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Condensed-Matter and Materials Physics: The Science of the World Around Us
the U.S. economy is now attributed to information technology. Seminal inventions such as the transistor, the hard disk drive, and the communications laser eventually enabled the rise of the Internet, a watershed event in modern history. Indeed, some have compared the impact of the Internet to that of the printing press in extending broad access to information that was previously available only to the privileged.
The great force behind this modern industrial revolution has been miniaturization—the repeated shrinking of the devices that process, store, and communicate information. To extend the IT revolution, new devices will have to be invented. Smaller devices tend to be faster. More important, they tend to be cheaper because many more devices can be manufactured at the same time. More devices per dollar mean more function per dollar, and ever-more-affordable function has enabled the industry to expand its products from yesterday’s mainframe computers to today’s bewildering array of consumer products. Virtually every electronic product contains at least one microprocessor, and game machines now pack the computational power of supercomputers from just a few years ago.
The devices of IT have not just “shrunk.” They have shrunk in an exponentially compounding fashion. In 1965, it was noted that the number of transistors that could be built on a single silicon chip was doubling every few years. This doubling trend has roughly persisted over the past 40 or so years, elevating the trend to the status of a “law”—Moore’s law. But Moore’s law is not unique in IT. Analogous exponential doubling trends have been noted for information storage capacities of hard disk drives, digital communication rates, and many other key performance indicators of IT. Over time, such powerful trends can change the world. For example, since the introduction of the hard disk drive about 50 years ago, the areal density of information storage has increased by a factor of roughly 100 million. The resultant dive in the cost of information storage has been a key enabler for the rise of the Internet and the explosive growth of electronic commerce.
What is required to drive such powerful trends? It is the repeated reinvention of the devices that store, process, and communicate information. For instance, the introduction to hard disk drives in 1998 of read-head sensors based on the giant magnetoresistance (GMR) effect greatly accelerated the above-mentioned doubling trend in information storage capacity. GMR is a subtle collective effect in magnetic materials, unknown to physics before 1988. It is striking that GMR was put to practical use in almost all computers manufactured worldwide by the late 1990s. Harnessing the effect for information technology involved the development of practical methods for manufacturing multilayered structures consisting of thin ferromagnetic films separated by metallic spacers of precisely controlled nanometer-scale thickness. A magnetic field passing through such a structure determines whether the magnetic moments of adjacent ferromagnetic films are aligned parallel or antiparallel. Through the phenomenon of spin scattering, this alignment gives rise to low and high states of electrical resistance, respectively, corresponding to the 0 and 1 states of binary digital information. Thus, this “spin valve sensor”