Measurement of local magnetic fields in the CuO₂ planes of CuBa₂YCu₂O_7-δ superconductors

A fully oxygenated Cu(1)Ba2YCu(2)2O7-δ sample having 0.167 at% of the Cu atoms replaced by enriched Fe57 was synthesized. As confirmed by Fe57 Mössbauer spectroscopy ∼16% of the Fe atoms entered the superconducting Cu(2)O2 planes. Mössbauer spectra recorded at various temperatures between 77 and 373 K exhibited the presence of a weak magnetic hyperfine interaction in the Fe atoms entering the CuO2 planes. The nonzero internal field obtained from the Mössbauer data increased following a Brillouin-type behavior, from the estimated Néel-like temperature of ∼400K down to 96 K where it reached ∼1.5T. Upon further cooling below Tc the internal field decreased fast down to ∼0.4T at 77 K. The presence of the internal field is believed to be related to magnetic correlations among the CuO2-plane atoms, i.e., to reflect local antiferromagnetic coupling between the Cu atoms.