Magnetism, like that of high-performance magnets, is one of the basic properties of matter. All substances are magnetic media, which can be divided into three types:
1. Paramagnetic substances: Such substances generate an additional magnetic field in the same direction as the external magnetic field under the action of the magnetic field. Most substances belong to this category.
2. Diamagnetic substances: These substances generate an additional magnetic field in the opposite direction to the external magnetic field under the action of the magnetic field, such as copper and inert gases.
3. Ferromagnetic substances: These substances generate a strong additional magnetic field in the same direction as the external magnetic field under the action of the magnetic field. For example, iron, cobalt, and nickel are ferromagnetic substances.
According to the hypothesis first put forward by Ampere, in the molecules of paramagnetic substances, there are molecular currents with certain magnetic moments. In the absence of an external magnetic field, due to the thermal motion of molecules, the orientations of these molecular currents are irregular. As a result, the magnetic fields they generate average out to zero, and no magnetism is exhibited externally. When an external magnetic field exists, these molecular currents are affected by the orientation effect of the external field, and their magnetic moments turn towards the direction of the external magnetic field, generating an additional magnetic field along the direction of the external magnetic field. This is the reason for the magnetization of paramagnetic substances.
The molecules that make up diamagnetic substances have an internal structure such that their molecular currents are zero in the absence of an external magnetic field. When an external magnetic field is introduced, just as an induced current is generated when a magnetic field is introduced into a closed circular wire, induced currents are also generated in the molecules of these diamagnetic substances. Since there is no resistance in the molecules, just like in superconductors, once the current is generated, it will circulate forever until the external magnetic field is removed and a reverse induced current is generated to cancel it out. Under the action of an external magnetic field, all magnetic media will generate induced molecular currents. That is to say, diamagnetism is common to all substances. However, in the molecules of paramagnetic substances, the magnetic moment of the molecular current is much larger than that of the induced current, so the diamagnetism of the substance is masked, and only paramagnetism appears.
Regarding the cause of ferromagnetism, there is a magnetic domain hypothesis. The magnetic moments of individual atoms of many substances are on the same order of magnitude. So it's not that the magnetic moments of atoms are affected by the magnetic field that causes the difference between ferromagnetic substances and other magnetic media. Instead, it's because the atoms of ferromagnetic substances are more easily arranged under the action of an external magnetic field. Why are the atomic magnetic moments in ferromagnetic substances so easily arranged? This is because there is a molecular field generated by the strong interaction between atoms (called exchange force) in ferromagnetic substances.
The effect of the molecular field is the same as that of a magnetic field, causing the orientation and arrangement of the atomic magnetic moments. The magnitude of the molecular field is much stronger than that of an ordinary magnetic field. For example, in iron at room temperature, more than 95% of the atomic magnetic moments are arranged in orientation due to the action of the molecular field. However, ferromagnetic substances do not exhibit magnetism before being magnetized. This is because each ferromagnetic substance is actually divided into many small regions, and we call such small regions magnetic domains. The molecular field makes the magnetic moments of each atom in each magnetic domain arranged in the same direction, but the directions of the magnetic moments of each magnetic domain are different from each other. Therefore, in the absence of an external magnetic field, although the atomic magnetic moments within each magnetic domain have almost all been arranged, the total magnetic moment of the ferromagnetic substance is still zero, and the entire ferromagnetic substance does not exhibit magnetism. After an external magnetic field is applied, the directions of the magnetic moments of each magnetic domain turn towards the direction of the external magnetic field, and the total magnetic moment of the ferromagnetic substance becomes non-zero.
Given that the atomic magnetic moments in each magnetic domain have already been oriented in the absence of an external magnetic field, ferromagnetic substances also exhibit strong magnetism in a relatively small external magnetic field. When the temperature exceeds a certain value (the Curie point), the magnetic domains break down, and the ferromagnetic substance loses its ferromagnetic properties and becomes the same as an ordinary paramagnetic substance.
