The nuclear and magnetic structure of Fe 1+y (Te 1-x ,Se x ) (0 ≤ x ≤ 0.20) compounds was analyzed between 2 K and 300 K by means of Rietveld refinement of neutron powder diffraction data.Samples with x ≤ 0.075 undergo a tetragonal to monoclinic phase transition at low temperature, whose critical temperature decreases with increasing Se content; this structural transition is strictly coupled to a long range antiferromagnetic ordering at the Fe site. Both the transition to a monoclinic phase and the long range antiferromagnetism are suppressed for 0.10 ≤ x ≤ 0.20. The onset of the structural and of the magnetic transition remains coincident with the increase of Se substitution. The low temperature monoclinic crystal structure has been revised. Superconductivity arises for x ≥ * Corresponding author: amartin@chimica.unige.it 0.05, therefore a significant region where superconductivity and long range antiferromagnetism coexist is present in the pseudo-binary FeTe -FeSe phase diagram.

CoFe(2)O(4) nanoparticles (D(NPD) ~6 nm), prepared by a thermal decomposition technique, have been investigated through the combined use of dc magnetization measurements, neutron diffraction, and (57)Fe Mössbauer spectrometry under high applied magnetic field. Despite the small particle size, the value of saturation magnetization at 300 K (M(s) ͠= 70 A m(2) kg(-1)) and at 5 K (M(s) ͠= 100 A m(2) kg(-1)) are rather close to the bulk values, making the samples prepared with this method attractive for biomedical applications. Neutron diffraction measurements indicate the typical ferrimagnetic structure of the ferrites, showing an inversion degree (γ(NPD) = 0.74) that is in very good agreement with cationic distribution established from low temperature (10 K) Mössbauer measurements in high magnetic field (γ(moss) = 0.76). In addition, the in-field Mössbauer spectrum shows the presence of a non-collinear spin structure in both A and B sublattices. The results allow us to explain the high value of saturation magnetization and provide a better insight into the complex interplay between cationic distribution and magnetic disorder in ferrimagnetic nanoparticles.

In this work we present a systematic experimental and theoretical study of the structural, transport and superconducting properties of Sm(Fe 1-x Ru x )As (O 0.85 F 0.15 ) polycrystalline samples as a function of Ru content (x) ranging from 0 to 1. The choice of Ru as isoelectronic substitution at Fe site of Fdoped compounds allows to better clarify the role of structural disorder in modifying the normal and superconducting properties of these newly discovered multiband superconductors. Two different regions are identified: the Fe-rich phase (x<0.5) where superconducting and normal state properties are strongly affected by disorder induced by Ru substitution; the Ru-rich phase (x>0.5) where the system is metallic and strongly compensated and the presence of Ru frustrates the magnetic moment on Fe ions. Here the lack of magnetic features and related spin fluctuations may be the cause for the suppression of superconductivity. 1.IntroductionThe recent discovery of high critical temperature superconductivity in iron based compounds 1 has attracted a great deal of attention as these compounds appear to be a glaring case of proximity between superconductivity and magnetisms. The parent compounds exhibit antiferromagnetic spindensity-wave (SDW) order that disappears upon doping, giving rise to superconductivity. It has been suggested by many authors that superconductivity in pnictides could be mediated by magnetic excitations which couple electron and hole pockets of the Fermi surface, favoring s-wave order parameters with opposite sign on different sheets of the Fermi surface (s ± coupling). 2The interplay between superconductivity and magnetisms can be investigated by varying magnetic and superconducting properties of the compounds through suitable substitutions. Moreover, scattering induced by substitutions is expected to affect superconductivity in very differently ways in the cases of conventional or unconventional coupling. 3 As a consequence, a thorough study of the behavior of T c vs structural disorder is crucial in order to probe different theoretical models. Similarly to cuprates, the pnictide compounds have a layered structure characterized by the stacking of insulating and FeAs-conducting layers with general formulas REFeAsO (RE being a rare earth)

We measure magnetotransport of F doped SmFeAsO samples up to 28T and we extract the upper critical fields, using different criteria. In order to circumvent the problem of criteriondependence H c2 values, we suggest a thermodynamic estimation of the upper critical field slope dH c2 /dT based on the analysis of conductivity fluctuations in the critical regime. A high field slope as large as -12T/K is thus extracted for the optimally doped sample. We find evidence of a twodimensional lowest Landau level (LLL) scaling for applied fields larger than μ 0 H LLL ∼8T. Finally, we estimate the coherence length values and we observe that they progressively increase with decreasing T c . In all cases, the coherence length values along the c axis are smaller than the interplanar distance, confirming the two-dimensional nature of superconductivity in this compound.

In this paper we carry out a direct comparison between transport and superconducting properties-namely resistivity, magnetoresistivity, Hall effect, Seebeck effect, thermal conductivity, upper critical field-of two different families of Fe-based superconductors, which can be viewed in many respects as end members: SmFeAsO 1−x F x with the largest T c and the largest anisotropy and Fe 1+y Te 1−x Se x , with the largest H c2 , the lowest T c and the lowest anisotropy. In the case of the SmFeAsO 1−x F x series, we find that a single-band description allows us to extract an approximate estimation of band parameters such as carrier density and mobility from experimental data, although the behaviour of the Seebeck effect as a function of doping demonstrates that a multiband description would be more appropriate. On the contrary, experimental data for the Fe 1+y (Te 1−x , Se x ) series exhibit a strongly compensated behaviour, which can be described only within a multiband model.In the Fe 1+y (Te 1−x , Se x ) series, the role of the excess Fe, tuned by Se stoichiometry, is found to be twofold: on one hand it dopes electrons in the system and on the other hand it introduces localized magnetic moments, responsible for Kondo like scattering and likely pairbreaking of Cooper pairs. Hence, Fe excess also plays a crucial role in determining superconducting properties such as the T c and the upper critical field H c2 . The huge H c2 values of the Fe 1+y Te 1−x Se x samples are described by a dirty limit law, opposed to the clean limit behaviour of the SmFeAsO 1−x F x samples. Hence, magnetic scattering by excess Fe seems to drive the system in the dirty regime, but its detrimental pairbreaking role seems not to be as severe as predicted by theory. This issue has yet to be clarified, addressing the more fundamental issue of the interplay between magnetism and superconductivity.