Magnetic ordering in57Fe-doped high-T c superconductors

The high-temperature superconductor, Tl2CaBa2(Cu1−xFe x )2O8+δ (the 2122 compound), has been investigated by a number of techniques, including X-ray diffraction, resistance and ac susceptibility measurements, and Mossbauer spectroscopy. The procedures followed to make close to single-phase samples are described. The decrease in the critical temperature for superconductivity,T c , is less than for the iron-doped 123 compounds. The Mossbauer spectra at 77 K and above consist of an asymmetric doublet. Below about 10 K magnetic hyperfine splitting occurs; relaxation effects are still present at 2.3 K. The spectra can be fitted with two overlapping patterns. Their origin is discussed: comparisons are made with other high-T c superconductors.