Table of contents

Nanocrystalline La_0.7Sr_0.3Mn_0.9M_0.1O₃ (M = Cr, Mn, Fe, Ni, Cu) powder samples were prepared by a sol-gel method. In the samples Mn atoms were partially substituted by a transition metal M. Pure perovskite phase nanocrystalline grains (~60 nm in diameter) were confirmed for all the samples through XRD and SEM observations. Magnetization and magnetoresistance of selected samples were investigated over a temperature range of 5 to 350 K, under a magnetic field of up to 5 T. Strong low field sensitive magnetoresistance was detected in Fe and Ni substituted samples. Spin polarized tunnelling and substitution modified non-balance magnetization processes are believed to be the origin of the enhanced low field sensitivity.

We have investigated the implantation of size-selected Ag₇^- clusters into the graphite surface as a function of kinetic energy (E) from 1.0 to 5.5 keV, via scanning tunnelling microscopy together with molecular dynamics simulations. By utilizing thermal oxidation of the graphite surface to expand laterally the defects created via implantation, we find that the cluster implantation depth is proportional to (E)^1/2 and hence the velocity of the bombarding cluster. By careful control of the oxidation temperature, we also demonstrate that even moderate temperatures (923-1123 K), considerably lower than the melting point of graphite (4450 K), lead to significant annealing of the defects formed through cluster impact, accounting for the shape of the experimental depth distributions observed.

By taking into account the chemical-potential fluctuations, we propose a generalization of the formalism elaborated by R V Chepulskii and V N Bugaev (1998 J. Phys.: Condens. Matter10 7309), which is based on the use of the thermodynamic fluctuation method in the first order of a modified thermodynamic perturbation theory under the choice of the inverse effective number of atoms interacting with one fixed atom as a small parameter of expansion. As a result, a new approximation for calculation of the short-range order parameters in the disordered (i.e. without the long-range order) state of alloys is obtained. Moreover, the rigorous equation for determination of the chemical potentials is derived to replace the heuristic one obtained earlier. Both analytical and numerical comparison of the results of old and new approximations is performed. By comparison with the results of the Monte Carlo simulation, the higher numerical accuracy of the new approximation is demonstrated. The importance of taking into account the chemical-potential fluctuations is shown at low temperatures.

The influence of Dzyaloshinskii-Moriya (DM) interaction on the spin-Peierls systems is studied by use of theoretical analysis and the Lanczos numerical method. The ground sate of the spin-Peierls system is still a dimerized phase when DM interaction is present. The results show that the uniform DM interaction is always against dimerization but the staggered DM interaction may act in favour of or against dimerization. There exists a critical point where the staggered DM interaction has no influence on dimerization.

We consider a `quantum dot' in the Coulomb blockade regime, subject to an arbitrarily large source-drain voltage V. When V is small, quantum dots with odd electron occupation display the Kondo effect, giving rise to enhanced conductance. Here we investigate the regime where V is increased beyond the Kondo temperature and the Kondo resonance splits into two components. It is shown that interference between them results in spontaneous oscillations of the current through the dot. The theory predicts the appearance of `Shapiro steps' in the current-voltage characteristics of an irradiated quantum dot; these would constitute an experimental signature of the predicted effect.

We have studied the electronic structure of liquid water using x-ray absorption spectroscopy at the oxygen K edge. Since the x-ray absorption process takes less than a femtosecond, it allows probing of the molecular orbital structure of frozen, local geometries of water molecules at a timescale that has not previously been accessible. Our results indicate that the electronic structure of liquid water is significantly different from that of the solid and gaseous forms, resulting in a pronounced pre-edge feature below the main absorption edge in the spectrum. Theoretical calculations of these spectra suggest that this feature originates from specific configurations of water, for which the H-bond is broken on the H-donating site of the water molecule. This study provides a fingerprint for identifying broken donating H-bonds in the liquid and shows that an unsaturated H-bonding environment exists for a dominating fraction of the water molecules.

We present a combined experimental/computational study of the near-edge x-ray absorption fine structure of the liquid water surface which indicates that molecules with acceptor-only hydrogen bonding configurations constitute an important and previously unidentified component of the liquid/vapour interface.

We address the century-old puzzle of the existence of a liquid-like layer at the ice surface near its melting point with new photoelectron spectroscopic tools using synchrotron radiation. Near-edge x-ray absorption shows that a liquid-like film exists at temperatures as low as -20 °C. Near 0 °C this film is about 20 Å thick, i.e., six bilayers. With high-pressure x-ray photoelectron spectroscopy, we have further investigated the effect of surface contamination, which is ubiquitous in natural environments, on the state of the ice. Our results show that the premelting of ice can be strongly enhanced by the presence of hydrocarbon contamination.

