Showing papers in "Journal of Physics F: Metal Physics in 1975"

Theory of spin glasses

Abstract: A new theory of the class of dilute magnetic alloys, called the spin glasses, is proposed which offers a simple explanation of the cusp found experimentally in the susceptibility. The argument is that because the interaction between the spins dissolved in the matrix oscillates in sign according to distance, there will be no mean ferro- or antiferromagnetism, but there will be a ground state with the spins aligned in definite directions, even if these directions appear to be at random. At the critical temperature the existence of these preferred directions affects the orientation of the spins, leading to a cusp in the susceptibility. This cusp is smoothed by an external field. Although the behaviour at low t needs a quantum mechanical treatment, it is interesting to complete the classical calculations down to t=0. Classically the susceptibility tends to a constant value at t=0, and the specific heat to a constant value.

Use of energy derivative of the radial solution in an augmented plane wave method: application to copper

Abstract: By a suitable combination inside the muffin-tin sphere of a radial solution to the Schrodinger equation and its energy derivative, the dependence of the APW matrix elements on the energy E used to construct the basis functions can be greatly reduced. This has a number of advantages. The authors present an application of the method, previously suggested by Marcus (1967) and analysed by Andersen to the Chodorow copper potential and examine its precision and its convergence properties. It was found the method converges about equally well or somewhat more slowly than the standard APW method in number of basis functions. The eigenvalue error of the method is proportional to (E-E0)4. The error of the wavefunction is proportional to (E-Eo)2. The d states limit the range of mod E-E0 mod <0.1 Ryd for acceptable wavefunctions. The limit for non-d states is larger than 1 Ryd.

First order transitions in ACo 2 compounds

Abstract: Some magnetic properties of ACo2 compounds, where A is a heavy rare earth metal, are shown to be related to properties of YCo2 In particular the existence of first order transitions for A=Er, Ho, Dy, but second order transitions for A=Tb, Gd, implies a special temperature dependence of the Landau parameter B which appears in the expansion of the free energy of YCo2 in powers of the magnetization B changes from negative to positive with increasing temperature and this is verified experimentally by measurements of the field dependence of the susceptibility of YCo2 at 42, 77 and 300K in field up to 150 kOe It is estimated that an applied field of about 1400 kOe would be required to produce itinerant electron metamagnetism in YCo2 and the magnetization of GdxY1-xCo2, where the 'applied' field is the molecular field due to Gd moments, is discussed from this point of view Stoner theory is used to relate the behaviour of B to the existence of a maximum in the susceptibility of YCo2 as a function of temperature

Magnetization of disordered cold-worked Fe-Al alloys up to 51 at.% Al

Abstract: The magnetization and high-field susceptibility of disordered cold-worked Fe1-cAlc alloys with 0

Conduction electron polarization of gadolinium metal

Abstract: The zero field moment of Gd has been measured at 4.2K for a single crystal prepared by solid state electrotransport. It is magnetically very soft: the moment is (7.630+or-0.010) mu B per atom in the b direction and is slightly smaller in the c direction. The conduction electron polarization is 15% higher than previously estimated. Due to its small value the conduction electron susceptibility could not be measured accurately.

Metallic glasses as relaxed Bernal structures

Abstract: A dense random packing of 888 hard spheres has been relaxed with respect to a realistic interatomic potential. The result is compared to the experimental pair distribution function of amorphous Co and an amorphous Ni76P24 alloy. The positions of the maxima of the pair distribution function of Co are accurately reproduced. The differences between Co and Ni76P24 are discussed.

Surface densities of states in cleaved transition metals

Abstract: The local densities of states on clean low index surface of fcc N, hcp Co and bcc Fe are studied within the tight binding scheme using a moment method and a continued fraction analysis. The d band degeneracy is fully taken into account. The method also allows one to handle charge transfer effects a surface perturbation potential is included and calculated selfconsistently. A fairly complete series of surface densities of states is presented and discussed with particular emphasis on modifications brought about the surface perturbation potential and occurrence of surface states. For the bcc structure of Fe, effects of crystal field integrals are also investigated. A comparison is made with recent spectroscopic observations. Some applications to variations of density of charge on the surface and to surface tension are briefly discussed.

