The limit imposed, by the fluctuations of magnetization resulting from thermal agitation, on the areal density of the information that can be stored in the groups of tiny magnets called bits in magnetic recording. When the areal density reaches around one trillion bits per square inch, the magnetic anisotropy energy holding the bits in place on the medium becomes equal to the ambient thermal energy. At this point, the bits no longer stay in a stable state and can flip between 0 and 1 randomly. One way to overcome the superparamagnetic limit is to use antiferromagnetically coupled media in which two magnetic layers with moments pointing in opposite directions reduce the effective magnetic volume.