In constructing a computer, or any other electronic device, it is important to store information on a temporary or a long-term basis. Information storage is done in memory devices, and different sorts of memories have been used since the introduction of the early magnetic core memories. Indeed, the increase in the capability of memory—that is the amount of information that can be stored in a given volume of space—has improved even faster than the Moore's law curve for transistor density.

The current state of the art is represented by hard-disk memories. These are based on magnetics: information is stored as magnetic polarization on a disk and is read or written by a special head as the disk spins. The phenomenon involved here is called ...

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