Theory of itinerant ferromagnetism

A ’’fluctuating band’’ picture of ferromagnetism is presented which agrees qualitatively with the entire range of phenomena in itinerant ferromagnets at energies comparable to or less than thermal, and at temperatures up to the Curie point and above, as well as providing a unified picture in which band theory clearly is correct at low temperature and magnetic fluctuations dominate the thermodynamics in the neighborhood of the Curie point. These phenomena include the insensitivity of the magnetic features of the band structure (the exchange splitting) to the temperature and phase transitions as found by Mook et al. and Lonzarich and Gold. The basic idea used is that the bands are for the most part locally determined, and therefore may be calculated in the presence of fairly long wavelength and slow fluctuations in the magnetisation before averaging thermodynamically over these fluctuations. The effect of the magnetic fluctuations on the single electron states in the band is calculated by a perturbation pro...