ABSTRACT
An open (closed) system, in which matter is (not) exchanged through surface diffusion, was realized via growth kinetics. Epitaxially grown Si-Ge:Si (001) islands were annealed in different environments affecting the diffusivity of Si adatoms selectively. The evolution of the driving forces for intermixing while approaching the equilibrium was inferred from Synchrotron x-ray measurements of composition and strain. For the open system, intermixing due to the Si inflow from the wetting layer (reservoir) caused a decrease in the Ge content, leading to a lowering of the elastic energy and an increase in the mixing entropy. In contrast, for the closed system, while keeping the average Ge composition constant, atom rearrangement within the islands led to an increase in both elastic and entropic contributions. The Gibbs free energy decreased in both cases, despite the different evolution paths for the composition profiles.
ABSTRACT
We present specific heat data on three samples of the dilute Ising magnet LiHoxY1-xF4 with x=0.018, 0.045, and 0.080. Previous measurements of the ac susceptibility of an x=0.045 sample showed the Ho3+ moments to remain dynamic down to very low temperatures, and the specific heat was found to have unusually sharp features. In contrast, our measurements do not exhibit these sharp features in the specific heat and instead show a broad feature, for all three samples studied, which is qualitatively consistent with a spin glass state. Integrating C/T, however, reveals an increase in residual entropy with lower Ho concentration, consistent with recent Monte Carlo simulations showing a lack of spin glass transition for low x.
ABSTRACT
A high-resolution X-ray diffraction beamline at the Brazilian Synchrotron Light Laboratory (LNLS) has been commissioned for the study of crystalline magnetic materials. The beamline optics is based on a Rh-coated vertical-focusing X-ray mirror and a sagittal-focusing double-crystal monochromator. The primary instrument is a six-circle diffractometer equipped with energy and polarization analysers and a closed-cycle He cryostat. The beamline source is a bending magnet of the 1.37 GeV storage ring of the LNLS, delivering approximately 4 x 10(10) photons s(-1) at 8 keV at the sample position. Resonant and non-resonant scattering are the main techniques used to study charge, orbital and magnetic structures. Examples of magnetic scattering in Ho and NiO single crystals, as well as orbital ordering in manganites thin films, are presented.
