Classification of magnetism and magnetic materials
Paramagnetic substance: It is a substance that can magnetize according to the direction of the magnetic field when moved close to it, but it is very weak and can only be measured with precision instruments; If the external magnetic field is removed, the internal magnetic field will also return to zero, resulting in its lack of magnetism. Such as aluminum, oxygen, etc. Antimagnetic substance: It is a substance 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 it produces is opposite to the direction of the external magnetic field. In other words, the direction of the magnetic field after magnetization is opposite to that of the external magnetic field. All organic compounds have diamagnetism, such as graphite, lead, water, etc. Ferromagnetic substance: It is a substance that can maintain its magnetic state even if the external magnetic field disappears after being magnetized by an external magnetic field. Iron, cobalt, and nickel are all ferromagnetic substances. Ferromagnetic material: Macroscopic magnetism is the same as ferromagnetism, except for a lower magnetic susceptibility. A typical ferromagnetic material is ferrite. The most significant difference between them and ferromagnetic materials is the difference in their internal magnetic structure. Antiferromagnetic material: Within an antiferromagnetic material, the spin of adjacent valence electrons tends in opposite directions. This substance has a net magnetic moment of zero and does not generate a magnetic field. This substance is relatively uncommon, and most antiferromagnetic substances only exist at low temperatures. Assuming the temperature exceeds a certain value, it usually becomes paramagnetic. For example, chromium, manganese, etc. all have antiferromagnetic properties. We call paramagnetic and diamagnetic substances weak magnetic substances, and ferromagnetic and ferrimagnetic substances strong magnetic substances. The commonly referred to magnetic materials generally refer to strong magnetic substances. Magnetic materials can be classified into soft magnetic materials based on their usage: they can achieve maximum magnetization with the minimum external magnetic field and are magnetic materials with low coercivity and high magnetic permeability. Soft magnetic materials are easy to magnetize and also easy to demagnetize. For example, soft magnetic ferrite and amorphous nanocrystalline alloys. Hard magnetic materials, also known as permanent magnetic materials, refer to materials that are difficult to magnetize and difficult to demagnetize once magnetized. Their main characteristic is high coercivity, including rare earth permanent magnetic materials, metal permanent magnetic materials, and permanent ferrite. Functional magnetic materials: mainly include magnetostrictive materials, magnetic recording materials, magnetoresistance materials, magnetic bubble materials, magneto-optical materials, and magnetic thin film materials.