Spin torque

Article By:

Åkerman, Johan Department of Physics, University of Gothenburg, Gothenburg, Sweden.

Last reviewed:2014

DOI:https://doi.org/10.1036/1097-8542.YB140218

All electronic devices utilize flows of electrons, carrying with them a negative charge that is used to both store and control the flow of information. Besides their negative charge, electrons also have a number of other physical properties that are transported with them, the most notable being spin. Spin is an intrinsic, quantized form of angular momentum that, for the purposes of this discussion, can be viewed as a direction associated with each electron, often pictured as an arrow. In charged particles, such as electrons, the presence of spin also gives the particle a magnetic moment, and thus a magnetic field oriented along the axis of the spin; in electrons, the magnetic moment points in the direction opposite to that of the spin. In nonmagnetic metals, the spins of the electrons are randomly oriented, and, as a result, currents in such materials typically carry no overall spin. In ferromagnetic metals, on the other hand, the equilibrium electronic state is spin-polarized; that is, the electron population is divided into two subpopulations with opposite spin directions and with more spins pointing in one direction than in the other. As a consequence, currents in ferromagnetic metals are inherently spin-polarized and transmit a flow of spin alongside the flow of charge, allowing spin to play a significant role in device function. A thin ferromagnetic layer can function as a very effective spin polarizer of unpolarized electrons entering from a nonmagnetic metal, a process that is broadly similar to the polarization of sunlight by polarizing lenses. While a growing number of spin-dependent phenomena that such currents can generate, control, and make use of are known, two fundamental effects stand out: (1) the flow of spin-polarized currents can effectively be controlled by manipulating magnetization configurations, and conversely, (2) spin-polarized currents can be used to control the magnetization direction and the dynamic properties of magnetic systems. The second effect is called spin torque, and is the focus of this article.

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