We propose a double exchange model to describe the RuO_2 planes of RuSr_2(Eu,Gd)Cu_2O_8. The Ru^+5 ions are described by localized spins, and additional electrons provided by the superconducting CuO_2 planes are coupled ferromagnetically to them by Hund rules coupling. We calculate the spin structure factor, magnetic susceptibility and magnetization as a function of magnetic field and temperature, using a Monte Carlo algorithm in which the Ru^+5 spins are treated as classical. Several experiments which seemed in contradiction with one another are explained by the theory.
@article{arxiv.cond-mat/0108286,
title = {Double exchange model for RuSr_2(Eu,Gd)Cu_2O_8},
author = {H. Aliaga and A. A. Aligia},
journal= {arXiv preprint arXiv:cond-mat/0108286},
year = {2009}
}