Showing papers in "Journal of Physics F: Metal Physics in 1981"

Study of the 4f and valence band density of states in rare-earth metals. II. Experiment and results

Abstract: For pt.I see ibid., vol.11, p.113 (1980). The 4f and valence states of all metallic rare earths have been studied using X-ray photoelectron spectroscopy (XPS) for the occupied part and bremsstrahlung isochromat spectroscopy (BIS) for the unoccupied part. It is found that the bandwidth increases from Gd to Lu, and that the valence band spectra are in fair agreement with APW calculations. The intensities of the 4f final-state multiplets are well described in terms of the coefficients of fractional parentage. There is a symmetry between the fn XPS and f14-n BIS spectra. The observed energies of the 4f excitations correspond to transitions to completely screened final states. These values enable one to predict the elements which are liable to interconfiguration fluctuation when their 4f levels are shifted to EF by the chemical environment or by compression. The large Coulomb correlation energies, U, which prevent the formation of 4f bands in these elements, are directly obtained from the authors' spectra, and are found to be in good agreement with recent calculations. The linewidths and singularity indices of the XPS and BIS 4f lines are determined and discussed.

Pressure and temperature dependence of electrical resistivity of Pb and Sn from 1-300K and 0-10 GPa-use as continuous resistive pressure monitor accurate over wide temperature range; superconductivity under pressure in Pb, Sn and In

Abstract: 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 (Tc) Pb manometers is excellent. Tc(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.

Density of states for the Anderson model

Abstract: The Anderson model is studied in the limit U to infinity using a Green function decoupling procedure. It is shown that the solution gives the correct results in the intermediate valence case, and in the Kondo limit, at low and high temperatures: for the intermediate valence case, the position of the virtual d level is obtained as a function of temperature; for the Kondo case it is shown that the density of states has a peak of width TK at the Fermi level, which disappears above the Kondo temperature.

Auger and Coster-Kronig broadening effects in the 2p and 3p photoelectron spectra from the metals 22Ti-30Zn

Abstract: The 2p and 3p core level widths for the metals 22Ti-30Zn have been studied by photoelectron spectroscopy. The L2L3M45 Coster-Kronig rate is found to have a maximum for Co and Ni. The Coster-Kronig broadening seems to be a simple function of the kinetic energy of the emitted electron. Absolute 2p level widths have been determined which in general show much higher accuracy than previously reported values. The presently determined widths are found to agree well with theoretical predictions. The M2M3M45 super Coster-Kronig decay channel is open in Co but not in Ni.

Nb-Ta metal superlattices

Abstract: The authors report the successful growth of almost perfect single crystal superlattices made from alternating layers of Nb and Ta with superlattice periodicities in the range 20-1000 AA

Amorphous yttrium-iron alloys. I. Magnetic properties

Abstract: The magnetic properties of amorphous yttrium-iron alloys Y1-xFex have been studied over a wide concentration range 0.32

The appearance of critical behaviour at the onset of ferromagnetism in AuFe alloys

Abstract: Reports measurements of the dynamic response, chi '( omega ), for AuFe alloys. It is shown that this experiment can indicate the infinite percolation cluster concentration Cp. The thermal critical behaviour of the zero field chi '( omega ) in alloys containing an infinite cluster is discussed in terms of the cross-over from isotropic short-range exchange-dominated to dipolar-dominated behaviour, although renormalisation of the critical exponents is necessary. Coexistence of finite clusters with the infinite cluster is also confirmed; the freezing temperature of these finite clusters well below Tc shows up very clearly in the simple magnetic response experiments. (These effects should not be associated with a phase transition from a ferromagnetic to a spin-glass regime.) For alloys below the percolation threshold the fluctuations within very large clusters of highly correlated spins are essentially quasi-critical and very similar to those in alloys containing an infinite cluster. The critical temperature Tc of the percolation cluster, however, was found to be finite and non-zero, suggesting a possible modification of the pure percolation processes by the long-range RKKY interactions between clusters.

Theory of photoemission from random alloys

Abstract: A theory of angle-resolved photoemission from the surface of a random alloy is described. It is argued that this theory contains a proper treatment of the disorder (via the Korringa-Kohn-Rostoker coherent potential approximation, KKR CPA), the surface, the LEED-type final state, and the electron-photon matrix elements. The results are illustrated by comparing calculations for pure Cu and Cu-Ni alloys.

