  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.

Condensed Matter