Magnetism and superconductivity in Y_1-χCa_χSr₂GaCu₂O_7-δ

Samples of Y_1-χCa_χSr₂GaCu₂O_7-δ (χ=0, 0.4), prepared under various oxygen pressures, have been studied by magnetometry. Samples doped with ⁵⁷Fe have been studied also by Mössbauer spectroscopy. The majority of the iron ions (Fe³⁺) enters the Ga (Cu(1)) site, a minority enters the Cu(2) site. For the χ=0 sample, the Mössbauer spectra of the iron nuclei in the Cu(2) sublattice display magnetic order of the Cu with TN=370 K. The moments order along the c-axis. The iron in the Ga site displays magnetic order only at low temperatures, probably induced from the Cu(2) site. At temperatures above 90 K, these iron ions display a single pure quadrupole doublet Mössbauer spectrum, with a splitting of 2.0 mm/s. The sample χ=0.4 prepared under ambient pressure of oxygen (APO) also displays magnetic order of the Cu(2) site, with T_N≈370 K. The iron nuclei in the Ga site display paramagnetic long spin relaxation times phenomena at 4.2 K. The χ=0.4 sample, prepared under 110 atm oxygen pressure (HPO), displays superconductivity according to the magnetometry measurements (Tc≈50 K). The Mössbauer spectra give evidence of the presence of two phases in this sample. One displays magnetic order like the APO sample, the other is paramagnetic. The latter is probably associated with the superconducting phase.