Local density of states of impurities in Al

Abstract: Using the KKR-Green function method, the electronic structure of impurities in Al is calculated self-consistently by applying the density functional theory in the local spin-density approximation of von Barth and Hedin (1972). The impurity is described by a single perturbed muffin-tin potential, which is determined self-consistently. The authors give results for the local density of states, scattering phase shifts and local charges for 3d impurities and sp-impurities (Si, P, S, Cl, Ga, Ge, As). The virtual bound states of some 3d impurities show strong deviations from a Lorentzian form arising from the band structure of Al. The impurities Cr, Mn and Fe turn out to be magnetic, with local moments of 2.0, 2.5 and 1.75 Bohr magnetons. The electronic structure of the sp impurities Si, P, Ge and As is characterised by a shift to lower energies of the s density of states, leading to localised s states for P and As, and by a filling up of the p density of states.

Neutron diffraction study of magnetic ordering in UPd2Si2, UPd2Ge2, URh2Si2 and URh2Ge2

Abstract: Neutron diffraction measurements on polycrystalline samples show that UPd2Si2, UPd2Ge2 and URh2Si2 become antiferromagnetic below temperatures of 150K, 140K and 137K respectively. The magnetic moment ordering in UPd2Si2 and UPd2Ge2 is represented by a static longitudinal wave propagating along the c axis of the tetragonal body-centred crystal lattice. The magnetisation wave is polarised in the direction of the c axis: q=(0, 0, 0.662+or-0.010)c* in UPd2Si2 and q=(0, 0, 0.748+or-0.010)c* in UPd2Ge2. At 40K, UPd2Si2 exhibits a magnetic phase transition: a commensurate magnetisation wave with q=(0, 0, 1)c* represents the growth of a simple antiferromagnetic ordering. URh2Si2 is a collinear antiferromagnet with coinciding magnetic and chemical unit cells. The sequence of ferromagnetic sheets piled up along the tetragonal axis is given by +-. The values of the root mean square of the magnetic moment on the U ion at 4.2K are found to be 1.96 mu B, 1.97 mu B and 1.96 mu B for UPd2Si2, UPd2Ge2 and URh2Si2 respectively. The observed magnetic ordering schemes are discussed in terms of the RKKY exchange interaction. Neutron diffraction measurements performed at zero magnetic field show that URh2Ge2 is paramagnetic at 4.2K.

The electronic structure of antiferromagnetic chromium

Abstract: The author has used the local spin density formalism to perform self-consistent calculations of the electronic structure of chromium in the non-magnetic and commensurate antiferromagnetic phases, as a function of the lattice parameter. A change of a few per cent in the atomic radius brings the calculated ground state properties into agreement with experiment. The magnetisation is studied as function of volume in several models, and it is shown that a Stoner picture provides an extremely accurate description of the full calculation provided the sp-d hybridisation is taken into account. It is found that the calculated sublattice magnetisation is extremely sensitive to the exchange-correlation potential used and to the quality of the calculated state density.

Amorphous yttrium-iron alloys. II. Mossbauer spectra

Abstract: For pt.I see ibid., vol.11, no.12, p.2727 (1981). Mossbauer data are presented for eight Y1-xFex alloys with 0.32

Theory of local magnetic moments in transition metals

Abstract: The authors consider whether a state of disordered local moments (DLM) can exist in an itinerant magnetic metal above Tc. They develop a criterion Il(EF)>1 for the non-magnetic state to be unstable with respect to the formation of DLM. This criterion is more easily satisfied than the Stoner criterion for ferromagnetism. In(EF)>1, near the centre of a tight-binding band and less easily satisfied near the band edges. They report calculations on tight-binding d bands of 3d transition metals by means of the recursion method. These suggest that BCC Mn and Fe do possess a DLM state, whilst Cr, FCC Mn and Ni do not. They present a method of calculation of the real part of the Green function by the recursion method.

Helimagnetic structure of the FexCo1-xSi alloys

Abstract: From low-field magnetisation measurements the authors give evidence for the existence of helimagnetism in magnetically ordered FexCo1-xSi alloys. They report small-angle neutron scattering experiments on two samples which are confirmed to be helimagnetic with a helix long period of 430 AA for Fe0.9Co0.1Si and 295 AA for Fe0.8Co0.2Si.

Study of the 4f and valence band density of states in rare-earth metals. I. Theory of the 4f states

Abstract: The partially filled 4f shell in rare earths gives rise to complex multiplet structure in both photoelectron spectroscopy (PES) and bremsstrahlung isochromat spectroscopy (BIS). The fractional parentage theory previously applied to PES is extended to BIS, and a similarity is predicted between the PES of a 4fn element and the BIS of a 4f14-n element. Fractional parentage arguments are also applied to the linewidths of the 4f multiplet levels as observed in electron spectroscopy.

