Phase diagrams for two- and three-dimensional Ising models with fluctuating exchange integrals

The Ising ferromagnet with fluctuating exchange integrals has been investigated using the two-spin cluster approximation. The phase diagrams were obtained for both two- and three-dimensional systems. They show a re-entrant behaviour only for a square lattice. It is argued that this behaviour is expected in disordered systems where one has competition between ferromagnetic and antiferromagnetic interactions in a certain range of the concentration of the bond mixture.