We show results from simulations of the Langevin dynamics of a two-dimensional scalar model with competing interactions for ultrathin magnetic films. We find a phase transition from a high temperature disordered phase to a low temperature phase with both positional and orientational orders. Both kinds of order emerge at the same temperature, probably due to the isotropy of the model Hamiltonian. In the low temperature phase orientational correlations show long range order while positional correlations show only quasi-long-range order in a wide temperature range. The orientational correlation length and the associated susceptibility seem to diverge with power laws at the transition. While at zero temperature the system exhibits stripe long range order, as temperature grows we observe the proliferation of different kinds of topological defects that ultimately drive the system to the disordered phase. The magnetic structures observed are similar to experimental results on ultrathin ferromagnetic films.Comment: extended introduction, added reference

This study investigated the synthesis of platinum nanoparticles (Pt NPs) in ethylene glycol using low cost and low toxicity chemicals as reducing (ascorbic acid) and stabilizing agents (polyvinylpyrrolidone and sodium citrate). By monitoring the changes in the local chemical environment of the Pt atoms in real time by in situ dispersive X-ray absorption spectroscopy, it is observed that the NP formation kinetics involved three different stages within 3 h 30 min of the reaction: a reduction-nucleation burst, followed by diffusion-limited Ostwald ripening growth and subsequent stabilization of the NPs. The resulting Pt NPs were analyzed by transmission electron microscopy and X-ray diffraction, revealing a monodisperse average size distribution of 2.7 ± 0.5 nm, characterized by highly crystalline and stable Pt clusters, showing no significant aging for at least nine months.

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We study two dimensional stripe forming systems with competing repulsive interactions decaying as r(-α). We derive an effective Hamiltonian with a short-range part and a generalized dipolar interaction which depends on the exponent α. An approximate map of this model to a known XY model with dipolar interactions allows us to conclude that, for α<2 long-range orientational order of stripes can exist in two dimensions, and establish the universality class of the models. When α≥2 no long-range order is possible, but a phase transition in the Kosterlitz-Thouless universality class is still present. These two different critical scenarios should be observed in experimentally relevant two dimensional systems like electronic liquids (α=1) and dipolar magnetic films (α=3). Results from Langevin simulations of Coulomb and dipolar systems give support to the theoretical results.

In the present study, in situ X-ray absorption spectroscopy (XAS) was used to monitor the structural evolution of isolated Pt 0.3 Pd 0.7 nanoparticles (NPs) subjected to different gaseous atmospheres. Time-resolved XAS measurements were performed at the Pt L 3 edge in the dispersive mode, during which X-ray absorption near edge spectra were sequentially collected. The NPs were initially activated under a reducing atmosphere at 300 °C (H 2 + He) and subsequently exposed to a sulfidation process at the same temperature (H 2 + He + H 2 S at 300 °C). Then, the sulfided NPs were thermally treated under a reducing atmosphere, and the reversibility of the sulfidation process was successfully accomplished, for the first time in these unsupported and well-characterized nanoscale systems. The atomic local order in the vicinity of the Pt atoms was investigated by extended X-ray absorption fine structure spectroscopy throughout all of these thermal treatments, monitoring the chemical stability of the metal−sulfur bonds and allowing kinetic modeling, from which the activation energies for the sulfidation process were estimated.