The nature of magnets
1. Antimagnetic
Solids exhibit diamagnetism when the magnetization M is negative. Metals such as Bi, Cu, Ag, and Au have such properties. In the external magnetic field, the magnetic induction intensity inside the magnetized medium is smaller than the magnetic induction intensity M in vacuum. The magnetic moment of the atoms (ions) of diamagnetic substances 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 originates from the change of electron orbital state in atoms. The diamagnetic ability of diamagnetic materials is generally weak,
2. Paramagnetic
The main feature of paramagnetic materials is that there is a permanent magnetic moment inside the atoms 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 the paramagnetic substance, the macroscopically non-magnetic Very weak magnetism. The magnetization is positive in the same direction as the external magnetic field, and is strictly proportional to the external magnetic field H. In addition to H, the magnetic properties of paramagnetic materials also depend on temperature. Its magnetic susceptibility H is inversely proportional to the external magnetic field H. absolute temperature T. Among them, C is called the Curie constant, which depends on the magnetization and the magnetic moment of the paramagnetic material. The magnetic susceptibility of paramagnetic materials is generally small, and H is about 10 at room temperature. Generally speaking, 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 that do not fill the shell, are paramagnetic substances.
3. Ferromagnetic
For Fe, Co, Ni and other materials, the magnetic susceptibility at room temperature can reach 10-3 orders of magnitude, and the magnetic properties of such materials are called ferromagnetism. Ferromagnetic materials can achieve extremely high magnetization even in weak magnetic fields, and remain strong when the external magnetic field is removed. The magnetic susceptibility is a positive value, but when the external magnetic field increases, its H becomes smaller due to the rapid saturation of the magnetization.
Ferromagnetic materials are very magnetic, mainly due to their strong internal exchange fields. The exchange energy of a ferromagnetic material is positive and large, so that the magnetic moments of adjacent atoms are oriented parallel (corresponding to a steady state), forming many small regions, or domains, inside the material. Each magnetic domain has about 1015 atoms. The magnetic moments of these atoms are aligned in the same direction, assuming a strong internal field called a "molecular field" inside the crystal, which is sufficient to automatically magnetize each magnetic domain to saturation.
This self-generated magnetization is called spontaneous magnetization. Due to its presence, ferromagnetic materials can be strongly magnetized in weak magnetic fields. Therefore, spontaneous magnetization is the basic feature of ferromagnetic materials, and it is also the difference between ferromagnetic materials and paramagnetic materials.
The ferromagnetism of ferromagnets only manifests below a certain temperature. Above this temperature, due to thermal turbulence inside the material, the spontaneous magnetization of the ferromagnet becomes zero and the ferromagnetism disappears. 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 Curie—the outer law is C, the Curie constant.
4. Antiferromagnetic
Antiferromagnetism means that the electron spins are aligned 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 opposite directions. The spontaneous magnetizations in the two sublattices are the same size and opposite in direction as the entire crystal. Antiferromagnetic materials are mostly non-metallic compounds such as MnO.
Spontaneous magnetization of antiferromagnetic materials cannot 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 decreases and gradually increases. At a certain temperature, the maximum value is reached. It is called the Curie point or Neil point of antiferromagnetic matter. The explanation for the existence of Neil points is that at very low temperatures, the magnetic moments are almost completely canceled due to the complete reversal of the spins of adjacent atoms, so the magnetic susceptibility is almost close to zero. The effect of spin inversion weakens and increases as the temperature increases.
