What are the categories of magnetism?

Date:2018-02-02 Author: Click:

In general, the magnetism of a substance can be divided into diamagnetic, paramagnetic, ferromagnetic, antiferromagnetic, and ferromagnetic.
1. Antimagnetism
When the magnetization M is negative, the solid appears to be diamagnetic. Bi, Cu, Ag, Au and other metals have this property. In an external magnetic field, the magnetic induction inside such a magnetized medium is less than the magnetic induction M in a vacuum. The magnetic moment of an Atom(Ion) of a diamagnetic material 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 senses a magnetic moment opposite the direction of the external magnetic field, which is characterized as diamagnetic. Therefore, diamagnetism comes from the change in the state of the electron orbit in the Atom. The diamagnetic properties of diamagnetic substances are generally weak, and the magnetization H is generally about -10 ^ -5, which is negative.
2. paramagnetism
The main feature of paramagnetic materials is that there is a permanent magnetic moment inside the atom regardless of the presence of an external magnetic field. However, in the absence of an external magnetic field, due to the irregular thermal vibration of the atoms of the paramagnetic material, there is no magnetism at the macro level; Under the action of an external magnetic field, each atomic magnetic moment is more regularly oriented, and the material shows extremely weak magnetism. The magnetization is consistent with the direction of the external magnetic field,
It is positive and strictly proportional to the external magnetic field H.
The magnetism of paramagnetic materials depends on temperature in addition to H. Its susceptibility H is inversely proportional to the absolute temperature T.
In the formula, C is called the Curie constant, depending on the magnetization and magnetic moment of the paramagnetic material.
The magnetization of paramagnetic substances is generally very small, and H is about 10 ^ -5 at room temperature. An atom or molecule that generally contains an odd number of electrons. The atoms or ions of electrons that are not filled with shells, such as transition elements, rare earth elements, steel elements, and aluminum platinum, all belong to paramagnetic substances.
3. Ferrimagnetic
For substances such as Fe, Co, Ni, the magnetic rate at room temperature can reach an order of 10 ^ -3, and the magnetism of such substances is said to be ferromagnetic.
Even if the ferromagnetic material is in a weaker magnetic field, it can obtain a very high magnetic susceptibility. Its magnetic susceptibility is positive, but when the outer field increases, its magnetization quickly reaches saturation, and its H becomes smaller.
The ferromagnetic materials have strong magnetism, 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 parallel(corresponding to the stable state), and many small areas -- magnetic domains -- are formed inside the material. Each domain has about 1015 atoms. The magnetic moments of these atoms are arranged in the same direction, assuming that there is a strong inner field called a "molecular field" inside the crystal, and the "molecular field" is sufficient to automatically magnetize each magnetic domain to saturation. This spontaneous magnetization is called spontaneous magnetization. Due to its existence, ferromagnetic materials can be strongly magnetized under weak magnetic fields. Therefore, spontaneous magnetization is the basic feature of ferromagnetic material, and it is also the difference between ferromagnetic material and paramagnetic material.
The ferromagnetism of the ferromagnet is only displayed below a certain temperature. Above this temperature, the spontaneous magnetization becomes 0 and the ferromagnetism disappears due to the thermal disturbance inside the material that destroys the parallel orientation of the electron spin magnetic moment. This temperature is called Curie Point. Above the Curie point, the material exhibits strong paramagnetism, and its relationship between susceptibility and temperature obeys Curie-Weiss's law.
Where C is the Curie constant.
4. Anti-ferromagnetism
Anti-ferromagnetism refers to the reverse parallel arrangement of electron spins. There is a spontaneous magnetization in the same sublattice, and the electron magnetic moments are arranged in the same direction; In different sublattices, the electron magnetic moments are arranged in reverse. The spontaneous magnetization in the two sublattices is the same in size, opposite in direction, and the entire crystal. Anti-ferromagnetic substances are mostly non-metallic compounds such as MnO.
At any temperature, no spontaneous magnetization of antiferromagnetic substances can be observed, so its macroscopic properties are paramagnetic, M and H are in the same direction, and the susceptibility is positive. When the temperature is very high, it is very small; The temperature decreases and gradually increases. At a certain temperature, reach the maximum value. The Nair temperature of an antiferromagnetic material. The explanation for the existence of the Nair point is that at an extremely low temperature, the magnetic moment is almost completely offset because the spin of the adjacent atom is completely reversed, so the susceptibility is almost zero. When the temperature rises, the effect of the spin reverse decreases and increases. When the temperature rises above the Nell point, the thermal disturbance has a greater impact. At this time, the anti-ferromagnet and the paramagnetic body have the same magnetization behavior.
5. Ferrimagnetic
Ferrimagnetic refers to a magnetic material formed with two sets of sublattices. The magnetic moment direction of different sublattices is the same as that of anti-ferromagnetic, but the magnetization of different sublattices is different and can not be completely offset, so there is a residual magnetic moment called ferromagnetic. Anti-ferromagnetic materials are mostly alloys, such as TbFe alloys. Ferromagnetic also has a critical temperature from ferromagnetic to paramagnetic, called Curie temperature.