Mössbauer emission study of57Co: YBa2Cu3O7−y HTSC

Mössbauer emission spectroscopy is performed on⁵⁷Co:YBa₂Cu₃O_7–y oxides in a temperature range from 300 K to 77 K. The spectra show, at least two different location for the⁵⁷Co(⁵⁷Fe) impurities. The isomer shift and quadrupole splitting values of the⁵⁷Fe daughter are coincident with those observed in⁵⁷Fe absorption experiments. From the analysis of theIS andQS values as well as from the relative location of Fe impurity levels in the HTSC matrix it is argued that:a) Co impurities enters into the lattice mainly in the Cu1 sites, but some of them have a higher coordination number.b) The daughter⁵⁷Fe exists as localized Fe⁴⁺ state.c) The parent⁵⁷Co enter as a localized Co³⁺ state. These conclusions appear consistent with the observed increase ofN(0) on doping with Fe or Co ions and with the existence of localized magnetic moments as determined from paramagnetic susceptibility measurements.