Some interesting magnetic properties of artificially layered metallic materials are strongly connected with the existence of electron standing waves, or quantum well (QW) states. One such property is the oscillation in exchange coupling between two ferromagnetic materials separated by a nonmagnetic spacer layer of varying thicknesses. This article summarizes the findings of an extended investigation of QW states and their relation to oscillatory magnetic interlayer coupling carried out using angle-resolved photoemission with synchrotron radiation and auxiliary techniques such as magnetic x-ray linear dichroism and surface magnetic optical Kerr effect. A key feature of the measurements was the use of wedge-shaped samples, which, in combination with the small spot size of the synchrotron source, permitted investigation of the entire layer thickness range in a single experiment. Single-wedge samples were used as well as double-wedge samples tapered in orthogonal directions. The systematics of QW formation are well understood in terms of the elementary quantum mechanics of a particle in a box. We treat a single well, a double well and a corrugated well. The work on single wells focused on the elucidation of the long and short magnetic oscillatory periods for a Cu spacer layer, and their relation to the belly and neck regions of the Cu Fermi surface. The effects of interfacial roughness and interfacial mixing were investigated. The studies on double wells focused on the controllable degree of tunnelling between the wells and the avoided crossings that occur when the QW energies in one well are swept through those of the other. Finally, we consider the QW wavefunctions and their envelope modulation. The latter can be understood in terms of Bragg diffraction within a corrugated well. With use of a double-wedge-shaped sample it has proved possible to pass a thin probe across the well and to detect experimentally the envelope modulation.

Kinetic thermal degradation of amorphous carbon and carbon nitride films is studied. Significantly improved thermal stability was observed for films intensified with C-N, C=N, and C≡N bonds. When the N₂% (percentage of nitrogen) in Ar/N₂ during film deposition was varied from 0 to 30 at pressures of 3×10^-3 and 16×10^-3 Torr, the onset decomposition temperatures increased from 396 to 538 and from 340 to 360 °C, while the apparent activation energy ΔE at 60% residual weight increased from 149 to 158 and from 96 to 120 kJ mol^-1, respectively. A change in the thermal stability was observed when the N₂% reached 50. The films of higher bonding ratio and structural integration showed a single-step decomposition mechanism. They had ΔE values decreasing with the decomposition process, following convex-trend isothermal weight-loss curves. Films deposited using low-energy carbon plasma had higher contents of loosely bonded molecules; this resulted in a seemingly two-step decomposition and ΔE increasing with weight loss over a certain range. They followed concave-trend isothermal weight-loss curves. The overall decomposition mechanism could best be expressed in terms of nth-order reactions with the value of n closely related to film structures and reaction temperatures. Lower n-values are related to densely packed structure and higher temperature due to the increased diffusion barrier of the products.

We study theoretically ultrasound focusing in periodic multilayered structures, or superlattices, by solving the wave equation with the Green function method and calculating the transmitted ultrasound amplitude images of both the longitudinal and transverse modes. The constituent layers assumed are elastically isotropic but the periodically stacked structure is anisotropic. Thus anisotropy of ultrasound propagation is predicted even at low frequencies and it is enhanced significantly at higher frequencies due to the zone-folding effect of acoustic dispersion relations. An additional effect studied is the interference of ultrasound (known as the internal diffraction), which can be recognized when the propagation distance is comparable to the ultrasound wavelength. Numerical examples are developed for millimetre-scale Al/polymer multilayers used recently for imaging experiment with surface acoustic waves.

Using the full-potential linear-muffin-tin-orbital method, we have performed molecular-dynamics simulations for oxygen adsorption on Si_n (n = 1-7) clusters. It is found that Si₁, Si₂ and Si₃ can react with the O₂ molecule directly to form small SiO₂, Si₂O₂ and Si₃O₂ molecules. For the Si clusters with more than four Si atoms, the O₂ molecule cannot be adsorbed on them directly, due to the potential barrier for dissociative chemisorption of O₂. In contrast, atomic oxygen favours reactions with all the silicon clusters considered here. The formation of strong Si-O bonds makes the structures of the small Si clusters obviously distorted and their stabilities decreased. As for the fragmentation, the processes from Si_nO₂ to Si_n-2 + 2SiO are found to be energetically favourable. The theoretical investigations can help us to understand the existing experimental results.

