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Fe Mössbauer spectra have been measured systematically in YBa2(Cu1-xFex)3Oy prepared by two different heat treatments: the usual treatment (O) and the heat treatment including that under lower oxygen fugacity at high temperatures followed by the oxidation process at low temperatures (NO). From the analysis of the results in terms of binomial random probability, it has been found that the Fe distribution in the Cu(1) site is almost random in the heat treatment (O). Here, the D1 and D2 components in the Mössbauer spectra, which have been assigned to the 4 and 6 oxygen coordinated Fe ions in the Cu(1) site respectively, should correspond to the monomer and cluster Fe ions. In the heat treatment (NO), some of the Fe ions are transferred to the Cu(2) site and there exist only cluster Fe ions in the Cu(1) site, resulting in the characteristic change of macroscopic properties of this system.

Abstract: The negative muon spin rotation method (μ− SR) has been applied to studies of electronic states at oxygen sites of oxide superconductors YBa2Cu3O7, Nd2−x Ce x CuO4−δ (x=0.15, oxygen reduced), LiTi2O4 and related oxide-insulators La2CuO4−δ, CuO, Cu2O. The paramagnetic shifts of μ− trapped at oxygen nuclei in these polycrystalline powder samples have been measured at 300 K. All the measured shifts are positive. In copper-oxides the paramagnetic shifts are of the order 10−3, while in LiTi2O4 is very small (8.43.3410−5). In YBa2Cu3O7, a fast μ− spin relaxation timeT 2* (∼ 200 ns) has been observed; the reason for this is unknown and further studies are now in progress.

The57Co emission Mössbauer spectra from YBa2Cu3O6.92(1-2-307) and YBa2Cu3O6.00(1-2-306) have been measured and compared with the57Fe absorption spectra from YBa2Cu2.95Fe0.05O7−δin order to clarify decisively the site assignments for the57Fe quadrupole-split doublets in these compounds. Mössbauer spectra obtained from both specimens consist of four components whose hyperfine interaction parameters well agree with each other. It is shown that the Co and Fe atoms mainly substitute at Cu1 chain sites in 1-2-307, but in 1-2-306 the Co atoms occupy randomly the Cu2 plane sites and indicate magnetically-split sextet which converts to a paramagnetic doublet of S-state Fe3+in 1-2-307 by a post-annealing in O2gas.

Abstract: Spin-glass like magnetic ordering of iron moments was observed in both orthorhombic and tetragonal YBa2(Cu1−xFex)3Oy (x=0.08) by μ+SR measurements. In a “Tetra” sample, all the muons sense the superconducting transition at 60 K and magnetic ordering at around 15 K, while in an “Ortho” sample they reveal that two magnetically different parts exist in the sample: about 40% of the sample is superconducting withT c ≈90K and the remaining part is magnetic withT M≈33K. These phenomena can be explained in terms of clustering of the Fe atoms in the “Ortho” sample.