Magnetic materials are composed of ferromagnetic materials or ferromagnetic materials. Under the action of an external magnetic field H, there must be corresponding magnetization m or magnetic induction B. their curve of change with magnetic field H is called magnetization curve (m~h or b~h curve). Generally speaking, the magnetization curve is nonlinear and has two characteristics: magnetic saturation and hysteresis. That is, when the magnetic field intensity H is large enough, the magnetization m reaches a certain saturation value MS, continues to increase h, and MS remains unchanged; And when the m value of the material reaches saturation and the external magnetic field H decreases to zero, m does not return to zero, but changes along the MSMR curve. The working state of the material is equivalent to a point on the m~h curve or b~h curve, which is often called the working point.