conditions. It is found that the shape of the hysteresis loops changes with the amplitude and

frequency of AC magnetic field strength. The changes of the shape of the AC hysteresis loops are

caused by the waveform distortion of magnetic field intensity H. In a weak AC magnetic

field，when magnetic induction intensity B is changed sinusoidally，the magnetic field

intensity H is also changed sinusoidally. At this time，the shape of the hysteresis loop is

elliptical. When the mag-netic field intensity increases，H-t curves show different degrees of distortion. The

stronger the magnetic field is， the more severe the distortion of H - t curves is. At

this time，the shape of hysteresis loops becomes bended and sharp，shows a fat“S”shape. These

changes in the shape of hysteresis loops are explained through theoretical anal-ysis and experimental fitting.