Hyperfine field systematics in Heusler alloys

Abstract: The present study establishes hyperfine field systematics at the non-magnetic sites in Heusler alloys and emphasizes the effect of local screening on the hyperfine field value. A range of Heusler alloy hosts were doped with 1 at.% of the isotope 119Sn for examination using the Mossbauer effect. The results reported indicate that the hyperfine field at a given site is strongly dependent in magnitude and sign on the magnetic character of the local screening charge. The fields at Sb sites in several alloys have been measured using the Mossbauer isotope 121Sb and at a Cd impurity site in Ni2MnIn using the method of time-differential perturbed angular correlations. The Sn site fields have been measured in the Co based alloys Co2TiSn, Co2ZrSn and Co2HfSn. The similarity of the values obtained is interpreted in terms of the similar outer electron structure of these alloys.

Spin glass versus ferromagnet

Abstract: An investigation is presented of the conditions for spin glass or ferromagnetic ordering in systems with exchange interactions distributed randomly about a positive mean, using a quantum extension of a recent method of edwards and anderson (see ibid., vol.5, no.5 p.965, (1975)). The results are related to the situation relevant to a metallic magnetic alloy where they lead to a phase diagram and susceptibility cusp in good accord with experiment.

X-ray excited Auger and photoelectron spectra of magnesium, some alloys of magnesium and its oxide

Abstract: KLL, and KLM Auger and X-ray photoelectron spectra for pure magnesium, for Mg2Cu, Mg3Au and Mg3Bi2, and for MgO are reported. Assignments are made for almost all of the observed features in the Auger spectra. For the pure metal, internal photoemission peaks and a plasmon gain peak are shown to be present in the X-ray excited spectra.

Mossbauer study of the intermetallic compounds (Fe1-xCox)2B and (Fe1-xCox)B

Abstract: Results of Mossbauer measurements on the intermetallic compounds (Fe1-xCox)2B(0

Helical magnetic order in Ni2MnAl

Abstract: Magnetic susceptibility, X-ray diffraction and neutron diffraction measurements have been made on quenched and slow-cooled samples of Ni2MnAl alloy. Above 300K the susceptibility is Curie-Weiss but on cooling below this temperature the susceptibility becomes field dependent. Neutron diffraction patterns indicate that the compound is chemically ordered into the B2 structure with some additional L21 (Heusler alloy) type of order, that is enhanced by slow cooling. At 4.2K magnetic scattering was observed in fundamental reflections and in pairs of satellite reflections which were displaced by +or-0.111a* from the reciprocal lattice points corresponding to the nuclear reflections. The positions and intensities of the magnetic reflections are consistent with an antiferromagnetic cone spiral structure associated with the Mn atoms. The collinear components are coupled ferromagnetically in (111) sheets with antiferromagnetic coupling between adjacent planes.

Size effects, peaks in concentration fluctuations and liquidus curves of Na-Cs

Abstract: Conformal solution theory applies to some binary alloys, for which the volume differences are not too large. The volume ratio r for Cs/Na=3, and the system is not conformal. A theory of the concentration fluctuations, valid for large r, is described. The liquidus curves are calculated, there is a rather flat portion around an Na concentration of 0.8, correlating with a strong peak in the concentration fluctuations around this composition. This is in agreement with observation. Na-Rb alloys and a number of other alloy systems show a similar feature, and these are briefly discussed.

Interstitial trapping and detrapping in electron irradiated dilute copper alloys

Abstract: The interaction of migrating copper interstitials with impurities has been investigated by damage rate measurements in dilute copper alloys with Au, Be, Ge, Ni, Pd, Sb, Zn at irradiation temperatures between 50 and 170K. By applying the diffusion theory of the three dimensionally migrating interstitial to the data, the resistivity contribution of trapped interstitials and the capture radii of impurities for migrating interstitials has been determined. The above mentioned resistivity contribution was found to be independent of the impurity species and of irradiation temperature. The capture radii of all impurities decrease with increasing irradiation temperature and become zero at about 100K except that of Be. The cross section of a trapped interstitial to become detrapped is about 106 barn.

