J. P. Goff

Patterning of sodium ions and the control of electrons in sodium cobaltate

Sodium cobaltate (NaxCoO2) has emerged as a material of exceptional scientific interest due to the potential for thermoelectric applications, and because the strong interplay between the magnetic and superconducting properties has led to close comparisons with the physics of the superconducting copper oxides. The density x of the sodium in the intercalation layers can be altered electrochemically, directly changing the number of conduction electrons on the triangular Co layers. Recent electron diffraction measurements reveal a kaleidoscope of Na+ ion patterns as a function of concentration. Here we use single-crystal neutron diffraction supported by numerical simulations to determine the long-range three-dimensional superstructures of these ions. We show that the sodium ordering and its associated distortion field are governed by pure electrostatics, and that the organizational principle is the stabilization of charge droplets that order long range at some simple fractional fillings. Our results provide a good starting point to understand the electronic properties in terms of a Hubbard hamiltonian that takes into account the electrostatic potential from the Na superstructures. The resulting depth of potential wells in the Co layer is greater than the single-particle hopping kinetic energy and as a consequence, holes preferentially occupy the lowest potential regions. Thus we conclude that the Na+ ion patterning has a decisive role in the transport and magnetic properties.

Periclase surface hydroxylation during dissolution

Abstract Periclase (001) surfaces were etched in dilute acid at pH 2 and 4. X-ray reflectivity measurements on a reference crystal constrained the initial roughness of these surfaces to be approximately 30 A. The reference crystal and the crystal reacted at pH 2 were analyzed by Elastic Recoil Detection Analysis (ERDA) for proton penetration. After reaction the etched sample showed proton penetration to a depth of at least 5000 A while the reference crystal showed no significant proton inventory. Down to 900 A, the H Mg ratio in the etched sample was approximately 2, consistent with near-surface protonation of the MgO to form hydroxylated brucite-like layers. Protonation is a far more likely mechanism to explain the proton profile than precipitation because this reaction was completed over 16 orders of magnitude below saturation with brucite. Formation of a hydroxylated near-surface layer on periclase during dissolution explains why the dissolution rates of periclase and brucite are identical in the pH range 2–5; the detachment rates are the same because the surface structures are the same. This suggests that even for this ionic solid in acid, the dissolution reaction involves a two-step mechanism with a rapid single protonation step of near-surface oxygen atoms and a slower, rate determining second protonation step. In general, product phases such as brucite are likely to be better developed under natural weathering conditions of near-neutral pH because the second step of protonation (and thus full hydration of the detaching cation, e.g., Mg+2) is much slower than in acid. Our proposed protonation mechanism relates field observations of the periclase weathering reaction to laboratory dissolution, hydration, and dehydration experiments.

Suppression of thermal conductivity by rattling modes in thermoelectric sodium cobaltate

The need for both high electrical conductivity and low thermal conductivity creates a design conflict for thermoelectric systems, leading to the consideration of materials with complicated crystal structures. Rattling of ions in cages results in low thermal conductivity, but understanding the mechanism through studies of the phonon dispersion using momentum-resolved spectroscopy is made difficult by the complexity of the unit cells. We have performed inelastic X-ray and neutron scattering experiments that are in remarkable agreement with our first-principles density-functional calculations of the phonon dispersion for thermoelectric Na(0.8)CoO2, which has a large-period superstructure. We have directly observed an Einstein-like rattling mode at low energy, involving large anharmonic displacements of the sodium ions inside multi-vacancy clusters. These rattling modes suppress the thermal conductivity by a factor of six compared with vacancy-free NaCoO2. Our results will guide the design of the next generation of materials for applications in solid-state refrigerators and power recovery.

Hydrogen concentration and mass density of diamondlike carbon films obtained by x‐ray and neutron reflectivity

Specular reflectivity of neutrons and x rays can be used to determine the scattering length density profile of a material perpendicular to its surface. We have applied these techniques to study amorphous, diamondlike, hydrocarbon films. By the combination of these two techniques we obtain not only the mass density, but also the concentration of hydrogen, which varies in our case between 0 and 30 at. %. This method is a new and nondestructive way to determine the concentration of hydrogen within an error of less than 2 at. % in samples with sharp interfaces. It is especially suited for diamondlike carbon films.

The antiferromagnetic structures of IrMn3 and their influence on exchange-bias.

