Magnetic order and superconductivity in RBa2Cu3Oz

Mössbauer studies Fe57 in RBa2-yKy(Cu1-xFex)3Oz, with R=Y and Pr, y=0 and 0.5, x=0.01,0.05, and 0.1, and z between 5.9 and 7.1, have been performed. A minority of the iron ions enter the Cu(2) site and reveal its magnetic order. For R=Y, y=0, and x=0.1, TN equals 280 and 415 K for z=6.5 and 6.1, respectively. The magnetic moments lie in the basal plane. In tetragonal, oxygen-rich PrBa2(Cu0.9Fe0.1)3O6.9, TN=325 K; in superconducting YBa2(Cu0.9Fe0.1)3O7.1 there is no magnetic order. In nonsuperconducting YBa1.5K0.5(Cu0.95Fe0.05)3O6.1 two distinctly inequivalent magnetic iron sites are observed, corresponding to iron in the Cu(2) site with different Ba-K neighbors. Moments of iron ions that have three Ba and one K as first-nearest neighbors have a different temperature dependence and TN (TN=450 K) from those with four Ba neighbors, where TN=415 K, showing that the antiferromagnetic exchange in the Cu(2) planes is strongly affected by the replacement of Ba2+ by K+, probably by repelling oxygen from the Cu(2) plane. In superconducting YBa1.5K0.5(Cu0.95Fe0.05)3O6.5 the iron site with TN=450 K remains magnetic. The implications of these findings on the valencies of the Cu ions are discussed.