ABSTRACT
It 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. že topics included in this chapter are chosen by association one might have with the words spin polarization. When we think about it, the ¢rst 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 di¥erent transport, magnetic, and optical properties of charge carriers (see Chapters 17 and 23 of this book), spin injection into paramagnetic materials (see Chapters 6, 18, and 29), transmission coe«- cients through interfaces and tunnel barriers (see Chapters 4, 10, and 11), and di¥erent electronic properties of surfaces and the bulk. žis indicates that the concept of spin polarization is not self-evident and must be de¢ned [3]. žese de¢nitions are naturally linked to spin measurement techniques, as we will discuss later.
