The hysteresis loop relationship between magnetization intensity (vertical axis) and H-field (horizontal axis).

Inside ferromagnetic materials, like Paramagnetism materials, there are many unpaired electrons. Because of Exchange interaction, the spins of these electrons tend to be in the same direction as the spins of adjacent unpaired electrons. Due to the fact that ferromagnetic materials are divided into many magnetic domains, although the spins of all electrons within the domains are unidirectional, resulting in a "saturation magnetic moment", the direction and magnitude of the magnetic moment are different between domains. Therefore, the net magnetic moment and magnetization vector of an unmagnetized ferromagnetic material are equal to zero.

Assuming an external magnetic field is applied, the magnetic moments of these domains tend to be in the same direction as the external magnetic field, thus forming a potentially strong magnetization vector and its induced magnetic field. As the external magnetic field increases, the magnetization intensity also increases until the "saturation point", where the net magnetic moment equals the saturation magnetic moment. At this point, increasing the external magnetic field will not change the magnetization intensity. Assuming that the external magnetic field is weakened now, the magnetization intensity will also decrease accordingly. But it will not have the same magnetization intensity as before for the same external magnetic field. The relationship between magnetization and external magnetic field is not a one-to-one correspondence. The curve of magnetization intensity compared to the external magnetic field forms a hysteresis loop.

Assuming that after reaching the saturation point again and removing the external magnetic field, ferromagnetic materials can still maintain some magnetization state, and the net magnetic moment and magnetization vector are not equal to zero. Therefore, the ferromagnetic material after magnetization treatment has a "spontaneous magnetic moment".