Iron by copper substitution in M-1212 cuprates: coexistence of superconductivity and magnetism

Overdoped cuprates are attracting a renewed interest in searching for clues about the pairing mechanism in cuprate superconductors and, in particular, for a better understanding of the factors controlling the Tc. Starting from the well-known YSr2Cu3O6+δ (YSCO) cuprate, total or partial replacement of copper at the charge reservoir block (CRB) by other transition metals (M) stands as a useful strategy to extend the hole doping range. The resulting MxCu1-xSr2YCu2O7+δ (M-1212 type) cuprates are suitable candidates in trying to achieve higher doping levels in cuprate superconductors. On the basis of this premise, the general objective of the present thesis concerns the study of the effect of iron by copper substitution in YSCO, giving FexCu1-xSr2YCu2O7+δ cuprates. Through a careful analysis of both the iron location, coordination geometry and oxidation state, structure- hole distribution-Tc relations have been established. The special interest of Fe by Cu substitution is sustained from the possibility of attaining high oxidation states for iron cations at the charge reservoir block (i.e. > 3). In particular, the stabilization of the unusual high-valent Fe4+ cations result in high doping levels in the superconducting planes, even if also brings further electronic complexity into these ferrocuprates. A very unusual coexistence of superconductivity and magnetism at the microscopic level in these Fe-1212 cuprates has been corroborated in this thesis. Furthermore, an intricate interplay between the magnetic and superconducting interactions has been highlighted by means of muon spin rotation spectroscopy. In the present work, we have first revisited the solubility of iron in the YSr2Cu3O6+δ compound at ambient pressure conditions and the iron substitution site, coordination, and oxidation state in the resulting FexCu1-xSr2YCu2O7+δ (0.25 ≤ x ≤ 1) materials. For selected FexCu1-xSr2YCu2O7+δ (x=0.5, 1) compositions, the samples have been annealed in different atmospheres, with special use of low temperature oxidation methodologies (topochemical oxidation). The structural evolution in varying the oxygen content in the CRB has been carefully characterized, and a complementary electronic characterization has been performed to analyse the charge distribution between the CRB and the superconducting block (SCB). To that purposes, a wide range of experimental techniques have been used: transmission electron microscopy, X-Ray and neutron diffraction, X-Ray absorption spectroscopy, Mössbauer spectroscopy and muon spin rotation spectroscopy..