The surfaces of wire-like silicon crystals grown by self-organization processes are characterized using electron microscopy of high resolution, scanning tunnelling microscopy, x-ray microprobe analysis, secondary-ion mass spectroscopy, and Auger electron spectroscopy. The results obtained have shown that the upper layer of each microcrystal is a nanoporous envelope. The composition of the envelope is in general similar to the composition of the bulk part of the crystal but with some differences: the density of the matter - due to the porous nature of the envelope - is much smaller, and the density of metal impurities is slightly increased on the external surface. More striking was the fact that the external surface of the envelope is passive under exposure to oxygen. In Auger spectra of crystals stored in open air for several years, only the LVV and KLL peaks that are specific to the atomically clean silicon surface were observed.

The polarization waves propagating in a slab-shaped or in a semi-infinite dielectric medium with spatial dispersion characterized by a volume free-energy density and by a boundary-surface energy density are studied, taking into account Maxwell's equations, in the framework of the Landau-Ginzburg formalism. It is shown that two independent extrapolation lengths providing for the required additional boundary conditions need to be specified at each surface limiting the medium. Complete calculations are performed in the electrostatic approximation: they provide evidence of the differences between the transverse in-plane polarized modes (s modes) and the sagittal plane polarized modes (p modes). True surface modes exist only in the case of negative extrapolation lengths. A detailed analysis of the symmetry properties of the surface and of the guided bulk modes in a slab is developed. Finally, our results are compared with those from previous models describing the boundary conditions in media where spatial dispersion is present.

A theoretical study of the piezoresistivity and magnetoresistivity in p-type heteroepitaxial diamond films is presented, based on the Fuchs-Sondheimer thin-film theory and the valence band split-off model. The Boltzmann transport equation in the relaxation time approximation was solved and a mixed scattering by lattice vibrations, ionized impurities and surfaces was considered. The analytical expressions for the piezoresistive and magnetoresistive effects has been developed in a parallel-connection resistance model for the light-hole, the heavy-hole and the split-off bands. The calculated results are discussed and compared with experimental data. It is found that the self-spin interaction of the heavy holes could be related to stress. Moreover, a possible mechanism is presented to explain the fact that the magnetoresistivity of the p-type heteroepitaxial diamond films is greater than that of the diamond bulk.

Fe-Tb multilayers and alloy films prepared by sputtering or evaporation are found to have perpendicular magnetic anisotropy. In this work, Fe-Tb alloy films with parallel magnetic anisotropy in a wide composition range, 3-30 at.% Tb, are obtained for the first time, prepared by the ion beam-assisted deposition technique. All the films have an easy magnetization axis in the film plane. A supersaturated bcc solid solution phase with enlarged lattice and increased saturation magnetization is obtained in the range of 6.5-13 at.% Tb; while in the range of 21-30 at.% Tb, the films are amorphous and have lower saturation magnetization. Thermodynamic calculation of the Gibbs free energy of the Fe-Tb system shows that both the bcc solid solution and the amorphous phase are energetically and dynamically favoured in the two composition regions, respectively.

In this work we have calculated the ferromagnetic (FM) resonance curves of Fe/Cr(100) thin films, which follow the Fibonacci sequence. Our approach is based on the equations of motion for the small-signal magnetization deviation from the equilibrium directions. The equilibrium positions of the magnetizations are calculated from the total energy, which includes the following contributions: Zeeman, bilinear exchange, biquadratic exchange, dipolar, cubic anisotropy and surface anisotropy terms. We also consider the presence of an external magnetic field applied in the plane of the films and parallel to the easy axes. The experimental parameters used in our calculations were recently reported and lie in three regions of interest, namely (i) near to the first antiferromagnetic (AF) peak (strong bilinear exchange coupling), (ii) near to the first FM-AF transition (moderate bilinear exchange coupling) and (iii) near to the second AF peak (weak bilinear exchange coupling comparable to the biquadratic exchange coupling). Our results show the effect of the quasi-periodic arrangement in the spin wave dispersion relation of these artificial structures.

