Mössbauer spectroscopy of ⁵⁷Fe in high T_c superconductors YBa₂Fe_3xCu_3(1-x)O_7-δ

Mossbauer spectroscopy of ⁵⁷Fe in both tetragonal and othorhombic phases of YBa₂(Fe_xCu_1-x)₃O_7-δ, with x = 0.01, 0.02 and 0.10, at temperatures 4.2 K, 75 K, 90 K, and 300 K have been performed. In all samples three major subspectra corresponding to iron in different local environments are observed. It is concluded that Fe substitutes mainly Cul. At 4.2 K, samples with x=0.01 in the "quenched" tetragonal phase exhibit magnetic hyperfine structure, due to slow spin relaxation rates, whereas in the orthorhombic superconducting phase, only samples with x=0.1 exhibit magnetic hyperfine structure, in this case probably due to spin glass magnetic order.