Neutron scattering has been used to study the magnetic correlations and long wavelength spin dynamics of La1−xCaxMnO3 in the ferromagnetic regime (0 ≤ x < 1 2 ). For x = 1 3 (TC = 250K) where the magnetoresistance effects are largest the system behaves as an ideal isotropic ferromagnet at low T, with a gapless (< 0.04meV ) dispersion relation E = Dq 2 and DT =0 ≈ 170 meV-Å2 . However, an anomalous strongly-field-dependent diffusive component develops above ∼ 200K and dominates the fluctuation spectrum as T → TC. This component is not present at lower x.

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We report a systematic study of the electronic transport properties of the metallic perovskite oxide LaNiO 3−δ as a function of the oxygen stoichiometry δ (δ 0.14). The electrical resistivity, magnetoresistance, susceptibility, Hall effect and thermopower have been studied. All of the transport coefficients are dependent on the value of δ. The resistivity increases almost exponentially as δ increases. We relate this increase in ρ to the creation of Ni 2+ with square-planar coordination. We find that there is a distinct T 1.5 -contribution to the resistivity over the whole temperature range. The thermopower is negative, as expected for systems with electrons as the carrier, but the Hall coefficient is positive. We have given a qualitative and quantitative explanation for the different quantities observed and their systematic variation with the stoichiometry δ.

The defect structure of oxygenated and reduced Nd2CuO~(y =4) was investigated by single-crystal neutron diffraction. Structural refinements indicate the presence of interstitial oxygen atoms in the "apical" O(3) position, directly above or below the copper atoms. The occupancy of the apical oxygen site is -0.10 (per formula unit) for the oxygenated sample and -0.04 for the reduced one. Both the in-plane

We describe experiments which demonstrate that carbon atoms introduced into a fused-silica substrate by means of MeV ion implantation can, after suitable annealing, form nanocrystalline diamond. Unlike other methods of creating diamond, the coalescence of the carbon into diamond nanocrystals occurs when the samples are heated in a conventional furnace and does not require the application of high external pressures, or any pre-existing diamond template. Following a dose of 5ϫ10 16 C/cm 2 into fused quartz and after annealing in forming gas ͑4% hydrogen in argon͒, perfect cubic diamond crystallites of 5-7 nm diameter are formed. For higher doses, the same annealing treatments produce larger crystallites which are comprised of other varieties of solid carbon phases. We conclude that diamond is the stable form of carbon provided that the crystallite size is sufficiently small ͑less than 7 nm͒ and that the nanocrystallites are appropriately surface passivated.