Undoped and boron-doped a-Si_1-xC_x:H, for x≈0.5, films have been prepared by means of plasma-enhanced chemical-vapour deposition using methyltrichlorosilane. The optical absorption spectra of these films demonstrate three characteristic peaks at about 1.6, 2.0 and 2.5 eV consistent with other experimental measurements. To explain the observed peculiarities of the spectra, the atomic and electronic structures of a-SiC have been investigated using both molecular dynamics simulations based on an empirical potential and the recursion method. The results of the calculations show that five-coordinated (T₅) atoms (floating-bond atoms), anomalous four-coordinated (T_4a) sites (weak-bond atoms), three-coordinated (T₃) defects (dangling-bond atoms) and normal four-coordinated (T_4n) atoms which are nearest neighbours of T₃, T_4a or T₅ atoms give rise to three gap peaks. It was established that three peaks in the low-energy region of the optical absorption spectra are due to the electronic transitions: the valence band → the empty gap peak and two occupied gap peaks → the conduction band. Boron doping effects upon the optical spectra was not revealed.

We report surface photovoltage (SPV) spectra of ZnSe thin films deposited on n⁺GaAs substrates in the wavelength range of 400-800 nm. In the above bandgap region of ZnSe (below 450 nm), we find that the major contribution to SPV comes from trapping and re-emission from the slow states at the ZnSe surface and ZnSe/GaAs interface. The effect of interference of light on the SPV spectra, has been analysed for subbandgap wavelength excitation of ZnSe (470-800 nm). In spite of the presence of a large number of subbandgap states in ZnSe, the major contribution to SPV in this wavelength range comes from the substrate. The difference in the magnitudes of the SPV between the bare n⁺GaAs and the ZnSe/n⁺GaAs is due to the reduction of surface recombination velocity (SRV) of the minority carriers in n⁺GaAs.

Temperature dependencies of Ni, Cu and Mo metals EXAFS spectra were studied in order to determine the anharmonic pair potential. The potential parameters for metals with cubic structure - Ni, Cu, Mo as well as for Pb (Stern E A et al 1991 Phys. Rev. B 43 8850), Au, Ag (Newville M and Stern E A http://krazy.phys.washington.edu/paper/ag-au.html) - obtained earlier, were analysed to find correlations with other physical characteristics. It was found that a, b potential parameter values correlate with cohesive energy and interatomic distance for face-centred-cubic structure metals. Obtained potential parameter values were used to determine thermodynamics parameters, including the linear coefficient of thermal expansion, the Debye temperature, the bulk modulus and the Grunesien parameter.

The stable molecular unit of trimethylaluminium is the dimer linked via bridging methyl groups. Neutron spectroscopy is used to determine internal and external modes from µeV (rotational tunnelling transitions of methyl groups, stochastic rotational jumps) to meV (deformations, librations, phonons etc). The dimer appears to be non-rigid with a very soft bend mode about an axis connecting the two bridging methyl groups. Rotational potentials are derived from tunnelling modes and barrier heights deduced from quasielastic spectra. Internal modes obtained from ab initio calculations for the isolated dimer are consistent with measured vibrations with respect to eigenenergies and band intensities calculated by the CLIMAX program. The bridging CH₃ group shows the strongest and almost purely intramolecular potential due to a loss of a single bond axis of its pentacoordinated carbon atom.

The dynamics of the icosahedral ZnMgY phase has been investigated on a single-grain quasicrystalline sample using inelastic neutron scattering. Similarly to other quasicrystals we observe well defined acoustic modes for values of the wavevector smaller than 0.3 Å^-1. We find a longitudinal sound velocity equal to 4800 m s^-1 and an isotropic transverse sound velocity equal to 3100 m s^-1. The acoustic character and the broadening of transverse excitations has been studied extensively. We find that transverse excitations remain acoustic up to q = 0.5 Å^-1. For q lying between 0.3 and 0.5 Å^-1 we observe a broadening, which is independent of the propagation or polarization direction, while the dispersion relation departs from linearity. For q = 0.6 Å^-1 there is a resonance with an optic-like excitation located at about 10 meV and the signal can no longer be described by a single acoustic mode. For larger wavevectors we observe a broad distribution of modes, which can be split into three dispersionless optical bands located at 8, 12 and 17 meV. The 8 and 12 meV bands are associated with the crossing of pseudo-zone boundaries by the acoustic branch. A detailed comparison with results obtained in the i-AlPdMn phase shows significant differences: in particular the broadening of transverse acoustic modes is much less pronounced in the ZnMgY phase than in the i-AlPdMn one.

