Abstract
absorption and emission Mössbauer spectroscopy was applied to iron- and cobalt-doped ( high-temperature superconductors (MFe, ; 3.5×≤x≤0.1, y=0 and y=0.8) between 4.2 and 295 K with and without external magnetic field. four iron species A, B, C, and D, are observed with different dominance at different x and y. Aftereffects of the electron capture are ruled out. At 4.2 K spontaneous antiferromagnetic order is shown by contributions A, B, and C in spectra of samples with . A model-independent separation of the magnetically split subspectra shows an asymmetry parameter of the electric field gradient above 0.8 for A and B. Temperature and external magnetic field dependence of the magnetic splittings is indicative of superparamagnetic relaxation of small clusters. The species A, B, and C are magnetically interrelated. We suggest a preferential Cu(1) substitution by Fe and Co. Three of the four iron species, viz. A, B, and D have been assigned to high-spin ; C is high spin . We provide evidence that Co is mostly in the four-valent state, too. Four-valent Fe and Co substitution in is in reasonable agreement with the dopant concentration dependence of . Possible crystallographic assignments of all four iron (cobalt) sites are discussed.
- Received 15 July 1988
DOI:https://doi.org/10.1103/PhysRevB.38.11373
©1988 American Physical Society