Table of contents

The effect of a demagnetizing field on the heat capacity of a ferromagnet is discussed. Mean field calculations of the contributions of demagnetization energies to the heat capacity of a uniaxial ferromagnet are presented and illustrated by application to heavy rare earth metals.

A variety of molecules reversibly self-assemble in solution, forming non-covalently bonded molecular aggregates. In many cases these aggregates are asymmetric in shape and are observed to form liquid-crystal phases. The nature of and mechanisms for liquid-crystalline ordering in such self-assembled systems is the focus of this review.

The molecular dynamics method was used to examine the B₂O₃ and borate glasses. The discontinuity in each of the thermodynamic factors such as the transition temperature, specific heat, thermal expansion and thermal compressibility is detected and checked by the so-called Prigogine-Defay ratio and it is found that this ratio exceeds unity for the systems studied. This behaviour indicates that multiple order parameters are required to characterize the glassy state. Also it is found that the formation of diborate groups in borate glasses reduces the value of the Prigogine-Defay ratio Pi , while the formation of groups with non-bridging oxygen ions increases the value of Pi , i.e. increases the number of order parameters.

A classical application of the Car-Parrinello method in the computer simulation of ionic systems is demonstrated. The induction effects in the interionic interactions are included in addition to the short-range repulsion and dispersion effects described by effective pair potentials. By representing the induced dipoles as a pair of dynamically variable charges fixed on the ends of a rod and extending the Lagrangian accordingly, the self-consistent induced dipoles at each time step are generated from the values at the previous time step, without the need for explicit minimization. Coulomb-field-induced and overlap-induced polarization effects are included and these are parameterized by ab initio electronic structure calculations. For simple ionic systems it is shown that the neglect of the overlap-induced dipoles leads to a poor representation of real systems.

Inelastic neutron scattering measurements on single-crystal LiTaO₃ in the temperature range from room temperature to 800 degrees C show evidence of overdamped scattering, both below and above the ferroelectric-to-paraelectric phase transition. The results are qualitatively discussed in comparison with the recent Raman studies. The measurements failed to observe any well defined underdamped soft mode in the experimental setting that favours detection of the ferroelectric mode (the lowest-frequency A₁(TO) mode), thereby not supporting a conventional displacive picture of phase transition in the crystal. The authors also measured some low-frequency phonon dispersions along two symmetry directions at room temperature.

The precipitation behaviour of a Cu-14.1 wt.% Al-4.2 wt.% Ni shape memory alloy and its effect on the phase constituents, fine structure and thermoelastic martensitic transformation have been studied using X-ray diffraction analysis, transmission electron microscopy and acoustic emission techniques. In the 700 K step-quenched specimens, a gamma ₂-phase with a complex cubic structure precipitates coherently from the parent phase. The gamma ₂ precipitate is rich in alloying elements which causes the retained parent phase to be poor in the alloying elements and hence increases the martensite transition temperature of the step-quenched specimens. The coherent strain field may influence the structure types and the fine structures of the transformed martensite. Finally, the cooling rate of the retained parent phase during quenching influences the domain size of the parent phase and the martensites transformed from the parent phase.

Mechanical milling was performed on the alloy Cr₇₂Fe₂₈. X-ray diffraction and Mossbauer spectroscopy were used to follow the progress of the structural variation. The experimental results can be interpreted by assuming that the milled powders are composed of a small amount of crystalline component and two disordered components: one with a random densely packed structure and the other with a random loosely packed structure. Moreover, differential scanning calorimetry was used to obtain the crystallization temperature. A mechanism was proposed to describe the formation of the disordered structures as well as the structural stability in this alloy.

The authors describe results of Raman and infrared studies of single crystals of pure C₆₀, CS₂ solvated C₆₀ (C₆₀.1.5CS₂) and of (BEDT-TTF)₂C₆₀ (BEDT-TTF is bisethylenedithio tetrathiafulvalene of ET). They have observed phonon shifts in the crystal surfaces using Raman scattering which they assign to diffusion of molecular oxygen into the crystals; the results indicate that the presence of molecular oxygen hinders rotation of C₆₀ molecules in the crystals. The four T_1u vibrational modes in the infrared spectrum of pure C₆₀ show fine structure which they attribute to Fermi resonances. They show that C₆₀ vibrations in C₆₀.1.5CS₂ crystals are less anharmonic than in pure C₆₀ crystals.

