Susan H. Kilcoyne

The magnetic structure of DyMn2

The evolution with temperature of the magnetic structure of the C15 Laves-phase compound DyMn2 has been studied using powder neutron diffraction and magnetization techniques. The Dy sublattice assumes a spin-canted ferromagnetic structure with 8.8 mu B per Dy atom. Although all Mn sites within the unit cell are chemically equivalent, only one Mn atom in four is found to possess a magnetic moment (of 1.4 mu B). These magnetic Mn atoms are located at sites with a strongly polarizing magnetic environment resulting from a near-neighbour configuration of ferromagnetically coupled Dy spins. A spin reorientation is observed at 36 K, and is accompanied by a small thermal expansion anomaly. The Curie temperature of DyMn2 is found to be 45 K.

The magnetic structure of HoMn2

The crystal and magnetic structure of the C15 Laves phase of HoMn2 was restudied using powder neutron diffraction. High-resolution spectra showed HoMn2 to remain cubic Fd3m below the magnetic transition temperature. The Ho spins assume a spin-canted ferromagnetic structure with 7.9 mu B per Ho atom. One out of four Mn sites carries a moment of 0.6 mu B induced by the strongly polarizing magnetic environment of ferromagnetically coupled near-neighbour (111) planes of rare-earth spins. A small thermal expansion anomaly accompanied by a spin reorientation is found at the Curie point of 25 K; the Neel point of the system lies at 31 K.

Distribution of enamel crystallite orientation through an entire tooth crown studied using synchrotron X‐ray diffraction

The biomineralization of human dental enamel has resulted in a highly anisotropic and heterogeneous distribution of hydroxyapatite crystallites, which in combination with its high mineral content has resulted in one of the most durable and hardest tissues in the human body. In this study, we used position-sensitive synchrotron X-ray diffraction to quantify the spatial variation in the direction and magnitude of the preferred orientation of enamel crystallites across a whole tooth crown. Two-dimensional synchrotron X-ray diffraction images were collected with 300 μm spatial resolution over a series of six sequential tooth sections obtained from a single maxillary first premolar and were analyzed using Rietveld refinement. Both the magnitude and the direction of the crystallite orientation were found to have a high spatial heterogeneity. Areas of high crystallite alignment were directed perpendicular to the biting surfaces, which is thought to meet the functional requirements of mastication. The results may assist in our understanding of the structure-function relationship and of the evolutionary development of enamel.

Crystallographic and magnetic identification of secondary phase in orientated Bi5Fe0.5Co0.5Ti3O15 ceramics

Oxide materials which exhibit both ferroelectricity and ferromagnetism are of great interest for sensors and memory applications. Layered bismuth titanates with an Aurivillius structure, (BiFeO<inf>3</inf>)nBi<inf>4</inf>Ti<inf>3</inf>O<inf>12</inf>, can possess ferroelectric and ferromagnetic order parameters simultaneously. It has recently been demonstrated that one such example, Bi<inf>5</inf>Fe<inf>0.5</inf>Co<inf>0.5</inf>Ti<inf>3</inf>O<inf>15</inf>, where n = 1 with half the Fe³⁺ sites substituted by Co³⁺ ions, exhibits both ferroelectric and ferromagnetic properties at room temperature. Here we report the fabrication of highly-oriented polycrystalline ceramics of this material, prepared via molten salt synthesis and uniaxial pressing of high aspect ratio platelets. Electron backscatter images showed that there is a secondary phase within the ceramic matrix which is rich in cobalt and iron, hence this secondary phase could contribute in the main phase ferromagnetic property. The concentration of the secondary phase obtained from secondary electron microscopy is estimated at less than 2.5 %, below the detection limit of XRD. TEM was used to identify the crystallographic structure of the secondary phase, which was shown to be cobalt ferrite, CoFe<inf>2</inf>O<inf>4</inf>. It is inferred from the data that the resultant ferromagnetic response identified using VSM measurements was due to the presence of the minor secondary phase. The Remanent magnetization at room temperature was M<inf>r</inf> ≈ 76 memu/g which dropped down to almost zero (M<inf>r</inf> ≈ 0.8 memu/g) at 460 °C, far lower than the anticipated for CoFe<inf>2</inf>O<inf>4</inf>.

Structural chemistry and magnetic properties of 6H and 15R hexagonal perovskites BaIrxFe1−xO3−δ

The crystal structures and magnetic properties of three compounds within the system BaIrxFe1−xO3−δ have been investigated using a combination of X-ray and neutron powder diffraction, high resolution transmission electron microscopy and d.c. SQUID magnetometry. BaIr0.2Fe0.8O2.932(5) and BaIr0.6Fe0.4O3 adopt the 6H structure (space group P63/mmc, a = 5.72 A, c = 14.15 A), whilst BaIr0.3Fe0.7O2.949(7) exists in the rare 15R modification (space group Rm, a ≈ 5.73 A, c ≈ 35.55 A). Both structures comprise dimers of face-sharing octahedra as well as octahedra which share only vertices with their neighbours. For 6H BaIr0.2Fe0.8O2.932(5) and 15R BaIr0.3Fe0.7O2.949(7) neutron diffraction experiments show an antiferromagnetically ordered state at 2 K. TN has been identified through variable temperature neutron diffraction to be 200(4) K for 15R BaIr0.3Fe0.7O2.949(7). In both of these samples a proportion of the spins remain decoupled from the magnetic backbone and there is evidence for a spin-glass freezing transition at low temperatures. Susceptibility data for 6H BaIr0.6Fe0.4O3 indicate a single transition to a spin-glass state.

