An Introduction to the Theory of Normal and Ferromagnetic Metals

Abstract

The proper understanding of spin electronics in metallic systems relies on an understanding of some basic metal physics. The material presented here is very brief and is covered in more detail in several excellent textbooks [1],[2]. The discussion starts with the definition of a metal and the justification for the frequently used independent particle model even when the interactions between the electrons are included. The role of impurities is also discussed and localized and extended states are defined. Because we are concerned with magnetic properties the most relevant perturbation will be that of a periodic magnetic field. This is characterized by the generalized susceptibility, which is a very useful concept as it allows us to consider the instability of a metal to ferromagnetism or to spindensity waves as well as the response of a paramagnetic metal to a magnetic impurity. Finally we consider strong coupling theory. This includes a discussion of the formation of local moments in a metal. Such a moment will be coupled to the conduction electrons and may be screened out by the Kondo effect. A dense array of Kondo type impurities will form a heavy Fermion compound, which is also described briefly. Alternatively, a dense array of such moments can interact with each other to form a spin glass or a ferromagnet.