Mössbauer investigation of amorphous-to-crystalline transformation of Fe62Ni16B14Si8 by isothermal annealing

Room temperature transmission Mössbauer spectra of Fe₆₂Ni₁₆B₁₄Si₈ ribbons, annealed in vacuum for time periods ranging from 5 to 60 minutes at the crystallization temperatureT _x=720 K, have been used to investigate the crystallization mechanism by isothermal annealing. Reorientation of the magnetic anisotropy almost normal to the ribbon plane was observed and correlated to the annealing time dependence of the mean hyperfine field (HF) of the amorphous component. Crystallization started at the surface before the bulk and was found to occur in two steps: a metastable equilibrium of the amorphous phase with (FeNi), (FeNi)Si and t-(FeNi)₃B, followed by the decomposition of t-(FeNi)₃B into t-(FeNi)₂B and (FeNi). The increase of the mean magnetic moment at the Fe sites as compared to related iron-based alloys was attributed to compositional small Ni additions.     

Species-specific and inhibitor-dependent conformations of LpxC: implications for antibiotic design

LpxC is an essential enzyme in the lipid A biosynthetic pathway in gram-negative bacteria. Several promising antimicrobial lead compounds targeting LpxC have been reported, though they typically display a large variation in potency against different gram-negative pathogens. We report that inhibitors with a diacetylene scaffold effectively overcome the resistance caused by sequence variation in the LpxC substrate-binding passage. Compound binding is captured in complex with representative LpxC orthologs, and structural analysis reveals large conformational differences that mostly reflect inherent molecular features of distinct LpxC orthologs, whereas ligand-induced structural adaptations occur at a smaller scale. These observations highlight the need for a molecular understanding of inherent structural features and conformational plasticity of LpxC enzymes for optimizing LpxC inhibitors as broad-spectrum antibiotics against gram-negative infections.     

Laser-induced periodic surface structures on polymers for formation of gold nanowires and activation of human cells

Frequently observed coherent structures in laser-surface processing are ripples, also denoted as laser-induced periodic surface structures (LIPSS). For polyethylene terephthalate (PET) and polystyrene (PS), LIPSS can be induced by irradiation with linearly polarized ns-pulsed UV laser light. Under an angle of incidence of θ, their lateral period is close to the laser wavelength λ divided by (n _eff ? sinθ). Here, n _eff is the effective refractive index which is 1.32 and 1.23 for PET and PS, respectively. We describe potential applications of LIPSS for alignment and activation of human cells cultivated on polymer substrates, as well as for formation of separated gold nanowires which show pronounced surface plasmon resonances, e.g., at 775 nm for PET.     

57Fe Mössbauer spectroscopy on disordered crystalline media with Ca-gallogermanate type structure. II. Below the magnetic ordering temperature

Magnetic phase transitions in disordered crystalline iron germanates of the type Sr₂LnFe₃Ge₃O₁₄ (Ln=La; Nd) were observed at low temperatures by means of⁵⁷Fe Mössbauer spectroscopy. The hyperfine interaction of Fe³⁺ magnetic moments ordered in one octahedral and two tetrahedral magnetic sublattices is discussed. A local environment computation on the octahedral sites confirms the assumption that the wide distribution found for the 4.2 K octahedral hyperfine fields is due to the random occupation by Fe³⁺ and Ge⁴⁺ ions of the nearest cation environment of the octahedral iron. The effect of substituting Ln rare earth ions in the Thompson cubes on the magnetic behaviour of these compounds is pointed out.     

Comparison of electron beam and pulsed laser irradiation effects in Fe80B20

The effects of high-energy electron beam and pulsed excimer laser irradiation on the magnetic properties of Fe₈₀B₂₀ amorphous alloy are investigated by means of transmission and conversion electron Mössbauer spectroscopy (CEMS) and scanning electron microscopy (SEM). Both types of irradiation were found to induce changes in the magnetic anisotropy of Fe₈₀B₂₀, but the influence of high-energy electron beam irradiation was weaker. Depending on the number of applied laser pulses and repetition rate, controlled magnetic anisotropy and/or relaxation of internal stresses could be obtained by excimer laser irradiation. The presence of molten zones subsequently solidified and of laser-induced internal stresses supports the model of closúre domain structure.