The low field static susceptibility of the spin glass Au0.96Fe0.04

Abstract: New measurements in fields of less than 20 Oe of the static susceptibility of Au0.96Fe0.04 are presented. A sharp peak chi (T) is obtained under non-equilibrium conditions. When however the magnetization is allowed to come into equilibrium at each temperature, a quite different behaviour of chi (T) is found. In this particular alloy the long time equilibrium state is apparently the same as the field cooled state.

Resistivity of spin glasses

Abstract: The main features of the resistivity of spin glasses as a function of temperature, in particular the T3/2 dependence at low temperature, are explained in terms of the scattering of the conduction electrons by elementary excitations of the system which are diffusive in character.

Electronic structure of metals: I. Energy independent model pseudopotential formalism?

Abstract: An energy independent model pseudopotential formalism for calculating the electronic structure of metals is developed so that uncertainties in the determination of the model potential parameters, such as those occurring in the Heine-Abarenkov type model potential due to the energy dependence, can be avoided. A model potential, in general more suitable than the square-well potential, with parameters determined by fitting to the energy levels of the low-lying atomic valence states is presented and is used in the construction of the model pseudopotential for the applications to metals. An expression for the form factor, including the effective mass and exchange-correlation effects, is derived in a way consistent with the proposed model pseudopotential.

The zero-frequency susceptibility of spin glasses and mictomagnets

Abstract: A model for magnetic clustering in spin glass alloys is presented. The susceptibility will show a discontinuous change of slope at the glass transition temperature Tg where an infinite cluster first forms, provided that cluster is superparamagnetically blocked. The statistics of magnetic clustering can be specified exactly on a suitably defined Bethe lattice. In the spin glass limit, i.e. infinite dilution of the magnetic species, it is found that (i) the susceptibility above Tg has the same Curie constant as for non-interacting spins and (ii) the change in slope at Tg is Delta K=2, where K=d(Inchi )/d(InT). The role of inter-cluster interactions and loose spins is briefly discussed.

Entropies and structure factors of liquid metals

Abstract: Recent theory, relating the thermodynamics for a real liquid to that of a hard sphere reference system suggests a correlation between the entropy and the structure factor. The extent to which this correlation exists in liquid metals is investigated by comparing, for each system and at various temperatures, the packing fractions deduced respectively from entropy and structure factor measurements. The quality of agreement is, overall, comparable with that obtained from previous comparisons of measured and hard sphere structure factors as functions of wave number. The standard ab initio method of calculating interatomic forces in simple metals uses pseudopotential theory. It is suggested that near the melting points entropies for all valencies, and specific heats for lower (

Limited ferromagnetism and other magnetic properties of Pd-Mn alloys

Abstract: Solid solutions of Mn in Pd have been studied by magnetization, paramagnetic resonance and electrical resistivity measurements and have been found to yield ferromagnetism only for solute concentrations below 5% and temperatures below 8K. At higher concentrations a susceptibility maximum and other effects characteristic of spin glass ordering appear, and in the transition region there is evidence for a large amount of freezing of spin entropy of a less well defined character. This behaviour is discussed in terms of the roles of solute-solvent interactions and solute-solute interactions for close and distant neighbours.

Determination of the three partial interference functions of an amorphous cobalt-phosphor ferromagnet by polarized-neutron scattering

Abstract: The three partial interference functions of a Co-P alloy, which are simply related to the static pair-correlation functions, have been determined using a polarized neutron technique which may be extended to other binary amorphous ferromagnets. The equations necessary for the interpretation of the measurements are presented and discussed.

Grain boundary contribution to the electrical conductivity of polycrystalline Cu films

Abstract: Cu films of 1000 AA were grown at various substrate temperatures with increasing grain sizes at 10-5 Torr. The grain boundary resistivity ( rho gb=3.58*10-12 Omega cm2) and reflection coefficient (R=0.2635) at room temperature (22 degrees C) remained almost the same for different grain sizes, suggesting that the potential barrier between adjoining grains remains the same. Impurities and other defects have negligible effect on these values of rho gb and R. Results support the Mayadas-Schatzkes (M-S) grain boundary model (1970) of electrical conductivity for polycrystalline films.

