1, diamagnetic
When the magnetization M is negative, the solid exhibits diamagnetism. Metals such as Bi, Cu, Ag, and Au have such properties. In an external magnetic field, the magnetic induction inside the magnetized medium is smaller than the magnetic induction M in the vacuum. The magnetic moment of the atom (ion) of the diamagnetic substance should be zero, that is, there is no permanent magnetic moment. When the diamagnetic material is placed in an external magnetic field, the external magnetic field changes the electron orbit and induces a magnetic moment opposite to the direction of the external magnetic field, which is expressed as diamagnetism. Therefore, diamagnetism is derived from changes in the state of the electron orbit in the atom. The diamagnetic resistance of the diamagnetic material is generally weak, and the magnetic susceptibility H is generally about -10-5, which is a negative value.
2, paramagnetic
The main feature of paramagnetic materials is that there is a permanent magnetic moment inside the atom whether or not the applied magnetic field is present. However, in the absence of an external magnetic field, due to the irregular thermal vibration of the atoms of the paramagnetic substance, macroscopically, there is no magnetism; under the action of an external magnetic field, the magnetic moment of each atom is relatively regularly oriented, and the substance exhibits extremely weak magnetic properties. The magnetization is consistent with the direction of the external magnetic field, is positive, and is strictly proportional to the external magnetic field H. The magnetic properties of paramagnetic materials depend on temperature in addition to H. Its magnetic susceptibility H is inversely proportional to the absolute temperature T. Where C is called the Curie constant, depending on the magnetization of the paramagnetic substance and the magnitude of the magnetic moment. The magnetic susceptibility of paramagnetic materials is generally small, and H is about 10 at room temperature. Generally, atoms or molecules containing an odd number of electrons, such as transition elements, rare earth elements, steel elements, and metals such as aluminum and platinum, which are not filled with shells, are paramagnetic substances.
3, ferromagnetism
For materials such as Fe, Co, Ni, etc., the magnetic susceptibility can reach 10-3 orders of magnitude at room temperature, and the magnetic properties of such materials are called ferromagnetism. Ferromagnetic materials can obtain extremely high magnetization even in a weak magnetic field, and retain strong magnetic properties when the external magnetic field is removed. The magnetic susceptibility is a positive value, but when the external field is increased, since the magnetization rapidly reaches saturation, its H becomes small.
Ferromagnetic materials are very magnetic, mainly due to their strong internal exchange field. The exchange energy of ferromagnetic materials is positive and large, so that the magnetic moments of adjacent atoms are oriented in parallel (corresponding to a steady state), and many small regions, magnetic domains, are formed inside the material. Each magnetic domain has approximately 1015 atoms. The magnetic moments of these atoms are arranged in the same direction, assuming that there is a strong internal field called a "molecular field" inside the crystal, and the "molecular field" is sufficient to automatically magnetize each magnetic domain to saturation.
This self-generated magnetization is called spontaneous magnetization. Due to its existence, ferromagnetic materials can be strongly magnetized in a weak magnetic field. Therefore, spontaneous magnetization is a basic feature of ferromagnetic materials, and it is also the difference between ferromagnetic materials and paramagnetic substances.
The ferromagnetism of ferromagnets only manifests below a certain temperature. Above this temperature, the spontaneous magnetization of the ferromagnetic body becomes zero due to thermal turbulence inside the material, and the ferromagnetic strength disappears. This temperature is called the Curie point. Above the Curie point, the material exhibits strong paramagnetism, and its relationship between magnetic susceptibility and temperature obeys Curie—the external law is C, which is the Curie constant.
4, antiferromagnetic
Antiferromagnetic means that the electron spins are arranged in antiparallel. In the same sublattice, there is spontaneous magnetization, and the electron magnetic moments are arranged in the same direction; in different sublattices, the electron magnetic moments are arranged in the reverse direction. The spontaneous magnetization in the two sublattices is the same in magnitude and opposite in direction to the entire crystal. Antiferromagnetic materials are mostly non-metallic compounds such as MnO.
No spontaneous magnetization of the antiferromagnetic material can be observed at any temperature, so the macroscopic properties are paramagnetic, M and H are in the same direction, and the magnetic susceptibility is positive. When the temperature is high, it is extremely small; the temperature is lowered and gradually increases. At a certain temperature, the maximum value is reached. It is called the Curie point or the Neil point of the antiferromagnetic substance. The explanation for the existence of the Neil point is that at very low temperatures, since the spin of the adjacent atoms is completely reversed, the magnetic moment is almost completely canceled, so the magnetic susceptibility is almost close to zero. When the temperature rises, the effect of the spin reversal is weakened and increased. When the temperature rises above the Neil point, the influence of the thermal turbulence is large, and the antiferromagnetic body has the same magnetization behavior as the paramagnetic body.