The phase diagram and the ground state properties of the R(Y, Pr)BaCuT(Fe, Co)O5+y perovskites, where y is the concentration of the extra oxygen, have been studied using the Hubbard model. We consider two chains of magnetic ions and allow electron hopping between sites on different chains, modeling in this way the effect of the extra oxygen atoms in the system. By doping the system with oxygen, narrow regions of competing magnetic structures appear in the phase diagram and the magnetic ordering is rapidly destroyed with increasing oxygen concentration y. This is in agreement with the experimental results which show that magnetic ordering exists only for low extra oxygen concentration y (y<0.08). The magnetic properties of the system are only locally affected by the oxygen impurities clustering.

The magnetic ordering of the compounds La1−yCayMn1−xFexO3 where 0.2<y<0.5 and 0<x<0.1 has been studied within the molecular field theory. We introduce a model based on the competition between ferromagnetic coupling between the Mn ions and strong antiferromagnetic coupling induced by the presence of Fe ions. The magnetization as a function of temperature and the critical temperature have been calculated for several values of the parameter x. We show that even for very small x, the magnetic order of the system is reduced. Our results are in good agreement with experimental findings on these systems.

The magnetic ordering of the compounds RBaCuTO5+y where R is a rare earth (Y or Pr) and T is a transition metal (Fe or Co) and 0<y<0.5 has been studied by molecular field theory and the Monte Carlo simulation technique. We introduce a model based on competition between weak ferromagnetic interlayer coupling and strong antiferromagnetic coupling induced by the presence of the oxygen defects. We show that even for small y, three-dimensional long range order is rapidly destroyed. (C) 1999 American Institute of Physics. [S0021-8979(99)41208-3].