We demonstrate that a special hybridization between the Al(s, p) and V(d) orbitals which is responsible for forming of a deep pseudogap near the Fermi level in the Al₃V compound is also associated with the formation of covalent bonds. We analyse the charge distribution in the elementary cell and find an enhanced charge density along the Al-V bonds and certain Al-Al bonds which is characteristic for covalent bonding. The role of the point-group symmetry and the character of the hybrid orbitals forming the covalent bonds are investigated. It is demonstrated that the deep pseudogap close to the Fermi level arises from the bonding-antibonding of the hybrid orbitals.

The specific heat of SrTiO₃ shows an anomaly at 105 K due to a structural transition from the tetragonal to cubic phase. The shape of the anomalous specific heat, ΔC_p(T), measured previously using quasi-static conduction calorimetry is confirmed and compared with results from rapid AC calorimetry (scanning rate ~30 K h^-1). The close agreement of both data sets proves that slow relaxational processes (e.g. the rearrangement of dense domain patterns) do not influence ΔC_p(T) significantly in SrTiO₃. Measurements at low temperatures do not reveal any further anomaly. The freezing temperature reported around 30 K due to quantum saturation of the order parameter does not produce any anomalous effect in the heat capacity.

High-resolution transmission electron microscopy observations on the structure of the interphase boundary between crystalline Al and amorphous Al₂O₃ coating reveal that an interfacial melting transition of Al occurs at 833 K, which is distinctly lower than the bulk melting point of Al. The crystalline lattice planes of Al near the interface bend or small segments of crystalline Al deviated from the matrix Al grains are formed. Stand-off dislocations formed at the interphase boundary are also observed. The amorphous Al₂O₃ coating plays an important role in retaining the evidence for structural transition at high temperature to room temperature, which makes it possible to make experimental observations.

The Naval Research Laboratory tight-binding (TB) method accurately describes the electronic and mechanical properties of many elemental systems, including semiconductors, simple metals, and transition metals. We report an extension of the method to an intermetallic compound, Fe_xAl_1-x. We found a set of non-orthogonal TB parameters which reproduced the electronic structure and total energy versus volume of a set of first-principles linearized augmented-plane-wave (LAPW) calculations for the B1 (NaCl), B2 (CsCl), D0₃ (Fe₃Al), C11_b (MoSi₂), and B32 (NaTl) structures. Structural properties, such as lattice constants and bulk moduli, and electronic properties, such as the densities of states and energy bands, are reproduced well. In addition, quantities that were not fitted, such as elastic constants, are found to be in very good agreement with independent LAPW calculations.

The main features of the phase diagram of the Fe-Pd system are characterized by phase separation in the Fe-rich region and appearance of two ordered phases in the Pd-rich region. The total-energy FLAPW electronic structure calculations are attempted on a set of selected ordered compounds as well as Fe and Pd to clarify the origin of these features. It is revealed that (i) magnetism plays a dominant role in the phase stability of the system and (ii) the system has an intrinsically large driving force for ordering, while the elastic energy originating from size mismatch causes the phase separation in the Fe-rich portion.

The collective electronic excitation (plasmon) modes of a three-dimensional distribution of quantum wires such as those produced in mesoporous silica (MCM-41) have been calculated. The plasmon frequencies are found to be strongly dispersive for momentum components both parallel and perpendicular to the axes of the quantum wires. The lowest subband plasmon modes are acoustic for momentum components parallel to the wires and they are optical for momentum components perpendicular to the wires. A comparison with the plasmon modes of a single quantum wire or of a uniform three-dimensional electron gas (3DEG) indicates that for plasmon wavelengths small (wavevector q large) compared with inter-quantum-wire distance a, i.e. for aq>>1, the calculated modes would tend to those of a single quantum wire, and they tend to those of the 3DEG for large wavelengths (aq<<1). The contribution of the lattice formed by the quantum wires in mesoporous silica is dominant for intermediate wavelengths (0<<aq<<∞).

The inelastic neutron scattering technique was employed to study the transitions between the low-lying levels of the ground-state J-multiplet of the Ho³⁺ ions in Ho_1-yCa_yBa₂Cu₃O_x (x≈6.3, 7.0; y≈0.1, 0.2) and Ho_1-yTh_yBa₂Cu₃O_x (x≈6.3; y≈0.07, 0.13) split by the crystalline electric field. For oxygen-rich Ca-doped samples, the observed energy spectra exhibit a fine structure, which reflects the behaviour of the local hole density near the doping centre. For the first time this experimental observation unambiguously confirms that the charge inhomogeneity, which was found to be a characteristic feature of the underdoped cuprates, still exists in the deeply overdoped regime with T_c = 56 K. On the other hand, the modification of the crystal-field spectra observed for the oxygen-deficient samples with both Ca²⁺ and Th⁴⁺ substitutions for Ho³⁺ is found to result from distortions of the copper-oxygen polyhedron around the Ho³⁺ ion in the direct neighbourhood of the implanted ions, rather than from the doping-induced variation of the carrier concentration in the CuO₂ planes.

