Electronic Spin Transport

1. Overview And Chapter Objectives

As well as mass and charge, an electron has another intrinsic property, called spin. The spin property of the electron was demonstrated experimentally by Stern and Gerlach (1922). The word ‘spin’ was coined by Wolfgang Pauli (1926) to explain the fine-structure of atomic spectra, following the proposal of the two students Uhlenbeck and Goudsmith (1926). They proposed that the spin angular momentum, obeys the same quantization rules as those governing orbital angular momentum of atomic electrons. It had been shown later by Paul Dirac (1928) that electron spin arises naturally in the relativistic treatment of quantum mechanics.

Figure 1.

Classical representation of the spin action of electrons. The electromagnetic field emanating from an electron can be considered to emanate from an idealized tiny bar magnet with north and south poles (dipole).

Unlike position and momentum, which have classical analogs, spin does not. But if we think of spin in classical terms, we can think of a spinning charged particle as a loop of current. Thus, if a particle spins about the z-axis, then the spin vector S points along the z-axis. Since the spinning charge is negative, the left-hand rule can be applied. When the fingers of the left hand rotate (curl) in the direction of spin, the thumb will point in the direction of spin. Therefore, a spinning charge carrier produces a magnetic field similar to that of a tiny bar magnet. In this case, the spin vector S points to the south pole of the bar magnet. If the spinning particle is placed in a magnetic field, it tends to align the spin vector in the opposite direction to the magnetic field lines, as shown in the above figure.

Therefore, electrons have intrinsic angular momentum, which is composed of two components, namely angular orbital and spin angular momentum:

Quantization of angular momentum had already known for orbital angular momentum, and if the electron spin behaves the same way, an angular momentum quantum number s =±½ is required to give just two states. The spin angular momentum S has the following magnitude:

Therefore, this intrinsic electron property gives an additional z-component for the angular momentum:

Spin electronics (or Spintronics) refers to the study of the role played by electron spin in solid state physics and devices. Spintronics utilizes the electron spin degree of freedom for information storage and transmission. The spintronic devices use the electron spin to carry and switch information, rather than the electron charge transport. The application of spintronics in information processing is a relatively new endeavor and is motivated by the belief that spintronics may offer a more power-efficient route compared to the traditional transistors. In fact, spin transport and spintronics have seen interesting developments in the past decade and have continued to attract attention.