Onsager cavity fields in itinerant-electron paramagnets

Abstract

For an itinerant system of electrons we develop a first-principles theory of spin fluctuations. It is based on the idea of a generalized Onsager cavity field which varies slowly compared to the motion of individual electrons. It gives an account of both the good local moment and the weak itinerant ferromagnet limits within the same framework. We illustrate its consequences by explicit calculations for Fe and Ni.