Table of contents

Hard magnetic behaviours of the composite materials (NdDy-FeCo-B)_1-xFe_x prepared by direct mixing of iron nanoparticles with HDDR NdDy-FeCo-B powders have been investigated. With increasing the content of iron nanoparticles in composite materials, the remanence first increases, undergoes a peak and then decreases, whereas the coercivity decreases monotonically. The squareness of the demagnetization curves decreases with increasing content of iron nanoparticles. Meanwhile, a high degree of reversibility in the minor loops along the demagnetization branch in the fields below coercivity has also been observed.

The structure and some thermodynamic properties of iron, cobalt and nickel are investigated in the framework of integral-equation theory with the hybridized mean spherical approximation. A local form factor, which combines the empty-core model and s-d mixing through an inverse scattering approach, is used to deduce effective-pair potentials suitable for transition metals via the second-order pseudopotential perturbation method. The resulting internal energy and thermodynamic quantities, necessary to achieve thermodynamic self-consistency, contain in addition a d-band contribution expressed by the Friedel rectangular model for the density of d states. The calculated structure factors S(q) of Fe, Co and Ni near the melting point are found to be in good agreement with experiment and the energies, obtained self-consistently, coincide reasonably well with other published theoretical data.

Charge polydisperse colloidal crystals have been recently reported to turn amorphous beyond a critical charge polydispersity (CPD). The resulting disordered (amorphous) state, due to its unusual structural properties, has been conjectured to be different from that obtained from quenching of atomic liquids and is probably a stable state. In order to confirm this, Monte Carlo simulations with different initial configurations are performed and properties of equilibrium states investigated. In contrast to a monodisperse system, which always evolves to a crystalline structure irrespective of the initial configuration, the charge polydisperse system with random charge distribution is found to remain disordered. On the other hand, for the same CPD an ordered crystalline state is found to remain in equilibrium when evolved from a charge-ordered initial configuration. Simulated annealing, by gradually decreasing impurity ion concentration n_i (which is equivalent to the lowering of temperature) and evolving at each n_i, is also carried out to obtain the low-temperature (ground) state. From the present investigations it is concluded that beyond a critical CPD the charge polydisperse system remains in a stable disordered state unlike atomic glasses (obtained by quenching) which are known to be metastable. Based on some common properties an analogy is made between a spin glass and the amorphous state. The stability aspects of this amorphous state and charge-ordered states are also discussed.

The capillary hysteresis for the drainage-imbibition process in correlated site-bond porous networks is studied. The degree of correlations between pore radii is shown to produce important modifications on the hysteresis loop which is wider for non-correlated networks and narrower for correlated ones. A relation between the degree of correlation and the type of solid is suggested.

The generalized mean spherical approximation model presented in the previous paper is extended to the case of the analytical solution of the Ornstein-Zemike equation with the closure relations which are given by Yukawa functions with different damping factors. The calculated result of the density profiles is compared with the recent computer simulation data.

Cr³⁺ and Fe³⁺ ions are used as paramagnetic probes of the local order in diamagnetic transition metal fluoride glasses (TMFG). EPR observations are made at S, X, K and Q band frequencies at different temperatures. It follows from the frequency dependence of the spectra that Cr³⁺ and Fe³⁺ ions are characterized by a continuous fine-structure parameter distribution in the range 0.06-0.55 cm^-1 for Cr³⁺ and 0.04-0.33 cm^-1 for Fe³⁺. The simulations of these spectra are computed with a fine-structure parameter distribution P(h₂⁰, lambda ) whose expression is determined by analogy with the distribution of electric field gradient P(V_zz, eta ) developed by Czjzek considering amorphous solids with random ionic coordination. An excellent agreement between observed and calculated spectra is obtained whatever the frequency band.

Propagation of plane acoustic waves in anisotropic media can be characterized by the slowness surface and ray surface which represent phase velocities and group velocities, respectively. The mathematical expressions for the slowness surface in terms of elastic constants are well established and provide a foundation for acoustic determination of the elastic constants of anisotropic elastic media. However, a direct mathematical formulation of the ray surface has not been fully developed because of the many-to-one correspondence between the ray surface and the slowness surface. Based upon the Stroh formalism for two-dimensional elastodynamic systems, we establish in this paper a direct and analytical formalism, the degeneracy analysis approach (DAA), for the construction of the ray surface and the recovery of elastic constants for anisotropic media. A special emphasis is put on the group velocities along symmetry directions with respect to symmetry planes, for which an explicit expression for the group velocity is given in terms of the elastic constants so that the inverse problem can be solved easily. Particularly, the complication of cuspidal points due to the presence of axial concavity in the slowness surface is treated and it is apparent that the presence of a cuspidal point becomes an advantage for the inverse problem of recovery of elastic constants.

The charge-transfer electronic structure of CuBr₄^2- in (N(CH₃)₄)₂CdBr₄ crystals is investigated through polarized optical absorption spectroscopy. The transition energy and the band polarization are explained in terms of the Jahn-Teller distortions of D_2d symmetry of the CuBr₄^2- complex. Some bands are split in the low-temperature spectra by the spin-orbit interaction of the Br ligands. The results are compared with those available for other CuBr₄^2- systems. We also investigate the dynamics of the inorganic CuBr₄^2- units in the 9.5-300 K temperature range through the intensity of the charge-transfer bands. Analogously to the CdBr₄^2- tetrahedra in the pure crystal, CuBr₄^2- experiences reorientational motions upon varying the temperature which are correlated with the temperature dependence of the monoclinic beta parameter below the Pmcn to P2₁c/ phase transition temperature T_cl=272 K. An important finding of the present work is the observation of anomalies in the CuBr₄^2- dynamics which are associated with the existence of a new first-order phase transition at T_c2=20 K with a thermal hysteresis Delta T=10 K. This new phase transition, which had not previously been detected in the ((CH₃)₄N)₂MBr₄ (M=Zn, Mn, Co or Cd) series, would correspond to the monoclinic P2₁c/ to orthorhombic P2₁2₁2₁ transition, confirming the predictions of the universal p-T phase diagrams of the title compounds.

