Spintronics, the science and technology of generating, manipulating and detecting spin currents rather than charge currents, is acknowledged as a plausible continuation of electronics [309, 310, 311]. In terms of encoding information, the two states of a spin with reference to a quantization axis, i.e. spin-up or spin-down, may be used to implement a system of binary logic. The technological advantage of spintronics over electronics lies chiefly in significant reductions of energy usage for proper operation. Novel low dissipation devices are envisioned that combine high speed, high density and endurance with non-volatile memory technology. The latter feature is among the major advantages of magnetic storage, involving the continued availability of information when the device is powered down. In the following, we will motivate the strong present interest in spintronics, introduce the basic concepts underlying this recent field of materials studies and discuss various applications of these concepts. By developing the elemental tenets of spintronics, this chapter serves as a preparation for the following one, where we describe actual or potential realizations of spin-based devices by use of carbon nanostructures as material constituents.

Section 9.1 introduces the notion of spin current and the associated causal concept of the spin-resolved quasi-chemical potential. In Section 9.2, essential devices of spintronics circuits are specified: the spin valve, the spin transistor and the spin filter. The rest of this chapter focuses on the crucial processes of spin injection, detection and relaxation. In particular, the mechanisms that limit the lifetime of spin-polarized currents in transmission media are discussed in Section 9.4.

Spin Current

For initial motivation, we clarify the term spin current. Electric current usually refers to charge transport without transfer of net spin polarization. Denoting the overall charge current with I and the spin-resolved alpha (up) and beta (down) contributions to I with I↑ and I↓, respectively, we have

The net spin currrent (Is), in contrast, is found from

In terms of the current density j, this quantity is defined for the charge current as

involving the velocity operator with respect to the electron wave function.