C. Adriano

Pressure and chemical substitution effects in the local atomic structure of BaFe2As2

The effects of K and Co substitutions and quasihydrostatic applied pressure ( P< 9G Pa) in the local atomic structure of BaFe2As2 ,B a(Fe 0.937Co0.063)2As2 and Ba0.85K0.15Fe2As2 superconductors were investigated by extended x-ray absorption fine structure (EXAFS) measurements in the As K absorption edge. The As-Fe bond length is found to be slightly reduced (0.01 u A) by both Co and K substitutions, without any observable increment in the corresponding Debye-Waller factor. Also, this bond is shown to be compressible [κ = 3.3(3) × 10 −3 GPa −1 ]. The observed contractions of As-Fe bond under pressure and chemical substitutions are likely related with a reduction of the local Fe magnetic moments, and should be an important tuning parameter in the phase diagrams of the Fe-based superconductors.

Electron spin resonance of Gd3+ in GdmMnIn3m+2n (M=Rh,Ir; n=0,1; m=1,2) antiferromagnets

We report electron spin resonance experiments of Gd3+ in the GdmMnIn3m+2n (M=Rh,Ir; n=0,1; m=1,2) intermetallic compounds. For T>TN∼45K, all compounds present a single Dysonian resonance and show a Korringa-like temperature dependence of the linewidth, ΔH=a+bT. The residual linewidth a is strongly affected by the transition metal M=Rh or Ir and/or by the layering (m=1 or 2) or change in structure (n=0,1). The residual linewidth is associated with an unresolved crystalline electrical field (CEF) fine structure. Consequently, a systematic evolution of the CEF in the GdmMnIn3m+2n compounds is inferred. A discussion to what extent our results can explain to the CEF effects observed in isostructural R-based compounds will be given.

Crystal structure and low temperature physical properties of Ho2CoGa8 intermetallic antiferromagnet

We have synthesized single crystalline samples of Ho2CoGa8 intermetallic compound using a Ga-flux method. This compound crystallizes with a tetragonal structure, space group P4∕mmm, and lattice parameters a=4.219(5)A and c=10.99(2)A. This structure is a bilayer version of the HoCoGa5 (1-1-5) which hosts a series of heavy-fermion superconductors and complex antiferromagnetic intermetallic systems. Measurements of magnetic susceptibility, heat capacity, and electrical resistivity revealed that Ho2CoGa8 is a metallic Curie–Weiss paramagnet at high temperature and presents an antiferromagnetic ordering below TN∼5K. The low temperature magnetic properties of this compound show the effects of tetragonal crystalline electrical field and the Ruderman–Kittel–Kasuya–Yosid interactions and the results presented here are compared with a broader description of the evolution of the low-T magnetic properties of structurally related series of rare-earth based tetragonal 2-1-8 and 1-1-5 compounds.