Spin Torque Effects in Magnetic Systems: Theory

Manipulating the magnetization direction of a magnetic layer without the use of an external magnetic field represents an outstanding opportunity and challenge for spintronic applications [1]. In the last decade, vast experimental data and many detailed theoretical results had been available. We do not intend to review all the development made to date. Rather we want to provide a coherent picture of this subject for various phenomena involving the interplay between the spin transport and magnetization dynamics. There are already a few excellent review papers [2-5], on this same subject and we refer some of the details to these articles. In this chapter, we address the theoretical aspects of current-induced magnetization control using the so-called spin torque (ST) effect from both the transport and magnetization dynamics points of view. Although numerous models have been developed, discussing them all is far beyond the scope of this chapter and we rather refer the reader to specific literature. After a brief overview of ST studies in Section 8.1, transport characteristics in a number of magnetic systems such as spin valves (SVs), magnetic tunnel junctions (MTJs), and magnetic domain walls (DW) are described in Section 8.2. At the end of the section, several unconventional STs will be briefly discussed. In Section 8.3, the ST-driven magnetization dynamics for these magnetic systems is studied. Finally, perspectives and conclusions of the ST effects are given in Section 8.4.