In magnetic disk drive storage technology, the fluctuation of magnetization due to thermal agitation. When the areal density -- the number of bits that can be stored on a square inch of disk media -- of a disk medium reaches 150 gigabits per square inch, the magnetic energy holding the bits in place on the medium becomes equal to the ambient thermal energy within the disk drive itself. When this happens, the bits are no longer held in a reliable state and can "flip," scrambling the data that was previously recorded.
Because of superparamagnetism, hard drive technologies are expected to stop growing once they reach a density of 150 gigabits per square inch.