Surface and bulk properties of cerium atoms in several cerium intermetallic compounds: XPS and X-ray absorption measurements

Abstract: The authors present the results of XPS (X-ray photoemission spectroscopy) and XRA (X-ray absorption) studies performed on several intermetallic cerium compounds (CeRh3, CeCo5, CeNi5, CeNi2, CeRu4B4, CePd3 and CeBe13). In CePd3 and CeBe13, the cerium is known to be in the so-called intermediate valence state; in CeRh3 and CeCo5, the cerium is thought to be in the tetravalent state: Ce4+ (4f05d2). Finally for CeNi5, CeNi2 and CeRu4B4 the situation is unclear. The main purpose of this paper is to show that the valence of cerium in these compounds cannot be deduced from XPS and XRA measurements in the same way as it is for other rare-earth compounds (Tm or Sm for instance). Indeed pure tetravalent compounds like CeRh3, CeCo5 or even insulating CeO2, present XPS and XRA spectra which are similar to those obtained for intermediate valence compounds like TmSe, SmB6, etc.... If one supposes that the final states starting from a Ce4+ initial state are essentially made of configurations which contain one more electron in the 4f shell, the authors show that the valencies of cerium at the surface and in the bulk, deduced from their XPS and XRA experiments, are in approximate agreement with those deduced from lattice parameter measurements. Then, the direct observation of replicates in the XPS or XRA spectra of these cerium intermetallic compounds cannot be considered as a proof of the existence of an intermediate valence state.

Chemical short-range order in liquid and amorphous Cu66Ti34 alloys

Abstract: It is demonstrated that the chemical short-range order (CSRO) which exists in the transition-metal glass Cu66Ti34 originates in the liquid state, and that this CSRO appears to be enhanced on vitrification. The structure factor S(Q) of the liquid alloy Cu66Ti34 has been obtained by neutron diffraction. The form of S(Q), and in particular the existence of a 'prepeak' at a Q value of approximately 1.9 AA-1, provides direct experimental evidence of CSRO in the liquid state. A comparison of the S(Q) curves of Cu66Ti34 for the liquid and glass states indicates that the CSRO is less well developed in the liquid. This qualitative analysis is supported by a rough estimate of the value of the CSRO parameter, alpha , which appears to be reduced by a factor of about two in the liquid state.

Elastic constants in Nb-Mo alloys from zero temperature to the melting point: experiment and theory

Abstract: The elastic constants of the Nb-Mo system have been measured and calculated theoretically over a wide range of temperature and composition. This study completes previous work on the elastic constants of the Nb-Zr system and contributes to a better understanding of the anomalous elastic behaviour of Nb. The agreement between experiment and theory is good, all anomalous effects appear in both of them, and their origin is understood as a bandstructure effect.

Dynamic corrections to Van der Waals potentials

Abstract: A discussion is given of the velocity-dependent forces which arise when two Van der Waals coupled systems move slowly relative to each other. Explicit calculations both for the cases of a molecule interacting with a metal and for two metals interacting with each other indicate that these forces are very small and may safely be neglected in analysing the results of dynamic experiments designed to measure the static Van der Waals force. A result due to Teodorovich (1978) for the case of two metal films in parallel motion is shown to be incorrect.

A quench and irradiation study of the vacancy parameters in alpha -Cu-30 at% Zn

Abstract: Resistivity measurements were used to monitor the short-range ordering rates associated with equilibrium, quenched-in or irradiation-produced vacancies in an alpha -Cu-30 at.% Zn solid solution. The procedure by which the ordering kinetics are analysed is presented and discussed. The quench studies give activation energies of 0.64+or-0.05 eV, 1.03+or-0.05 eV and 1.59+or-0.05 eV for the migration and formation of single vacancies and the relevant self-diffusion, respectively. Radiation studies yielded a vacancy migration energy of 0.65 eV. Previous quenching work is reinterpreted in the light of these results and a consistent picture for the quench and irradiation-enhanced ordering effects is proposed.

On the Fermi surfaces of paramagnetic CucNi1-c alloys

Abstract: The authors have studied the Fermi surfaces of CucNi1-c alloys for c=87, 77, 62, 39 and 19 at.% within the atomic sphere approximation to the KKR-CPA scheme. They found that all these alloys had well defined Fermi surfaces with typical lifetimes tau approximately 1013 s. In particular they have investigated the variation of the density of states at the Fermi energy and the shape of the Fermi surface with concentration. Moreover, they discuss the momentum distribution functions relevant to two-dimensional positron annihilation (ACAR) measurements. The results are rich in observable details and are consistent with experiments where these are available.

The vacancy formation enthalpy in Ni determined by positron annihilation

Abstract: The monovacancy formation enthalpy, H1vF, was determined for Ni using the Doppler broadening technique. The 58Co positron source was ion implanted in Ni of 99.995 wt.% purity. A 103Ru gamma -ray line (497 keV) was recorded simultaneously with the positron annihilation 511 keV line, and was used in stabilising the data. Measurements were performed on three similar Ni samples, which were annealed in an ultra-high vacuum chamber (10-8-10-9 Torr) at approximately 30 degrees C below their melting point for at least two hours prior to the in situ measurements. For one of the samples, a comparison was made of H1vF values deduced prior to and after in situ melting. A value of H1vF=1.8+or-0.1 eV was determined from the present data. In addition, a value for the monovacancy migration enthalpy, H1vM=1.1+or-0.1 eV, has been deduced from a comparison of the present H1vF measurement with previous self-diffusion results.

