Ferromagnetism of metallic systems, especially those including transition metals, has been a controversial subject of modern science for a long time.
This controversy stems from the apparent dual character of the rfelectrons responsible for magnetism in transition metals, i.e., they are itinerant electrons described by band theory in their ground state, while at finite temperatures they show various properties that have long been attributed to a
system consisting of local magnetic moments. The most familiar example of these properties is the Curie-Weiss law of magnetic susceptibility obeyed by almost all ferromagnets above their Curie temperatures.