The six elastic constants of BaFCl which is a crystal of the matlockite family have been measured using ultrasonic and Brillouin scattering techniques. The values obtained are compared with those of BaF₂ crystals which exhibit some structural similarities with BaFCl. Because of the layer structure of BaFCl, the 'transverse' elastic constants C₆₆, C₄₄, C₁₂ and C₁₃ and the 'longitudinal' elastic constants C₃₃ and C₁₁ are smaller than the corresponding constants of BaF₂. The linear and volume compressibilities are deduced from these measurements. An elastic Debye temperature is calculated ( Theta _el=246.5 K) and compared with other experimental (calorimetric) and theoretical (shell model) determinations. Finally, the effect of the dipole moments of the halide ions on the elastic constants for materials of the matlockite family is discussed.

The pressure P derivative dT_C/dP of the Curie temperature T_C, the linear magnetostriction Delta l/l at 4.2 K, the lattice parameters a and c as functions of temperature T and the high-temperature paramagnetic susceptibility chi of CrTe have been measured. The value of dT_C/dP is found to be -7.0+or-0.2 K kbar^-1. Thermal expansion anomalies of both a and c below T_C=350+or-2 K are observed upon cooling. The inverse of chi above T_C shows a break at about 600 K. The field derivative d( Delta l/l)/dH in the interval 40 kOe<H<78 kOe at 4.2 K is found to be 1.2*10^-9 Oe^-1. The results obtained are discussed on the basis of an appropriate thermodynamic relation, together with the reported results on dT_C/dP and the thermal expansions of Cr_{1- delta} Te.

The authors study the gap-labelling properties of the energy spectrum for the on-site model of one-dimensional Fibonacci quasi-periodic lattices; they have obtained the occupation probabilities on subbands of the hierarchical energy spectrum and the step heights of the integrated density of states. It is analytically proved that the step height is equal to (m tau ), where the braces denote the fractional part, and m is an integer which can be used to label the corresponding energy gap. Numerical simulations confirm these results.

A comprehensive study of the magnetic properties of non-interacting single-domain particles with cubic magnetocrystalline anisotropy is presented. These numerical calculations extend the zero-temperature predictions of Joffe and Heuberger and enable the effects of thermally activated magnetization reversal on the hysteresis loop at finite temperatures to be determined. Variations in particle size distribution are also considered. Calculations indicate a reduction in the intrinsic coercivity as the mean particle diameter is reduced and an increase in coercivity for small particle diameter systems as the standard deviation of the size distribution is increased.

For pt.I see ibid., vol.5, p.2779-92 (1993). A comprehensive study of the remanence curve properties of an assembly of noninteracting single-domain particles with cubic magnetocrystalline anisotropy has been performed. The theoretical calculations presented represent an extension of the zero-temperature analysis of Joffe and Heuberger to include the effects of thermally activated magnetization reversal and particle size distribution. The numerical validity of these computations is completed through the analysis of interaction-sensitive Henkel plots, which confirm the non-interacting nature of the independently calculated remanence curves. The model system shows behaviour with intuition and analogous to similar models for the uniaxial case, with the exception of the expected increase in maximum remanence and decrease in energy barrier, i.e. remanence coercivity.

NMR magnetic dipolar spectral density functions are obtained for some lattice diffusion models for two-dimensional lattice diffusion and compared with the results for the BPP and continuum models. The systems considered are dipolar interactions between spins diffusing in a plane, and interactions between diffusing spins in a plane with fixed spins in a separate parallel plane. Numerical results and analytic approximations are obtained for spins diffusing on square lattices. The BPP model is unsatisfactory in both magnitude and functional form in two-dimensional systems. The continuum and lattice diffusion models agree for interactions between well separated planes, but there are significant differences between the continuum and lattice models otherwise. Results for the longitudinal spin relaxation rates in the laboratory and rotating frames are obtained for square lattices, and show a strong dependence on the direction of the applied magnetic field relative to the crystal axes.

The authors have studied a biased double-barrier resonant tunnelling structure using time-resolved and continuous-wave photoluminescence (PL) spectroscopy and identified the principal mechanisms which contribute to the evolution of luminescence from the quantum well (QW) region. They find that in the structures investigated the PL intensity is dominated by the diffusion and tunnelling of minority holes, in contrast with several earlier studies in which the electron behaviour was suggested to control the PL characteristics. The processes dominating the variation of PL intensity with bias are: field-driven accumulation of holes in a layer adjacent to one barrier, tunnelling of holes from this layer into the QW and the escape of holes from the QW by tunnelling at low bias, and at high bias by direct escape over the emitter barrier. Additionally, the absence of luminescence corresponding to recombination from the upper electron level when the device is biased at the second tunnelling resonance implies fast inter-subband scattering for charge carriers in the QW.