Demagnetization Curve

The part of the maximum hysteresis loop of the permanent-magnetic material in the second quadrant is called the demagnetization curve as shown in the next figure. It is the basic characteristics curve of the permanent-magnetic material. On the demagnetization curve, the magnetic flux density has positive values, but the magnetic field intensity has negative values. It means that the permanent-magnetic material is applied with demagnetization magnetic field intensity. Since H_m and B_m are in opposite directions, when the magnetic flux goes through the permanent-magnetic material, the magnetic potential difference along the direction of the magnetic flux does not drop, but rises. Therefore, the permanent-magnetic material is a magnetic source, similar to the electric source in the electric circuit.

The two extreme positions on the demagnetization curve are the two significant parameters to represent the magnetic characteristics of the permanent-magnetic material. On the demagnetization curve, the value of the magnetic flux density corresponding to zero magnetic field intensity H is termed residual flux density B_r, on the other hand, the value of the magnetic field intensity corresponding to zero magnetic flux density B is termed coercive field force H_c. The produce of the magnetic flux density and the magnetic field intensity at any point on the demagnetization curve is termed magnetic energy product (BH), which is proportional to the magnetic energy density possessed by the permanent magnet at the given operating situation. At the two extreme positions (B = B_r, H = 0) and (B = 0, H = H_c), the magnetic energy product is equal to zero. Somewhere at an intermediate position, the magnetic energy reaches its maximum value and is termed maximum magnetic energy product (BH)_max, which is another significant parameter to represent the magnetic characteristics of the permanent-magnetic material. To some permanent-magnetic materials with linear demagnetization curve, it is obvious that at (B = B_r / 2, H = H_c / 2), the magnetic energy product reaches its maximum value, i.e. (BH)_max = B_r  H_c / 4.