Magnetic ordering of YBa2(Cu1−x Fe x )3O7

The Mössbauer spectra of YBa₂(Cu_1?x Fe _x )₃O₇ at room temperature show several doublets attributed to Fe in Cu(1) sites with different oxygen configurations. Here we present a systematic study performed at 4.2 K forx=0.005, 0.01, 0.03, 0.05, 0.10, 0.15. To obtain information about the magnetic ordered state two samples, withx=0.005 andx=0.15, have been studied at 4.2 K underB _ext=5 T. The Mössbauer spectra indicate that the iron moments are polarized forx=0.005, while in the ordered state (x=0.15) they have an antiferromagnetic or spin-glass-like arrangement with high anisotropy.     

Shifting the Superparamagnetic Limit of Nanosized Copper Iron Spinel

In this paper we analyze trend of EFG values measured at Cd impurity in a group of semiconducting delafossites with chemical formula CuBO₂ (B = Al, Fe, Cr, Nd). We conclude that this trend reveals one of the most subtle details in electronic spectrum of the compounds: if impurity states are formed within or out of the band gap. In CuAlO₂ and CuFeO₂ the Cd EFG exhibits larger value than in CuCrO₂ and CuNdO₂, when Cd substitutes the Cu atom. This occurs because in the first two compounds the Cd forms shallow band within the gap, and in the second two compounds does not. When Cd occupies the B position it exhibits almost the same EFG in all delafossites. In this case, Cd does not form its states within the gap in none of the compounds. To arrive to these conclusions we analyzed and calculated various systems (Cd-doped CuAlO₂ and CuCrO₂ compounds, fictitious molecules), using the FP-LAPW method.     

Calcium doping effects in57Fe : La2−x Ca x CuO4−y oxides

La_2–x Ca _x Cu_0.99Fe_0.01O_4–y samples with 0.00 x 0.30 were studied by X-ray diffraction and Mössbauer spectroscopy after heat treatment under different atmospheres (air, oxygen and helium). The variation of the relative population and the hyperfine interactions of the sites as well as the change in the lattice parameters were obtained.On leave from National Center for Scientific Research, PO Box 6990, Havana, Cuba.     

Fe2+-Mg order–disorder study in orthopyroxenes from São João Nepomuceno (IVA) iron meteorite

São João Nepomuceno (SJN) is an iron meteorite belonging to IVA chemical group. It consists of Fe-Ni metal matrix and silicate inclusions made of a coarse-grained mixture of tridymite, orthopyroxene and clinopyroxene. In spite of the extensive work performed on the IVA group there is still no consensus about their origin and its thermal history is subject of ongoing debates. In this work, we report preliminary results on Fe^2?+? distribution in the non-equivalent octahedral sites in orthopyroxene crystals extracted from São João Nepomuceno in order to infer on the thermal history of this meteorite and therefore acquire more information related to the origin of the IVA iron meteorite group.     

Possible oxygen hopping in the YBa2Cu3O7 system observed by Mössbauer spectroscopy

In situ high temperature⁵⁷Fe Mössbauer measurements have been performed on samples of YBa₂ (Cu_1?xFe_x)₃O_7?δ withx=0.005 andx=0.010, in pure oxygen or in air, in the range 295 K≤T≤640 K. TheT-dependence of Fe species, of the Debye-Waller factorf(T) as well as of the hyperfine parameters is analysed.     

Provenance study of obsidians from the archaeological site of La Maná (Ecuador) by electron spin resonance (ESR), SQUID magnetometry and 57Fe Mössbauer spectroscopy

Obsidians from major Ecuadorian sources (outcrops) were analyzed by electron spin resonance, SQUID magnetometry and ⁵⁷Fe Mössbauer spectroscopy. If the last technique allows to discriminate obsidians from the Quiscatola source, an association of ESR with SQUID magnetometry permits to differentiate obsidians from the sources of Cotopaxi volcano, from the Quiscatola and Mullumica-Callejones sources of the Chacana caldera and to infer that the 12 analyzed obsidians from the pre-Hispanic site of La Maná come from the Mullumica-Callejones source.     

Tetrataenite in metallic grains of the Antarctic L6 chondrite ALHA 76009

Mössbauer X-ray diffraction and electron microscopy studies revealed that segregation and ordering occurred in the metal particles of the Antarctic L6 chondrite ALHA 76009. The ordered crystal structure of AuCu type (tetrataenite) was detected coexisting with the disordered 50-50 taenite.     

High temperature Mössbauer studies of 57Fe: YSr2Cu3O7 related compounds containing oxyanions

Oxyanion substitutions at the copper sites can stabilize the Ba free phase YSr₂Cu₃O₇ and allow synthesis to occur under ambient conditions (P.R. Slater and C. Greaves, 1991; P.R. Slater et al., 1993). Here we report in situ ⁵⁷Fe Mössbauer spectroscopy experiments done in Y--Sr--Cu--O related oxides containing sulphate (SO₄ ^2-), phosphate (PO₄ ^3-) and borate (BO₃ ^3-), in the temperature range 300 K ? T ? 723 K. From the results obtained it has been possible to study the kinetics of oxygen loss, as well as the temperature dependence of the Fe species.     

Magnetic properties and electron spin resonance of Ecuadorian obsidians. Application to provenance research of archeological samples

Obsidians from major Ecuadorian sources were analyzed by SQUID magnetometry and electron spin resonance (ESR). The association of these two techniques permits to differentiate obsidians from the sources of Cotopaxi volcano and from the Quiscatola and Mullumica–Callejones sources of the Chacana caldera, taking into account various parameters arising from the M vs. H cycles and the X-band ESR spectra. The analysis of 27 archeological samples coming from the prehispanic site of La Maná allowed us to infer that most of them come from the Mullumica–Callejones source.     

Characterization by Mössbauer Spectroscopy and Electron Paramagnetic Resonance of Peruvian Obsidians for Provenance Studies: A Preliminary Study

We report on an investigation of several ancient clays which were used for pottery making in northern coastal Peru at a kiln site from the Formative period (ca. 2000–800 BC) in the Poma Canal and at a Middle Sicán pottery workshop in use between ca. AD 950 and 1050 at Huaca Sialupe in the lower La Leche valley. Neutron activation analysis, ⁵⁷Fe Mössbauer spectroscopy and X-ray diffraction were used for the characterisation of the clays. The changes that occur in iron-bearing compounds in the clays depending on the kiln atmosphere and on the maximum firing temperature were studied by Mössbauer spectroscopy and X-ray diffraction. Laboratory firing series under varying controlled conditions were performed to obtain a basic understanding of the different reactions taking place in the clays during firing. The results can be used as models in the interpretation of the Mössbauer spectra observed in ancient ceramics from the same context.