We report theoretical studies of electron and hole lifetimes in a free-electron gas as well as in real metals Al and Nb by means of the ab initio many-body method and semi-empirical scattering theory approach. We show that for free-electron-like excitations a simple density-of-state convolution model of the scattering theory approach (STA-DOS) is a good approximation to the ab initio lifetimes, the energy dependence of the STA-DOS transition matrix element being rather unimportant. The approximation of energy-independent matrix element appears to also be reasonable for Nb. A good correspondence between the STA-DOS and ab initio momentum-averaged results is achieved for Nb with an adjusted matrix element, although the ab initio lifetimes disperse greatly with respect to the averaged value.

In this work we study the magnetic moments and the hyperfine fields at rare-earth impurities diluted in Fe, Co and Ni ferromagnetic hosts. We use an Anderson-like model in which a degenerate localized 4f energy level of the rare-earth impurity is hybridized with a spin-polarized d-electron band. We use the Hartree-Fock approximation to deal with the electron-electron interactions and include the nearest-neighbour perturbation, due to the d translational invariance breaking introduced by the impurity. Our numerical results show that the systematics of the hyperfine fields changes sign in the middle of the rare-earth series and exhibits a strong orbital contribution for most of the rare-earth impurities, in good agreement with experimental data.

The magnetization distribution due to the Cr³⁺ ion in Cr₂O₃ has been determined using spherical neutron polarimetry. The magnetic structure factors of h 0 l reflections have been measured out to sin θ/λ = 0.75 Å^-1. The results show that the Cr³⁺ magnetic moment is reduced by the zero-point spin deviation and by covalent mixing to 2.48 µ_B. They are consistent with the Cr d electrons being in the trigonally symmetric a₁ and e orbitals derived from the cubic orbitals with t_2g symmetry. There is a small but significant magnetization which is not accounted for by these orbitals, which is attributed to covalent overlap. Its symmetry is consistent with the magneto-electric susceptibility.

We report an extended x-ray absorption fine-structure investigation on the Mn K absorption edge in La_1-xCa_xMnO₃ as a function of temperature and magnetic field. The results provide microscopic evidence that the modifications in the local structure around Mn atomic sites, as a function of temperature and applied magnetic field, are directly related to the magneto-transport properties of these materials.

In the present paper, a high energy milling process has been used to alloy Fe and B in the proportion 2:1. X-ray diffraction and Mössbauer spectroscopy were used to follow the solid state reaction among the alloy components and phase formation during thermal treatments of the final milled alloy, at distinct temperatures and environments. X-ray, DSC and Mössbauer data of the 310 h milled alloy show the presence of three phases: small fractions of α-Fe (n-Fe) and Fe₂B (n-Fe₂B) nanocrystalline phases and a large amount of amorphous Fe₂B (a-Fe₂B) phase (73%). Heat treatment of the milled alloy was done in a high vacuum furnace, followed by in situ Mössbauer measurements, or by annealing the sample sealed in a quartz tube. It is shown that segregation and crystallization effects thermally induced in the samples have been enhanced by the presence of oxygen in the residual atmosphere, being less effective in the case where the sample was continuously pumped during the annealing. These effects may have occurred due to boron oxidation. Also, the presence of a tet-Fe₃B phase in the annealed samples has been observed. The high temperature Mössbauer spectra for the sample annealed at 823 K indicate that the n-Fe₂B, n-Fe₃B and n-Fe phases exhibit superparamagnetic behaviour, with estimated blocking temperatures of 723 and 823 K, respectively. The amorphous a-Fe₂B phase has a T_C value estimated at about 823 K that fits into the system reported for amorphous Fe/B ribbons.

