We all know that the four parameters Br/Hcb/Hcj/Bhmax are the main parameters of magnet material performance. The engineer of magnet application design selects the appropriate magnetic material according to the electromagnetic conversion corresponding to these parameters. After the magnet is prepared, we can obtain these parameters by testing the demagnetization curve, and there are some other parameters on the demagnetization curve that are also very important for the application of the magnet, such as HK.

The second quadrant of the demagnetization curve is the commonly used J-H curve, which is the correlation curve between the magnet's magnetic polarization intensity J and the external magnetic field intensity H, which can reflect the changes in the internal magnetic properties of the magnet. When the magnetic polarization intensity J on the J-H curve is 0, the corresponding magnetic field intensity is called the intrinsic coercivity Hcj. The value of intrinsic coercivity reflects the size of the anti-demagnetization ability of the permanent magnet material. From the J-H graph, we can find that when the external magnetic field keeps increasing, the magnetic induction intensity/magnetic polarization intensity of the magnet decreases very slowly, but when the external magnetic field is greater than a certain value, the magnetic induction intensity of the magnet decreases rapidly. We reduce this by 10% (or 5%) of the magnet’s magnetization, which requires the application of a reverse magnetic field, called Knee coercive force, which is HK. After the external magnetic field exceeds the tolerance of HK, it will cause a large Irreversible magnetic loss, magnet application designers are very concerned about this point.

Usually we can see the ratio of HK/Hcj from the demagnetization curve. This ratio is called squareness. Generally, we consider magnets with squareness greater than 90% to be qualified.

Hk/Hcj is also one of the important magnetic properties of permanent magnets. Like μrec, it characterizes the stability of the magnet under dynamic working conditions.

HK/Hcj=1/μrec, the squareness is inversely related to the magnet's recovery permeability μrec. The larger the HK/Hcj, the closer the recovery permeability μrec is to 1, and the material's ability to resist interference from external magnetic fields and environmental temperature factors is better. The stronger, the better its stability. Therefore, increasing HK has greater practical significance for high-temperature applications such as motors.