The linear muffin tin orbital method has been used to calculate the energy-momentum distribution of valence electrons in graphite along major symmetry directions and as the spherical average over the irreducible wedge of the Brillouin zone. These data bridge the gap between existing band structure calculations and the emerging electron momentum spectroscopy of solids. The calculation has been validated by comparison to the most accurate experimental data on highly oriented laser-annealed carbon films.

Transient photoinduced absorption spectra of one-dimensional excitons and biexcitons coupled with lattice vibrations are calculated with a two-band model by molecular dynamics. It is shown that the absorption peaks corresponding to the transition from photoexcited B_u excitons to A_g excitons and the lowest biexciton shift as self-trapping proceeds, where the former shifts to higher energy and the latter to lower energy. This difference is understood by means of the relaxation energy of B_u, A_g excitons and the biexciton. The transient absorbance change near the B_u to A_g peak is also discussed.

Low-field Hall effect (LFHE) data obtained at 4.2 K, and data on the deviations from Matthiessen's rule (DMR) obtained between 4.2-140K, of dislocated high-purity silver, gold and copper are compared in order to complete and extend a previous investigation of copper. The corrections to the two-group model (TGM) necessary to bring the anisotropy parameters for electron-dislocation scattering obtained from LFHE and DMR into coincidence are discussed in terms of the different Fermi surfaces and Fermi velocities as well as relaxation-time distributions. New insight is gained into the fact that these corrections are moderate, although the assumptions of the TGM are rather far from being fulfilled. The systematic changes of the anisotropy parameters estimated from the corrected TGM (0.09 for Ag, O.11 for Cu and 0.13 for Au) are in qualitative agreement with the Watts model of electron-dislocation scattering.

Emission of ballistic acoustic phonons by quantum edge states of one spatial quantization subband has been studied in a two-dimensional electron gas formed at the GaAs/AlGaAs heteroface. The phonon emission due to the deformation and piezoelectric interactions are considered. An analytic expression for the emission intensity distribution in phonon momenta has been derived. Detailed analysis of the intensity distribution is made in low- and high-temperature regimes. Different positions of the Fermi level are considered. It is shown that at low temperatures the phonon emission is predominantly concentrated within a narrow cone around the direction of the edge state propagation, while at high temperatures it is around the magnetic field normal to the electron plane. The emission intensity decreases when the Fermi level falls. This diminution is exponential at low temperatures.

Results of magnetic, medium-resolution neutron diffraction and small-angle neutron scattering (SANS) measurements on a polycrystalline LuFe₁₀Mo₂ intermetallic alloy show that ferromagnetic (FM) interactions exist between 5 and 325 K, but finite magnetic spin correlations emerge in the examined temperature range. It is observed that in zero applied magnetic field there is no well defined critical point T_c indicating a transition from a paramagnetic to a long-range ordered magnetic phase. A linear variation of magnetization as a function of T^3/2 remarkably persists up to a temperature that is a large fraction of the T_c for the LuFe_12-xMo_x alloys.

A combined nuclear magnetic resonance and Mossbauer spectroscopy study has been performed to investigate the effect of Y substitution on the phase composition of crystallized Nd₄Fe_77.5B_18.5 alloy. The annealing temperature dependence of the phase components for Y₄F_77.5B_18.5 was studied as well. It was shown that the substitution of Y has a different effect on the phase components from that of Gd substitution in some cases. A full substitution of Y results in the appearance of a new metastable phase Y₃Fe₆₂B₁₄.

Raman scattering investigations were performed over a large temperature range from 10 to 700 K on pure and Ti-doped PbHfO₃ single crystals. The modifications of the Raman spectra were precisely observed in the vicinity of two structural phase transitions occurring in the crystal: an antiferroelectric (A₁) to antiferroelectric (A₂) phase transition at 436 K and antiferroelectric (A₂) to paraelectric (P) phase transition at 484 K. The characteristic parameters of the Raman structures were determined throughout the whole phase transition sequence. The softening of the two lowest-frequency Raman lines seems to have a dominating influence on the slight increase in dielectric permittivity at the A₁ to A₂ phase transition in both pure and doped samples suggesting a displacive nature for this transition. For Ti-doped samples the form of the A₁ to A₂ transition is smoother than for undoped samples while the A₁ to P transition appears to be unaffected by doping with Ti.

Theoretical and experimental Rh L edge X-ray absorption near-edge spectra (XANES) in the compounds RhCl₃, Rh₂O₃ and (Rh(CO)₂Cl)₂ are presented. L₁ and L_2,3 pre-edge features (bound to bound state transitions) and near-edge features (continuum final states) are analysed and used to interpret experimental L₃ spectra. The self consistent-field local density theory was used to calculate charge densities and potentials, as well as densities of states in an embedded cluster approach. Multiple-scattering theory was used to determine the final states, and the absorption cross-section was calculated in the dipole approximation. Quantitative modelling of the absorption spectra permits derailed assignment of individual absorption features due to effective atomic configurations and local geometrical effects. The off-site Rh 4f resonance is a notable feature of the XANES which may be useful in characterizing Rh-Rh interaction.