Ferromagnetism of disordered systems

This review concerns disordered ferrotnagnets in which the magnetic atoms are randomly located on sites of a crystal lattice. Attention is given principally to magnets with strong spatial fluctuations of the exchange interaction. Naturally the magnetism of such systems differs substantially from the properties of ordinary ferromagnets; and for them, as for strongly disordered nonmagnetic systems, the methods of perturbation theory are inapplicable. The general concepts of the theory of disordered magnetic systems are presented with application to two models: disordered ferromagnets with interaction between nearest neighbors in the lattice, and ferromagnets in which the exchange interaction depends exponentially on the distance between the magnetic atoms. An example of ferromagnets of the second type is provided by dilute alloys of palladium with iron, cobalt, and manganese. The principal experimental results for these alloys are presented, and it is shown that the theory of disordered magnetic systems describes their properties well.