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Promising results reported in our previous works led us to think that production of B powder plays a crucial role in MgB2 synthesis. A new method for boron preparation has been developed in our laboratory. This particular process is based on magnesiothermic reaction (Moissan process) with the addition of an initial step that gives boron powder with nano-metric grain size. In this paper we report our efforts regarding optimization of PIT method for these nanometric powders and the resolution of problems previously highlighted such as the difficulty in powder packaging and the high friction phenomena occurring during cold working. This increases cracking during the tape and wire manufacturing leading to its failure. Packaging problems are related to the amorphous nature of boron synthesized in our laboratory, so a crystallization treatment was applied to improve crystallinity of B powder. To prevent excessive friction phenomena we synthesized non-stoichiometric MgB2 and using magnesium as lubricant. Our goal is the Jc improvement, but a global physical-chemical characterization was also made to analyze the improvement given by our treatments: this characterization includes X-ray diffraction, resistivity vs. temperature measurement, SEM image, besides magnetic and transport Jc measurements., Comment: 15 pages, 11 figures

Local structure of SmFe$_{1-x}$Ru$_x$AsO$_{0.85}$F$_{0.15}$ ($x$ = 0.0, 0.05, 0.25 and 0.5) superconductors has been investigated by temperature dependent As $K$-edge extended x-ray absorption fine structure. The effect of Ru substitution remains confined to the iron-arsenide layer but neither the static disorder nor the Fe-As bond strength suffers any change for $x \le$ 0.25. With further Ru substitution the static disorder increases while the Fe-As bond strength remains unchanged. Also, the Ru-As distance ($\sim$2.42 \AA), different from the Fe-As distance ($\sim$2.39 \AA), does not show any change in its force constant with the Ru substitution. These observations suggest that the SmFe$_{1-x}$Ru$_x$AsO$_{0.85}$F$_{0.15}$ system breaks down to coexisting local electronic phases on isoelectric substitution in the active FeAs layer.

We investigate the effect of external pressure on magnetic order in undoped LnFeAsO (Ln = La, Ce, Pr, La) by using muon-spin relaxation measurements and ab-initio calculations. Both magnetic transition temperature $T_m$ and Fe magnetic moment decrease with external pressure. The effect is observed to be lanthanide dependent with the strongest response for Ln = La and the weakest for Ln = Sm. The trend is qualitatively in agreement with our DFT calculations. The same calculations allow us to assign a value of 0.68(2) $\mu_B$ to the Fe moment, obtained from an accurate determination of the muon sites. Our data further show that the magnetic lanthanide order transitions do not follow the simple trend of Fe, possibly as a consequence of the different $f$-electron overlap., Comment: 16 pages, 11 figures

The effect of isoelectronic substitutions on the microscopic properties of LaFe1-xRuxAsO, for 0< x< 0.8, has been investigated by means of muSR and 139La NMR. It was found that Ru substitution causes a progressive reduction of the N\`eel temperature (T_N) and of the magnetic order parameter without leading to the onset of superconductivity. The temperature dependence of 139La nuclear spin-lattice relaxation rate 1/T_1 can be suitably described within a two-band model. One band giving rise to the spin density wave ground-state, while the other one is characterized by weakly correlated electrons. Fe for Ru substitution yields to a progressive decrease of the density of states at the Fermi level close to the one derived from band structure calculations. The reduction of T_N with doping follows the predictions of the J_1-J_2 model on a square lattice, which appears to be an effective framework to describe the magnetic properties of the spin density wave ground-state., Comment: 6 pages, 8 figures