ABSTRACT
37It is generally accepted that spin polarization plays a major role in spintronics [1, 2]. Indeed, one cannot adequately discuss spin-dependent transport without addressing the question of spin imbalance in a given system. The topics included in this chapter are chosen by the association one might have with the words spin polarization. When we think about it, the first thing that comes to mind is probably the asymmetry in the density of states (DOS) in magnetic materials. However, spin polarization may also be related to different transport, magnetic, and optical properties of charge carriers (see Chapters 2 and 6, Volume 2), spin injection into paramagnetic materials (see Chapter 5, Volume 1, Chapter 3, Volume 2, and Chapter 4, Volume 3), transmission coefficients through interfaces and tunnel barriers (see Chapters 4, 11, and 12 in Volume 1), and different electronic properties of surfaces and the bulk. This indicates that the concept of spin polarization is not self-evident and must be defined [3]. These definitions are naturally linked to spin measurement techniques, as we will discuss later.
