Jun-ichiro Inoue
Published: 2013-10-07
Total Pages: 85
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Novel magnetotransport phenomena appear when magnet sizes become nanoscale. Typical examples of such phenomena are giant magnetoresistance (GMR) in magnetic multilayers, tunnel magnetoresistance (TMR) in ferromagnetic tunnel junctions, and ballistic magnetoresistance (BMR) in magnetic nanocontacts. In this chapter, we first briefly review the relationship between spin-dependent resistivity and electronic structures in metals and alloys, and describe microscopic methods for investigating electrical transport. We then review the essential aspects of GMR, TMR, and BMR, emphasizing the role of the electronic structures of the constituent metals of these junctions and the effects of roughness on the electrical resistivity (or resistance). The important factors that control GMR are shown to be the spin-dependent random potential at interfaces and band matching/mismatching between magnetic and nonmagnetic layers. For TMR, several factors are shown to be important in determining the MR ratio, including the shape of the Fermi surface of the electrodes, the symmetry of the wave functions, electron scattering at interfaces, and spin-slip tunneling. An interpretation of TMR in Fe/MgO/Fe and of an oscillation of TMR is presented. TMR in granular films and in the Coulomb-blockade regime is also described. We also provide a brief explanation for other MR effects, such as normal MR, anisotropic MR (AMR) and colossal MR (CMR) in order to clarify the essential difference between these MRs and GMR, TMR, and BMR. These MR effects are attributed to the spin-dependent electrical currents produced in metallic ferromagnets. After the discovery of these different MR effects, the role of spin current was proposed, for example, spin Hall effect and the effects of spin transfer torque, which will be briefly explained in this chapter. The former orginates from the spin–orbit interaction, and can be observed even in nonmagnetic metals and semiconductors. It is closely related to the anomalous Hall effect observed in ferromagnetic metals. The spin transfer torque is an inverse effect of the MR. The MR is the resistivity change produced by magnetization rotation in ferromagnetic junctions, while the spin transfer torque is an effect in which spin-polarized current makes the magnetization rotate. Finally, we briefly introduce the coupled effects of spin, charge, and heat transport, which are called spin caloritronics.