Classification of matter magnetism
1.Antimagnetic
When the magnetization M is negative, the solid appears diamagnetic. Metals such as Bi, Cu, Ag, and Au have this property. In an external magnetic field, the magnetic induction intensity inside such a magnetized medium is smaller than the magnetic induction intensity M in a vacuum. The magnetic moment of the atom (ion) of the diamagnetic substance should be zero, that is, there is no permanent magnetic moment. When a diamagnetic substance is placed in an external magnetic field, the external magnetic field changes the electronic orbit, induces a magnetic moment opposite to the direction of the external magnetic field, and behaves as diamagnetic. So diamagnetism comes from the change of the state of the electron orbit in the atom. The diamagnetic properties of diamagnetic materials are generally weak, and the magnetic susceptibility H is generally about -10-5, which is a negative value.
2. Paramagnetic
The main feature of a paramagnetic substance is that there is a permanent magnetic moment inside the atom, regardless of the presence or absence of an external magnetic field. However, in the absence of an external magnetic field, due to the irregular thermal vibration of the atoms of paramagnetic substances, there is no magnetism at a macroscopic view; under the action of an external magnetic field, the magnetic moments of each atom are more regularly oriented, and the substance shows extremely weak magnetic properties. The magnetization is consistent with the direction of the external magnetic field,
Is positive and strictly proportional to the external magnetic field H.
In addition to the magnetic properties of paramagnetic substances, H is also dependent on temperature. Its magnetic susceptibility H is inversely proportional to the absolute temperature T.
In the formula, C is called the Curie constant, which depends on the magnetization and magnetic moment of the paramagnetic substance.
The magnetic susceptibility of paramagnetic materials is generally very small, and H is about 10-5 at room temperature. Generally, atoms or molecules containing an odd number of electrons, atoms or ions that do not fill the shell, such as transition elements, rare earth elements, steel elements, and metals such as aluminum and platinum are paramagnetic substances.
3.Ferromagnetic
For materials such as Fe, Co, and Ni, the magnetic susceptibility can reach the order of 10-3 at room temperature. The magnetic properties of such materials are called ferromagnetism.
The ferromagnetic substance can obtain extremely high magnetization even in a weak magnetic field, and can retain extremely strong magnetism even after the external magnetic field is removed. Its magnetic susceptibility is positive, but when the external field increases, its H becomes smaller because the magnetization quickly reaches saturation.
Ferromagnetic materials have very strong magnetism, mainly due to their strong internal exchange fields. The exchange energy of a ferromagnetic substance is positive and larger, so that the magnetic moments of adjacent atoms are aligned in parallel (corresponding to the steady state), and many small regions—magnetic domains—are formed inside the substance. Each magnetic domain has approximately 1015 atoms. The magnetic moments of these atoms are aligned in the same direction. It is assumed that a strong internal field called a "molecular field" exists inside the crystal. The "molecular field" is sufficient to automatically magnetize each magnetic domain to a saturated state. This spontaneous magnetization is called spontaneous magnetization. Due to its existence, ferromagnetic materials can be strongly magnetized in a weak magnetic field. Therefore, spontaneous magnetization is the basic feature of ferromagnetic materials, and also the difference between ferromagnetic materials and paramagnetic materials.
Ferromagnetism only manifests below a certain temperature. Above this temperature, the spontaneous magnetization becomes zero and the ferromagnetism disappears because the thermal disturbance in the material destroys the parallel orientation of the electron spin magnetic moment. This temperature is called the Curie point. Above the Curie point, the material exhibits strong paramagnetism, and the relationship between its magnetic susceptibility and temperature obeys the Curie-Weiss law,
Where C is the Curie constant.
4.Antiferromagnetic
Antiferromagnetism refers to the fact that electron spins are aligned in antiparallel. In the same sublattice, there are spontaneous magnetizations, and the electronic magnetic moments are aligned in the same direction; in different sublattices, the electronic magnetic moments are aligned in the opposite direction. The spontaneous magnetizations in the two sublattices have the same magnitude and opposite directions. Antiferromagnetic substances are mostly non-metallic compounds such as MnO.
At any temperature, no spontaneous magnetization of the antiferromagnetic substance can be observed, so its macroscopic characteristics are paramagnetic, M and H are in the same direction, and the susceptibility is positive. When the temperature is high, it is extremely small; the temperature decreases and gradually increases. At a certain temperature, it reaches the maximum value. It is called the Curie point or Neil point of the antiferromagnetic substance. The explanation for the existence of the Neil point is that at extremely low temperatures, the magnetic moments of the adjacent atoms are almost completely cancelled because the spins of adjacent atoms are completely reversed, so the susceptibility is almost close to zero. When the temperature rises, the effect of reversing the spin decreases and increases. When the temperature rises above the Neil point, the influence of thermal turbulence is greater. At this time, the antiferromagnet and the paramagnetic body have the same magnetization behavior.