遲滯現象

thumb|right|Electric displacement field of a ferroelectric material as the [[electric field is first decreased, then increased. The curves form a hysteresis loop.]] Hysteresis is the dependence of the state of a system on its history. For example, a magnet may have more than one possible magnetic moment in a given magnetic field, depending on how the field changed in the past. Such a system is called hysteretic. Plots of a single component of the moment often form a loop or hysteresis curve, where there are different values of one variable depending on the direction of change of another variab