Scanning transmission electron microscopy: microanalysis for the microelectronic age

Abstract: In this critical review, a brief account is presented of the new techniques of 'analytical electron microscopy': that is to say, the chemical and structural analysis of very small amounts of matter. Image formation, diffraction analysis, electron energy loss spectroscopy, and X-ray analysis are all discussed. The discussion is sufficiently detailed to allow the reader to appreciate what can be done with the techniques but it is intended to be easily understood by the non-specialist.

Interstitial cluster in FCC metals

Abstract: Interstitial clusters in FCC metals have been studied using computer simulation techniques and three different interatomic potentials. The potentials were fitted to both ideal and defect crystal data. Clusters containing more than 10-13 interstitials were found to take the form of two-dimensional platelets whereas the smaller clusters had a three-dimensional nature. The most stable 2D and 3D configurations were always those which preserved the closest packing.

Local structure and topology of a model amorphous metal

Abstract: A dense random packing model of amorphous iron was analysed in terms of the Voronoi polyhedral statistics and the recently proposed local structure parameters, in particular stresses and site symmetry coefficients (Egami et al., 1980). The purpose of this study is to clarify the relationship between these two types of methods of describing the local structure of amorphous solids. It has been shown that the topology of the local structure represented by the Voronoi polyhedra is related to the local stresses and symmetry only through the coordination number. On the other hand, the degree of distortion of the environment, which is of primary importance to many physical properties and is well characterised by the local parameters, is practically independent of the polyhedral topology. It is concluded, therefore, that the topological description based upon the polyhedral statistics does not provide an unambiguous microscopic basis for relating the local structure of the dense random packing model to its physical properties and could even be misleading.

The pressure dependence of the Curie temperature of heterogeneous ferromagnetic alloys

Abstract: The empirical evidence for a systematic dependence of the laws regarding the pressure dependence of the Curie temperature of alloys on their structural and magnetic heterogeneity is presented. The materials covered include both crystalline and amorphous alloys. Previous interpretations of the results are given, followed by a discussion on the basis of the Landau-Ginzburg formalism which is concerned with concentration fluctuations and which reduces the magnetovolume problem to a purely magnetic one. A reasonable approach to the problem gives results in semi-quantitative agreement with the empirical evidence.

AC susceptibility and resistivity of the ferromagnetic phase of PdMn

Abstract: 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 Tc. The electrical resistivity data, taken on the same samples, are in good overall agreement with the results obtained from the susceptibility study.

Thermoelectric powers of amorphous transition metal alloys and electron-phonon enhancement

Abstract: The authors have measured the thermoelectric powers of a series of amorphous transition metal alloys and find, contrary to expectation, that they are not simply proportional to temperature below about 200K. They argue that the results can be best understood as being due to electron-phonon enhancement effects and show that there is agreement between the experimentally deduced electron-phonon enhancement factors and those calculated from superconductivity data.

A molecular dynamics study of surface self-diffusion

Abstract: A molecular dynamics study of surface self-diffusion on four different crystal surfaces was accomplished for atoms interacting via a Lennard-Jones 12-6 potential energy function. Both the surface and adsorbed atoms experienced realistic thermal motion which included all anharmonic effects. The formation of vacancy-ad-atom defect pairs leading to surface layer melting was observed below the bulk melting temperature for the (111), (113), (110) and (100) surfaces. Diffusion activation energies and exponential prefactors were calculated. Details for unusual atomic transport mechanisms on the (111) and especially (110) surfaces are also reported. Additional features include the analysis of velocity correlations and vibrational spectra associated with different types of atomic motion at surfaces.

The use of a pair potential for the study of defects and disorder in aluminium

Abstract: The authors have examined the usefulness of the Dagens, Rasolt and Taylor (DRT) interionic potential (1975) for calculating disorder and defect properties in Al from the point of view of a molecular dynamics (MD) computer simulation. The problem was complicated by the strong long-range oscillations present in the Al potential and careful attention was paid to this detail. The authors confirmed that the low-temperature phonons generated by the MD program agreed very well with the quasi-harmonic phonons of DRT and then proceeded to generate the high-temperature phonons. They also calculated the liquid structure factor obtaining satisfactory agreement with experiment. They found, not too surprisingly, that the calculated vacancy formation energy for Al was much smaller than experiment by about 0.5 eV. However they noted that if one allows for re-screening effects in the vicinity of the vacancy a subtle three-body force comes into play which, on a crude argument, accounts for this difference.