Magnetic information is stored as 'bits' on the hard disc of a computer, or on magnetic tape used for backing up computers. These bits correspond to small, uniformly magnetized regions 'written' into the magnetic medium by an applied field which defines the orientation of the magnetization. In the read process, which is the opposite of the write process, the magnetic orientation of the bits is 'read' by detecting the stray fields they generate using a read head sensor. A key advantage of magnetic media is that they are write/erasable, i.e. the writing process can be repeated at will and in the absence of a field the information is stored with the need for any power consumption (i.e. non-volatile).
The heart of a personal computer's data storage is the hard disc found in laptops and desktop machines. The hard disc consists of a glass plate on which a transition metal alloy film has been deposited (the magnetic medium) and is spun at typically 7200 RPM -- corresponding to a speed about 75 mph. The read head sensor is held on a moveable arm slider and flies at a height of about 100 nm above the surface of the disc -- equivalent to flying a 747 airliner 0.1 inches above the disc! Future increases in storage density will require even smaller fly heights and probably greater speeds.
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