We have determined the magnetic structures of single-crystal thin-films of IrMn3 for the crystallographic phases of chemically-ordered L12, and for chemically-disordered face-centred-cubic, which is the phase typically chosen for information-storage devices. For the chemically-ordered L12 thin-film, we find the same triangular magnetic structure as reported for the bulk material. We determine the magnetic structure of the chemically-disordered face-centred-cubic alloy for the first time, which differs from theoretical predictions, with magnetic moments tilted away from the crystal diagonals towards the face-planes. We study the influence of these two antiferromagnetic structures on the exchange-bias properties of an epitaxial body-centred-cubic Fe layer showing that magnetization reversal mechanism and bias-field in the ferromagnetic layer is altered significantly. We report a change of reversal mechanism from in-plane nucleation of 90° domain-walls when coupled to the newly reported cubic structure towards a rotational process, including an out-of-plane magnetization component when coupled to the L12 triangular structure.

Vacancy defects and monopole dynamics in oxygen-deficient pyrochlores

The idea of magnetic monopoles in spin ice has enjoyed much success at intermediate temperatures, but at low temperatures a description in terms of monopole dynamics alone is insufficient. Recently, numerical simulations were used to argue that magnetic impurities account for this discrepancy by introducing a magnetic equivalent of residual resistance in the system. Here we propose that oxygen deficiency is the leading cause of magnetic impurities in as-grown samples, and we determine the defect structure and magnetism in Y2Ti2O7-δ using diffuse neutron scattering and magnetization measurements. These defects are eliminated by oxygen annealing. The introduction of oxygen vacancies causes Ti(4+) to transform to magnetic Ti(3+) with quenched orbital magnetism, but the concentration is anomalously low. In the spin-ice material Dy2Ti2O7 we find that the same oxygen-vacancy defects suppress moments on neighbouring rare-earth sites, and that these magnetic distortions markedly slow down the long-time monopole dynamics at sub-Kelvin temperatures.

Interplay between superconductivity and magnetism in Gd/La superlattices

Abstract A [Gd 30 /La 10 ] 60 superlattice has been studied using SQUID magnetometry and polarised neutron reflectivity. Zero-field cooling results in the coexistence of an antiferromagnetic alignment of the ferromagnetic Gd blocks, and 3D superconductivity at low temperature. Field cooling from room temperature results in ferromagnetic coupling between the Gd blocks, and under these conditions, the superconducting transition in the La is suppressed.

Neutron powder diffusion study of the fast-ion transition and specific heat anomaly in beta -lead fluoride

A powder sample of beta -PbF2 has been studied by neutron diffraction using the high resolution powder diffractometer at the Rutherford-Appleton Laboratory. Rietveld profile refinements have been performed on data collected at roughly 100 temperatures from ambient to well above the transition to the fast-ion phase, which occurs at Tc approximately 710 K. The lattice parameter, a0(T), the concentration of Frenkel defects, and thermal parameters for both the fluorine and the lead ions have been determined as a function of temperature. Precise values of a0(T) enable the linear expansivity to be calculated, for which there is a pronounced peak centred on Tc. It is possible to calculate contributions to the specific heats, Cp, from harmonic and anharmonic lattice vibrations, Frenkel disorder an lattice dilation; a broad peak is obtained centred on Tc, which is in good agreement with the directly determined experimental data.

Exchange-bias stabilization of the magnetic nanoparticles in a granular alloy grown by reactive sputtering

The superparamagnetic blocking temperature of a series of granular Co22Ag78 alloys grown by reactive sputtering under oxygen atmosphere exhibits a minimum when plotted as a function of the oxygen pressure. The magnetic stabilization observed above this minimum is found to arise from the exchange-bias between the Co core and a CoO shell, which appears to be spin disordered. The initial decrease in the blocking temperature at low oxygen pressures is mainly attributed to the inhibition of RKKY-like interactions by the formation of an insulating oxide layer, too thin to give rise to exchange bias, around the ferromagnetic cores.

Spin correlations and exchange in square-lattice frustrated ferromagnets

The J1-J2 model on a square lattice exhibits a rich variety of different forms of magnetic order that depend sensitively on the ratio of exchange constants J2/J1. We use bulk magnetometry and polarized neutron scattering to determine J1 and J2 unambiguously for two materials in a new family of vanadium phosphates, Pb2VO(PO4)2 and SrZnVO(PO4)2, and we find that they have ferromagnetic J1. The ordered moment in the collinear antiferromagnetic ground state is reduced, and the diffuse magnetic scattering is enhanced, as the predicted bond-nematic region of the phase diagram is approached.