Giant Magnetoresistance. Spin Valves

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With different areas of magnetism being incorporated these days into conventional electronics, there is no surprise that magnetoresistance (MR) has found its way into common everyday usage, like the read head in personal computers. But read heads are not the only devices employing the magnetoresistive effect. In fact, MR is one of those fields that result in many other technologically exploitable applications, proving that the magnetoresistive effect cannot be underestimated and has yet to be used at its full potential. Whether rotational speed control devices, high current monitoring devices for power lines, or positioning control devices in robotic systems, MR has been incorporated into systems that require a high sensitivity to magnetic fields. Unfortunately, for many readers it is not always obvious where the consequences of MR are demonstrated within a device, especially when overshadowed by other more dominant physical phenomena. Therefore, this chapter discusses the physical basics of the magnetoresistive effect and emphasizes a few material aspects of what characterizes magnetoresistive configurations. In the end, it is hoped that the existence of the magnetoresistive effect in devices other than read heads is unveiled to the reader who will more readily recognize MR configurations in other applications.In principle, magnetoresistive structures are usually composed of several layers of different materials that can be combinations of ferromagnetic, antiferromagnetic, or nonmagnetic materials, metals or semiconductors, or even organic compounds. Predicted by calculations based on spin-dependent energy bands, and confirmed through a variety of experiments, GMR can be explained by a spin-dependent conductivity in these multilayer stacks, highly influenced by scattering at interfaces between ferromagnetic and nonmagnetic layers. A common feature of MR structures is that their layers are coupled in a specific way, rendering them certain magnetic and electric properties while allowing selective passage for one spin component of the electronic current density. Changes in electrical resistance occur when a varying external magnetic field overcomes the coupling between the layers of the compound. Only those variations in magnetoresistance that are significant enough are of technological interest, and these large MR variations are commonly known as giant magnetoresistance (GMR). In view of the spin-dependent conductivity variation, the GMR mechanism is different from the better known MR phenomena studied in the past. GMR is also different from colossal magnetoresistance (CMR) for which a variety of mechanisms have been proposed such as electron–phonon coupling, double exchange, electron–magnon interactions, or phase- and charge-segregation.Historically, GMR was initially reported in the late 1980s when the results of now celebrated experiments were first published [1, 2]. However, it was not until November 1997 that they made their appearance on the market, when IBM introduced commercial multilayer GMR sensors as magnetic recording read heads. These were incorporated into disk drive products Deskstar 16GP where extremely small magnetic bits at an areal density of 2.69 Gb∕in2 were read. Deskstar 16GP contained 95-mm-diameter disks, each with a storage capacity of more than 3.2GB, resulting in a total data storage capacity of 16.8GB. The current flowed parallel to the layers in the device in a so-called current-in-plane (CIP) geometry, requiring the sensor to be electrically insulated from the conducting magnetic shields. Since then, many advances have occurred in the world of GMR sensors, and it may be easy to forget how it started and where technological progress has taken us in the meantime.