Fundamentals of Magnetoresistance Effects

Giant magnetoresistance (GMR) effect is the most fundamental phenomenon in the field of spintronics. This chapter starts with a brief explanation of the conventional MR in ferromagnetic materials that had been known before the discovery of GMR. Here, it is noted that the word of GMR means a giant magnetoresistance (MR) effect, and also contains a giant tunnel magnetoresistance (TMR) effect (once called the tunneltype GMR), a colossal magnetoresistance (CMR) effect observed in magnetic oxides, and further different types of giant MR effects in a broad sense. However, GMR normally concerns a giant MR effect observed in "metallic systems" composed of ferromagnetic and nonferromagnetic metals in a nanometer scale. The metallic systems are discussed here. GMR in metallic superlattices is caused by spin-dependent scattering of conduction electrons at the interface. The principle and the recent progress of TMR studies are reviewed.