Behaviour of collective volume excitations in aluminum near the 'critical wavevector' (transition into single particle excitation)

Abstract: From energy loss experiments with 51 keV electrons the dependence of the aluminum plasmon energy on the wavevector q has been derived. The measurements are extended to q>qc=1.3 AA-1 (qc=critical wavevector) and reach q approximately=2.2 AA-1. For q qc, however, the loss energy shows only a weak dependence on the wavevector. The results are compared with calculations of the loss function. Im(- epsilon ( omega ,q)-1), using a dielectric function epsilon ( omega , q) that takes into account finite electron lifetime and local field corrections. It is shown that for large q local field effects have a strong influence. The halfwidth of the loss peak has also been obtained as a function of q. It shows a nearly steplike increase near the critical wavevector.

Off-diagonal disorder and energy of formation in transitional alloys

Abstract: The authors show that in most cases the energy of formation of disordered transitional alloys is mainly determined by the deformation of the electronic bands upon alloying and that the difference in bandwidth between the metals must be taken into account together with the difference between their energy levels

57Fe mossbauer studies of the (RE)Fe3 intermetallic compounds

Abstract: See also ibid., vol.5, no.5, p.1037 (1975). The methods described in the previous paper are used to analyse the complicated 57fe mossbauer spectra of the (re)fe3 intermetallic compounds, where re=gd, tb, dy, ho, er and y. The resultant hyperfine parameters are then used, in conjunction with local dipolar field calculations, to infer directions of preferred magnetization. In particular, it is shown that at 77k, hofe3 magnetizes along an a axis, tbfe3 along a b axis and erfe3 either along or close to a c axis. The case of dyfe3 however is slightly more complex and it appears that this compound does not magnetize along a principal hexagonal axis. Some similarities between the mossbauer spectra of the (re)fe2 and (re)fe3 compounds are also noted.

An image force theorem for a dislocation near a crack in an anisotropic elastic medium

Abstract: Using the sextic formalism developed by Stroh (1958, 1962), and Malen and Lothe (1970), the interaction between a semi-infinite crack and a straight dislocation parallel to the crack plane is treated. The following theorem is proved: If r' is the radial distance from the crack tip to the dislocation, then the image (interaction) force exerted by the crack on the dislocation is given by Fr=-Einfinity /r' where Einfinity is the prelogarithmic energy factor of the same dislocation in the infinite uncracked medium. The image force is thus independent of the dislocation's angular position. Anisotropy influences the image force only through Einfinity . This result is of importance in estimating the local plastic deformation near the tip of a crack.

Antiferromagnetic domains in rare earth metals and alloys

Abstract: Anomalies in the c-axis elastic properties of antiferromagnetic Dy 50% Tb-Ho and 60% Gd-Y are reported. The anomalies are only present when the sample is cycled from the ferromagnetic to the antiferromagnetic state and are attributed to domains in the helical regime.

Magnetic properties of LaCo5xNi5-5x

Abstract: The magnetic properties, the pressure dependence of the Curie temperature (Tc) and the initial compressibilities of the series of compounds LaCo5xNi5-x were determined. The compounds with x>or=0.6 are ferromagnets whilst for the compounds with 0.2

Lattice dynamics of noble and transition metals

Abstract: A simple model for the calculation of the phonon dispersion curves of the noble and transition metals is proposed. The model uses the concept of a rigid shell moving relative to its nucleus to simulate the response of the d electrons to the nuclear motion, short range central forces for the interaction between the ion cores, and a screened pseudopotential for the conduction electron response. The most general form of the model considered for f.c.c. metals contains six parameters, three of which may be obtained from the adiabatic electric constants and the remainder from a least square fit to the experimental frequencies. Preliminary applications of the model containing five parameters made for copper and nickel have resulted in fits to the experimental dispersion curves which are within 2 and 2.5%, respectively.

Perturbation theory and three body forces in hcp metals

Abstract: An analysis is presented of the role of the non pair forces in the lattice dynamics of sp bonded hcp metals: Be, Mg and Li. By expanding the total energy up to third order in the electron-ion pseudopotential, a dynamical matrix is derived which contains terms arising from central pairwise interactions between the ions as well as contributions due to sums of three body forces. The phonon dispersion relations are calculated using a local form of the model potential, which allows for a realistic evaluation of the third order contributions. The results show that the three body forces are very important in Be and are responsible for the peculiarities of the spectrum, as is most clearly seen from the analysis of the frequencies at the K point of the Brillouin zone.