Magnetism and Magnetic Materials

    When it comes to magnetic materials, we have to talk about "magnetism" first. Experiments show that any substance can be more or less magnetized in an external magnetic field, but the degree of magnetization is different. According to the properties of substances in external magnetic fields, substances can be divided into five categories: paramagnetic substances, diamagnetic substances, ferromagnetic substances, ferrimagnetic substances, and antiferromagnetic substances:

Paramagnetic substance: It is a substance that can be magnetized according to the direction of the magnetic field when it is moved close to the magnetic field, but it is very weak and can only be measured with a precision instrument; if the external magnetic field is removed, the internal magnetic field will also return to zero, cause it to be non-magnetic. Such as aluminum, oxygen, etc.

 Diamagnetic material: It is a material with a negative magnetic susceptibility. When subjected to an external magnetic field, an induced electron circulation is generated in the molecule, and the magnetic moment generated by it is opposite to the direction of the external magnetic field. The magnetic field is in the opposite direction. All organic compounds are diamagnetic, graphite, lead, water, etc. are diamagnetic substances.

Ferromagnetic material: It is a material that is magnetized under the action of an external magnetic field, even if the external magnetic field disappears, it can still maintain its magnetized state and has a magnetic material. Iron, cobalt, and nickel are all ferromagnetic materials.

 Ferrimagnetic material: The macroscopic magnetism is the same as that of ferromagnetism, but the magnetic susceptibility is lower. The typical ferrimagnetic material is ferrite. The most significant difference between them and ferromagnetic substances is the difference in the internal magnetic structure.

 Antiferromagnetic substances: Inside antiferromagnetic substances, the spins of adjacent valence electrons tend to be in opposite directions. This substance has a net magnetic moment of zero and produces no magnetic field. This material is relatively uncommon, and most antiferromagnetic materials exist only at low temperatures. Assuming the temperature exceeds a certain value, it usually becomes paramagnetic. For example, chromium, manganese, etc. are all antiferromagnetic.