Superconductivity and magnetism in Y_1-xCa_xSr₂GaCu₂O_7-δ

Samples of Y_1-xCa_xSr₂GaCu₂O_7-δ (x=0, 0.4) doped with⁵⁷Fe, prepared under various oxygen pressures, have been studied by magnetometry and Mössbauer spectroscopy. Most of the iron ions (Fe³⁺) enter the Ga(Cu(1)) site. For the x=0 sample, the Mössbauer spectra of the iron nuclei in the Cu(2) sublattice display magnetic order of Cu, T_N=370 K. The iron ions in the Ga site display magnetic order only at low temperatures. At temperatures above 90 K, these iron ions display a pure quadrupole doublet Mössbauer spectrum. The sample x=0.4 also displays magnetic order of the Cu(2) site, T_N≈370 K. A sharp drop in the hyperfine field is observed at T_N, probably associated with a first-order phase transition or two-dimensional ordering. The iron nuclei in the Ga site display paramagnetic long spin relaxation time phenomena at 4.2 K. The x=0.4 sample prepared under 110 atm oxygen pressure, displays superconductivity, T_c≈50 K. The Mössbauer spectra give evidence of the presence of two phases. One displays magnetic order, the other is paramagnetic, the last is probably associated with the superconducting phase.