Fe_1-xSi_x (0.15<x<0.375) alloys have been prepared by a rapid-quenching technique. X-ray diffraction patterns show that these alloys consist of submicron grains between 100 and 300 nm. The DO₃ cubic symmetry could be obtained for Si content up to 34 at.% Si. This feature is particularly interesting since alloys containing more than 26 at.% Si and cast with usual techniques consist of a mixture of Fe₃Si and Fe₅Si₃. The magnetization, Curie temperature and resistivity data for these fine-grained alloys are consistent with those for bulk alloys for Si content below 25 at.% Si. In contrast, the structural and physical data for single-phase alloys with 28-34 silicon content are reported for the first time. Within this composition range, the magnetization and Curie point values roughly correspond to those suggested by the extrapolation of existing data. However, the resistivity shows an unexpected marked increase above 25 at.% Si. The values of resistivity, magnetization and effective anisotropy of soft-magnetic nanocrystalline Fe₂Si are found to be 200 µΩ cm, 0.6 T and 15 kJ m^-3 respectively, suggesting that this alloy has potential for high-frequency applications.

A two-dimensional Ising system composed of ferrimagnetically ordered chains coupled weakly and ferromagnetically to each other is discussed by the use of differential operator technique, in order to clarify the possibility of two compensation points in a molecular-based ferrimagnetic magnetic material. For this possibility, we find that each chain must consist of at least three magnetic atoms with spin ½ and integer spins as well as particular negative uniaxial anisotropy values. Two possible cases are proposed.

Electron tunnelling in a double-barrier junction with ferromagnetic external electrodes and a magnetic quantum dot as a spacer is analysed theoretically in the framework of the nonequilibrium Green function technique. The considerations are restricted to spin-conserving tunnelling processes through a quantum dot with a single spin-split discrete level. The Coulomb correlations on the dot are taken into account in terms of the Hubbard Hamiltonian. Transport characteristics, including tunnelling magnetoresistance due to rotation of the magnetic moments of external electrodes, are calculated selfconsistently.

Electron spin-resonance experiments were performed on Co²⁺ substituting for Zn²⁺ or Mg²⁺ in powder samples of Zn₂(OH)PO₄ and Mg₂(OH)AsO₄. These two compounds are iso-structural and contain the Co²⁺ in two environments with approximately octahedral and trigonal bipyramidal structures. The observed resonances are described using a theoretical model that considers the departures from the two perfect structures. It is shown that resonance in the penta-coordinated complex is allowed, and the crystal fields that would reproduce the resonance of the Co²⁺ in the two environments are calculated. The low intensity of the resonance in the penta-coordinated complex is explained by assuming that this site is much less populated than the octahedral one; this assumption was verified by a molecular calculation of the energies of the two environments, with both Co and Zn as central ions in Zn₂(OH)PO₄.

In order to investigate the effect of A-site substitution on phase transitions in barium titanates, dielectric permittivity in Ba_1-x(La_0.5Na_0.5)_xTiO₃ was measured in the temperature range up to 400 K with frequency of 100 Hz to 100 kHz by changing x from 1/8 to 4/8. The transition from classical non-relaxor ferroelectric (x = 1/8) to relaxor ferroelectric (x⩾3/8) occurs, through the intermediate between classical non-relaxor and relaxor ferroelectrics (x = 2/8). There is a strong correlation between the ionic mass at the A site M and the temperature of the permittivity maximum. It is found that M is one of the important parameters which dominates the dielectric permittivity and phase transitions in perovskite titanates. The diffusive displacement of Ti⁴⁺ due to the random fields created by inhomogeneity which A-site substitution introduces, besides the displacement suppressed by the shrinkage of lattice constant, reduces the permittivity maximum when x⩾3/8.

Using the Hartree-Fock and single configuration interaction model in conjunction with the local field method, the dielectric tensor, refractive index, circular dichroism, birefringence coefficient and the effects of spatial dispersion on the spectrum of the transverse normal polariton waves of the simple cubic phase of the C₆₀ crystal are calculated. Our results on magneto-optical circular dichroism in the energy range 3-4 eV almost reproduces the reported experimental results. Also, our results on the effect of spatial dispersion, compared with the non-dispersive case, indicate that the first branch of the transverse normal polariton in the C₆₀ crystal is due to the coupling of the second excitonic state of the C₆₀ molecule with the electromagnetic field and the first excitonic state has very weak coupling.

Spectroscopy measurements of optical constants of molecular-beam-epitaxy-grown PbSrSe thin films have been carried out for mid-infrared laser application. A new and simple empirical model for the refractive index and extinction coefficient of Pb_1-xSr_xSe thin films, in relation to the band-gap energy, is presented. The model of the optical constants is based on the Lorentz calculation of the imaginary part of the dielectric function. In contrast to other models, our model not only can explain well all of our experimental results with Sr compositions as high as 0.276 and at different temperatures from 10 to 300 K, but also is in good agreement with the reported results for PbSrSe thin films in the literature. Furthermore, we obtain, for the first time, the SrSe thermal expansion coefficient: (7.3±0.3)×10^-6 K^-1, based on the model and experimental data. The results clearly show promise of being applicable to PbSrSe thin films as well as their microstructures.

