Phase diagram for Ca1-xYxMnO3 type crystals

We present a simple model to study the electron doped manganese perovskites. The model considers the competition between double exchange mechanism for itinerant electrons and antiferromagnetic superexchange interaction for localized electrons. It represents each Mn4+ ion by a spin 1/2, on which an electron can be added to produce Mn3+ we include a hopping energy t, a strong intratomic ferromagnetic interaction exchange J (in the limit J/t --> infinity), and an interatomic antiferromagnetic interaction K between the local spins. Using the Renormalized Perturbation Expansion and a Mean Field Approximation on the hopping terms and on the superexchange interaction, we calculate the free energy. From it, the stability of the antiferromagnetic, canted, ferromagnetic, and novel spin glass phases can be determined as functions of the parameters characterizing the system. The model results can be expressed in terms of t and K for each value of the doping x in phase diagrams. The magnetization m and canting angle theta can also be calculated as functions of temperature for fixed values of doping and model parameters. (C) 1999 Elsevier Science Ltd. All rights reserved.