Local-band theory of itinerant ferromagnetism. I. Fermi-liquid theory

Abstract

Itinerant ferromagnetism, particularly in iron, nickel, and cobalt, is described by a fluctuating-local-band theory, a new concept which generalizes the old band theory. In this description, the old unexplained localized-itinerant complementarity is resolved. It is noted that band structure itself is determined locally, on the scale of a few neighbors, and that the main coordinate which changes in space and time is $\hat{M}$, the collective variable representing the magnetization direction. Because these changes are relatively slow, their effects on band structure can be calculated in a perturbation theory. It is stressed that the classical variable $\hat{M}$ is appropriate, even though quantization is in some cases eventually necessary. In this paper a ferromagnetic-Fermi-liquid theory is set up and compared with previous versions which it generalizes. The range of validity of the theory is shown to extend well above $T_C$, the Curie temperature. Some estimates of $T_C$ on the basis of the new theory are obtained, which are encouraging. Agreement with a range of experiments is found. Papers II and III discuss nonlinear magnetization-fluctuation interactions, and their bearing on experiment.