The room-temperature Raman spectra of Sr_1-xCa_xTiO₃ (SCT) for x = 0.06 (SCT06), 0.12 (SCT12) and 0.30 (SCT30) are shown to contain first-order Raman lines corresponding to E_g and B_1g modes, which appear below 105 K in pure SrTiO₃. The E_g line shows slight asymmetry, indicating the orthorhombic structure of SCT for x⩾0.06. The low-temperature Raman spectra of SCT06 and SCT12 contain Raman-active modes of symmetry B_2g and E_g, predicted by group theory but not observed in SrTiO₃ or SCT0.7 (x = 0.007) by previous workers. The polar hard modes TO₂ (175 cm^-1) and TO₄ (550 cm^-1) are present even at room temperature for SCT06 and SCT12 due to Ca²⁺-centred ferroelectric microregions. The intensity of these modes starts increasing below 55 and 80 K for SCT06 and SCT12 respectively, which are nearly 2 T_c, where T_c is the temperature of the smeared dielectric peak. The presence of such an extended precursor region is akin to relaxor behaviour. For SCT30, two new lines at 79 and 128 cm^-1 appear below the paraelectric to antiferroelectric phase transition temperature of 230 K. It is shown that Raman lines characteristic of both ferroelectric and antiferroelectric phases are present in SCT12 below 100 K, confirming the frustration model for the highly smeared dielectric response for this composition.

The fundamental optical absorption of Cs₄PbBr₆ crystals, which are built up of nearly regular Pb²⁺(Br^-)₆ octahedra mutually bound by Cs⁺ ions, exhibits novel features: despite the crystalline entity of Cs₄PbBr₆, it shows oscillator-like absorption peaks and a wide window just above the first peak. The Cs⁺ ions prevent the Pb²⁺ 6s and 6p states from taking part in the construction of extended states. These states form a set of localized states confined to within respective octahedra, similar to the case of isolated Pb²⁺ ions doped in face-centred cubic alkali halide crystals.

Nanostructured Al₂O₃ powders were prepared by high-energy ball milling of corundum. Both the solid state nuclear magnetic resonance spectra of the Al³⁺ ions and the solid state electron paramagnetic resonance spectra of incorporated Fe³⁺ ions are governed by noticeable spectral changes dependent on the duration of the mechanical treatment. The quadrupolar parameters of the ²⁷Al nuclei and the zero-field splitting parameters of the Fe³⁺ ions as well as their statistical distributions were determined as functions of the milling time. Structural changes of the Al₂O₃ matrix were also followed by powder x-ray diffraction and transmission electron microscopy measurements. Direct relations between the structural disorder as obtained by x-ray data and the spin Hamiltonian parameters of both ions could be established.

These results suggest that the milled powders consist of nanocrystalline grains embedded in amorphous grain boundaries even for the longest milling time. The grains can be described in terms of ordered AlO₆-octahedra as in the starting crystalline material exhibiting a slight rhombic distortion. The grain boundaries look like random arrangements of these octahedral units. The specific behaviour of the environment of the Fe³⁺ paramagnetic probe points out that such a point defect acts as an activation centre of the amorphization process.

Inadvertently, the results calculated using the VASP-PAW method and presented in figures 3 and 4 were obtained for the theoretical equilibrium lattice constant. The correct figures with the values at the experimental lattice constant are displayed below. The conclusions of the paper are not modified by these changes.

Figure 3. The variation of the energy difference between the SDW and the commensurate AFM1 state of Cr as a function of the wavevector. Full dots: LMTO-ASA results; empty circles: VASP-PAW results; squares: FLAPW results of reference [1]. All calculations use the GGA.

Figure 4. The variation of the amplitude m₀ of the SDW as a function of the wavevector q as calculated in the GGA. Full dots: LMTO-ASA results; empty circles: VASP-PAW results; squares: FLAPW results (reference [1]).

We are grateful to S Cottenier, B De Vries, J Meersschaut and M Rots for bringing this mistake to our attention.

References

[1] Bihlmayer G, Asada T and Blügel S 2000 Phys. Rev. B 62 11937