Spintronics: The Future of Data Storage?
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Spintronics: The Future of Data Storage?
S.A. Wolf, Daryl Treger, and Almadena Chtchelkanova Abstract Reasearch and technology developments in the field of spintronics have grown tremendously in the past 10-15 years and already have had a major impact on the data storage industry. The future looks even brighter, as many new spintronic discoveries have been recently made that promise an even bigger impact in the future. This article summarizes the past accomplishments, describes some of the major discoveries that will have a lasting impact on the field, and discusses some of the technologies that may revolutionize data storage in the next decade. Keywords: magnetic, memory, spintronic.
Introduction The last 20 years have seen the emergence of significant new developments in magnetic data storage. The very rapid growth in magnetic disk drive capacity has been fueled by a series of discoveries and their rapid conversion into technology.
Giant Magnetoresistance This article’s starting point is the important discoveries in the late 1980s of giant magnetoresistance (GMR) in alternating magnetic–nonmagnetic multilayers independently by Fert1 and Grunberg.2 These discoveries were the first steps on the path toward the integration of spintronic devices into information technology. These early discoveries, however, were done at low temperatures and relatively high magnetic fields. It was not until the discoveries of GMR in Co/Cu multilayers at room temperature and low magnetic fields,3 oscillatory interlayer coupling through Cu and other nonmagnetic noble and transition metals,4 and interface engineering to create large GMR values in very small fields5 that spintronics became a technological reality.
Spin Valve Technology The first really significant technological discovery was the spin valve,6 illustrated in Figure 1. This is a multilayer structure
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incorporating a “magnetically hard,” or pinned, ferromagnetic layer on top (consisting of a bilayer of an antiferromagnet strongly coupled to a ferromagnetic layer), a nonmagnetic conductor layer (typically copper) in the middle, and a “magnetically soft,” or “free,” layer on the bottom just above the substrate. The pinning of the top ferromagnetic layer significantly biases the switching field for this layer far away from zero field, so it is not free to rotate at low fields. Thus, “pinning” means that this layer is always pointing in the same direction relative to the substrate. If the magnetic moments in the pinned and free layers are parallel, the current can flow easily throughout the structure, and the resistance is low. However, if the layers are magnetized oppositely, the current is impeded, and the resistance is high. A spin valve can function as either a magnetic field sensor or a hysteretic memory device, depending on how easy it is to rotate the moment of the free layer from parallel to antiparallel with respect to the pinned layer. In the early 1990s, IBM started a project to develop such GMR devices as read-head sensors for magnetic disk drives and introdu
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