G. Kalonji

The cooling rate dependence of cation distributions in CoFe2O4

The room‐temperature cation distributions in bulk CoFe2O4 samples, cooled at rates between <10−2 and ∼103 °C s−1, have been determined using Mossbauer spectroscopy in an 80‐kOe magnetic field. With increasing cooling rate, the quenched structure departs increasingly from the mostly ordered cation distribution ordinarily observed at room temperature. However, the cation disorder appears to saturate just short of a random distribution at very high cooling rates. These results are interpreted in terms of a simple relaxation model of cation redistribution kinetics. The disordered cation distributions should lead to increased magnetization and decreased coercivity in CoFe2O4.

A grain boundary phase transition studied by molecular dynamics

We have conducted a thorough investigation of the high temperature behaviour of crystalline interfaces in 2-dimensional close-packed and 3-dimensional f.c.c. Σ = 7 bicrystals using the atomistic computer simulation technique of molecular dynamics. Gibbs free energies of the bicrystals were computed using the determinant technique and a computer calorimetry technique which we describe. The thermodynamic properties of the boundary were monitored over a wide range of temperatures, up to the bulk melting point. We observed a first order phase transition of the grain boundary in the 2D and 3D bicrystals at temperatures well below the bulk melting temperature. At the transition temperature the crystalline grain boundaries are replaced by highly disordered liquid-like layers.

A molecular dynamics study of grain boundary phase equilibria: The case of the Σ = 13 boundary

Abstract Computer molecular dynamics simulations of a two-dimensional Σ = 13 bicrystal, interacting through a Lennard-Jones potential, have been performed. Grain boundary phase equilibria were studied through the temperature dependence of the bicrystal enthalpy. Two phenomena were studied: a transition to a liquid-like layer at approximately 80% of the bulk melting point, and a reorientation of the grain boundary from the Σ = 13 misorientation of 27.80 ° to a misorientation of approximately 44 °.

Spinel ferrite-silica glass obtained by splat quenching☆

Abstract The compositions (in mol%) 40 MnFe 2 O 4 , 60 SiO 2 , and 42.8 CoFe 2 O 4 , 57.1 SiO 2 have been melted and splat-quenched. The resulting materials have been analyzed using X-ray diffraction, transmission electron microscopy, scanning transmission electron microscopy, and room temperature Mossbauer spectroscopy. The quenched Mn-containing material was completely amorphous. Its Mossbauer spectrum contains two doublets, indicating Fe 2+ and Fe 3+ in distorted octahedral sites. The quenched Co-containing composition contained (Co, Fe) 2 SiO 4 (olivine and (Co, Fe) 2 O 4 (spinel) precipitates, 150–400 A in diameter, in a glassy matrix. The Mossbauer spectrum contains three doublets, indicating octahedral Fe 2+ in the olivine, distorted octahedral Fe 2+ in the glass, and distorted octahedral Fe 3+ in the glass. The spectrum also shows trace hyperfine splitting, attributed to the spinel ferrite.

Calculation of free energies of Lennard-Jones crystals via molecular dynamics

The effectiveness of the determinant method and of energy distribution methods for the calculation of free energies with molecular dynamics sampling in the isothermal–isobaric ensemble has been demonstrated. The accuracy of the methods employed has been characterized. The quasiharmonic and anharmonic free energies of two‐dimensional and three‐dimensional crystals have been estimated over a wide range of temperatures.

Structure and dynamic properties of twist boundaries in NaCl through a molecular dynamics study

Abstract NaCl Σ = 5 [001] twist boundaries, in which the ions interact through the rigid-ion model of Tosi and Fumi, have been studied by a constant-pressure molecular dynamics simulation. The Tasker-Duffy structure, in which Schottky defects are introduced on the boundary plane, was found to be stable up to the bulk melting temperature T m. Excess grain-boundary thermodynamic properties, including the excess entropy and excess free energy, were calculated from low temperatures up to the bulk melting temperatures. The boundary stress changes significantly and even changes sign as the system is heated. An analysis of the temperature dependence of the local structural and thermodynamic properties near the boundary revealed that the thickness of the region affected by the boundary is very narrow, for the pure stoichiometric boundaries studied. Although significant disorder due to vacancy migration occurs above about 1013 K (0-95T m), no grain-boundary melting transition was observed well below T m. The value...

Magnetic susceptibility of rapidly solidified YBa2Cu3O7−x superconductors

The superconducting YBa2Cu3O7−x phase has been produced through a novel rapid solidification processing route yielding a high‐quality granular superconducting phase. Features in the magnetic susceptibility as measured at various fields can be attributed to important transitions in the superconducting behavior of these materials. At low fields (∼20 Oe) anomalies in the susceptibility arise from decoupling of the grains in these polycrystalline materials. Susceptibility response at higher fields (∼100 Oe) is representative of randomly oriented superconducting grains which show local superconducting current densities comparable to those of single crystals. At even higher fields features consistent with Muller’s superconducting glass state model are observed.

Melting and solidification behavior of YBa2Cu3O7−x

We have prepared the superconducting compound YBa2Cu3O7−x through a melt process. Solidification of this material at cooling rates <105 K/s yields BaCuO2 and a previously unreported metastable phase which indexes to a cubic pattern with a lattice constant of 0.701 nm. Faster solidification rates yield only the cubic phase and glassy materials. Annealing these materials in O2 gives rise to a single‐phase material of the YBa2Cu3O7−x orthorhombic structure. The material has zero resistance at 91.6 K with a transition width of 1.4 K. The grain size of the materials after annealing averages 10 μm in length and 2–3 μm in width, and the grains are compositionally homogeneous. Twins are seen with spacings on the order of 20 nm. The irreversibility of the magnetization curve at low temperatures can be ascribed to strong flux pinning mechanisms.

The stacking-fault tetrahedron

Abstract From symmetry arguments it is concluded that what is called a stacking-fault tetrahedron is really a self-inclusion in which a portion of the same crystal is shifted by 1/4 〈111〉 relative to the enclosing matrix. The included crystal has fewer atoms than would have been required to fill the hole in the matrix crystal with perfect material, and this is a way of accommodating clusters of vacancies. Symmetry shows that, for this shift, the energy is at an extremum and that the form of the inclusion must conform to the point group 43m (tetrahedral). This is a simpler model than the usual description, and even the unrelaxed version has lower energy for small sizes. For intermediate sizes it would relax to the same description as the relaxed version of the defect as conventionally described, and at large sizes the stacking-fault tetrahedra are unstable with respect to the formation of Frank loops.

Local moments, diamagnetism, and time‐dependent magnetic response of high‐temperature superconductors

The magnetic properties of high‐Tc superconductors RBa2 Cu3O7−δ with R=Y, Gd were investigated in the temperature range 4.2–300 K. To first order, no interaction was found between superconductivity and paramagnetism in the zero‐field‐cooled state of the Gd‐doped samples: after correction for the Curie–Weiss magnetization, the diamagnetism of these samples is very close to that of the Y‐doped samples. This suggests that the superconducting screening of an applied field has little effect on the field seen by the R species. The diamagnetic screening is found to be strongly time dependent due to flux penetration, whereas the magnetic ions see the applied field without delay.