific heat of ErAg corresponds to an ionic moment of 7.6+or-0.1 mu B for Er3+; (ii) there are magnetic transitions of some complexity in ErAg in the range 10 to 18 K, and the entropy associated with them is close to R in 4, confirming that the crystal-field ground state of Er3+ is Gamma 8(3); (iii) the electronic specific heat coefficients of ErAg and TbAg are 11+or-1 and 7.5+or-0.5 mJ K-2 mol-1; (iv) the hyperfine specific heat of TbAg appears to be anomalously large.

257

, and

Low-temperature specific heats for ten amorphous and four crystalline Co-Y alloys have been measured in the temperature range 1.5-6 K. Both the linearly temperature-dependent specific heat coefficient gamma and the Debye temperature theta D in amorphous and crystalline phases exhibit a sharp increase at the composition where the ferromagnetism collapses. The enhancement in gamma is ascribed to the presence of the linearly temperature-dependent magnetic specific heat, which is often observed in the range where the ferromagnetism becomes unstable. The band-structure profile over a wide composition range is discussed in the light of recent band-structure calculations. The density of states at the Fermi level does not differ appreciably between the amorphous and crystalline phases and decreases gradually with increasing Y content. The internal magnetic field Hint was deduced from the 59Co nuclear specific heat coefficient. The authors found that Hint follows well a linear relationship with the magnetic moment per transition-metal atom.

267

Experimental data indicate that in the isostructural series the easy axis of the spontaneous magnetisation within the hexagonal plane alternates with the sign of the third Stevens factor. This confirms that the magnetocrystalline anisotropy of the rare-earth ions in the RCo5 and R2Co17 intermetallics has a single-ion origin. It is clearly shown that Co positive charges play the dominant role in the creation of the crystal electric field in the 4f Co-rich compounds. The basal-plane anisotropy of Tb, Dy and Ho has also been examined. The point-charge model calculation gives good agreement with the experimental data for trivalent ions, especially for Dy and Ho. The large contribution of conduction electrons to the CEF has been evaluated by analysis of the available experimental data for RCo5 and R2Co17.

277

and

Measurements are presented of the optical constants of a thin gold film over primarily the visible region of the spectrum using the surface plasmon-polariton resonance condition in the attenuated total-reflection configuration. Using this technique precise results are obtained and these are compared with those found using other experimental methods and also with the classical free-electron model.

289

, and

An expression for T( mu ) for the B2-R transition in NiTi has been derived, and the values of the parameters for which the B2-R transition is correctly reproduced have been found. Using a method for calculation of the electronic energy structure of binary substitutional alloys, a calculation of the density of electronic states for some compositions of NiTi has been performed. On the basis of these calculations a TR(C) diagram has been produced. The theory proposed agrees satisfactorily with the experimental results for the observed B2-R transition.