Neutron diffraction studies of magnetostrictive Fe–Ga alloy ribbons

Melt-spun Fe–Ga ribbons were prepared and some ribbons were annealed at 1000 °C for 1 h then slowly cooled to room temperature. X-ray diffraction patterns revealed no evidence of texture and only bcc phase in the as-quenched ribbons. However, high-resolution neutron diffraction patterns gave more information on the structure of these ribbons. Only diffractions from the disordered bcc A2 phase were found in as-quenched ribbons with 15, 17.5, and 19.5 at. % Ga content, without any trace of satellite peaks or splitting peaks from the proposed Ga–Ga pairing superlattice structure. The broadening of the base of the �110� peaks for all samples except the as-quenched 15 at. % Ga ribbon might indicate the existence of some kind of short range ordering. Ribbons developed L12 phase after annealing especially in the Fe 19.5 at. % Ga ribbon where the formation of L12 phase reduced the Ga content in the remaining A2 phase and decreased its lattice parameter dramatically. D03 phase formed in the as-quenched 22.5 at. % Ga ribbon and the following annealing treatment transformed more A2 phase into D03 phase.

Anharmonic Behavior in the Multisubunit Protein Apoferritin as Revealed by Quasi-Elastic Neutron Scattering

Quasi-elastic neutron scattering (QENS) has been used to study the deviation from Debye-law harmonic behavior in lyophilized and hydrated apoferritin, a naturally occurring, multisubunit protein. Whereas analysis of the measured mean squared displacement (msd) parameter reveals a hydration-dependent inflection above 240 K, characteristic of diffusive motion, a hydration-independent inflection is observed at 100 K. The mechanism responsible for this low-temperature anharmonic response is further investigated, via analysis of the elastic incoherent neutron scattering intensity, by applying models developed to describe side-group motion in glassy polymers. Our results suggest that the deviation from harmonic behavior is due to the onset of methyl group rotations which exhibit a broad distribution of activated processes ( E a,ave = 12.2 kJ.mol (-1), sigma = 5.0 kJ x mol (-1)). Our results are likened to those reported for other proteins.

Electron transfer in dextran probed by longitudinal field muon spin relaxation

Electron-transfer processes play a crucial role in bio-nanobattery design, the electron transfer rate through the organic material being a key parameter in determining the resistance, maximum current, power density, discharge rate and duty cycle of the cell. The labelled electron method using positive muons allows such transfer processes in macromolecules, such as polymers and proteins, to be probed on a microscopic level. Here we present the results of an experiment using the labelled electron method with longitudinal field muon spin relaxation (LF-μSR) to investigate electron-transfer processes in dextran. The data are well described using the Risch–Kehr model and the results suggest intra-chain diffusion is the dominant transport process in this systembetween 15 and 250 K. Intra-chain diffusion rates of 1013 s−1 have been determined.

Kinetic neutron diffraction and SANS studies of phase formation in bioactive machinable glass ceramics.

Bioactive fluormica-fluorapatite glass-ceramic materials offer a very encouraging solution to the problem of efficient restoration and reconstruction of hard tissues. To produce material with the desired crystalline phases, a five-stage heat treatment must be performed. This thermal processing has a large impact on the microstructure and ultimately the final mechanical properties of the materials. We have examined the thermal processing of one of our most promising machinable biomaterials, using time-resolved small angle neutron scattering and neutron diffraction to study the nucleation and growth of crystallites. The processing route had already been optimized by studying the properties of quenched samples using x-ray diffraction, mechanical measurements and differential thermal analysis. However these results show that the heat treatment can be further optimized in terms of crystal nucleation, and we show that these techniques are the only methods by which a truly optimized thermal processing route may be obtained.

Three Dimensional Mapping of Texture in Dental Enamel

We have used synchrotron x-ray diffraction to study the crystal orientation in human dental enamel as a function of position within intact tooth sections. Keeping tooth sections intact has allowed us to construct 2D and 3D spatial distribution maps of the magnitude and orientation of texture in dental enamel. We have found that the enamel crystallites are most highly aligned at the expected occlusal points for a maxillary first premolar, and that the texture direction varies spatially in a three dimensional curling arrangement. Our results provide a model for texture in enamel which can aid researchers in developing dental composite materials for fillings and crowns with optimal characteristics for longevity, and will guide clinicians to the best method for drilling into enamel, in order to minimize weakening of remaining tooth structure, during dental restoration procedures.