<P> Magnetic hysteresis occurs when an external magnetic field is applied to a ferromagnet such as iron and the atomic dipoles align themselves with it . Even when the field is removed, part of the alignment will be retained: the material has become magnetized . Once magnetized, the magnet will stay magnetized indefinitely . To demagnetize it requires heat or a magnetic field in the opposite direction . This is the effect that provides the element of memory in a hard disk drive . </P> <P> The relationship between field strength H and magnetization M is not linear in such materials . If a magnet is demagnetized (H = M = 0) and the relationship between H and M is plotted for increasing levels of field strength, M follows the initial magnetization curve . This curve increases rapidly at first and then approaches an asymptote called magnetic saturation . If the magnetic field is now reduced monotonically, M follows a different curve . At zero field strength, the magnetization is offset from the origin by an amount called the remanence . If the H-M relationship is plotted for all strengths of applied magnetic field the result is a hysteresis loop called the main loop . The width of the middle section along the H axis is twice the coercivity of the material . </P> <P> A closer look at a magnetization curve generally reveals a series of small, random jumps in magnetization called Barkhausen jumps . This effect is due to crystallographic defects such as dislocations . </P>

Does magnetic hysteresis cause a field to be non-conservative