Classification of magnetism and magnetic materials
When it comes to magnetic materials, we have to talk about "magnetism" first. Experiments show that any substance can be magnetized to a greater or lesser extent in an external magnetic field, but the degree of magnetization is different. According to the characteristics of the substance in the external magnetic field, the substance can be divided into five categories: paramagnetic substances, diamagnetic substances, ferromagnetic substances, ferrimagnetic substances, and antiferromagnetic substances.
Paramagnetic substances: 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 precision instruments; if the external magnetic field is removed, the internal magnetic field will also return to zero, resulting in no magnetism. Such as aluminum, oxygen, etc.
Diamagnetic substances: substances with negative magnetic susceptibility. When affected by an external magnetic field, induced electron circulation is generated in the molecules, and the magnetic moment it generates is opposite to the direction of the external magnetic field, that is, the direction of the magnetic field after magnetization is opposite to the direction of the external magnetic field. All organic compounds have diamagnetic properties, and graphite, lead, water, etc. are all diamagnetic substances.
Ferromagnetic material: a material that is magnetized under the action of an external magnetic field and can maintain its magnetized state even if the external magnetic field disappears. Iron, cobalt, and nickel are all ferromagnetic materials.
Ferrimagnetic material: The macroscopic magnetism is the same as ferromagnetism, but the magnetic susceptibility is lower. The typical ferrimagnetic material is ferrite. The most significant difference between them and ferromagnetic materials is the difference in internal magnetic structure.
Antiferromagnetic material: Inside the antiferromagnetic material, the spins of adjacent valence electrons tend to be in opposite directions. The net magnetic moment of this material is zero and no magnetic field is generated. This material is relatively uncommon, and most antiferromagnetic materials only exist at low temperatures. Assuming that the temperature exceeds a certain value, it usually becomes paramagnetic. For example, chromium, manganese, etc. are antiferromagnetic.
We call paramagnetic and diamagnetic materials weak magnetic materials, and ferromagnetic and ferrimagnetic materials strong magnetic materials. The magnetic materials usually referred to generally refer to strong magnetic materials.
Magnetic materials can be divided into soft magnetic materials according to their use:
Soft magnetic materials: can achieve the maximum magnetization intensity with the smallest external magnetic field, and are magnetic materials with low coercivity and high magnetic permeability. Soft magnetic materials are easy to magnetize and easy to demagnetize. For example: soft ferrite, amorphous nanocrystalline alloy.
Hard magnetic materials: also called permanent magnetic materials, refer to materials that are difficult to magnetize and difficult to demagnetize once magnetized. Their main feature is high coercivity, including rare earth permanent magnetic materials, metal permanent magnetic materials and permanent magnetic ferrites.
Functional magnetic materials: mainly magnetostrictive materials, magnetic recording materials, magnetoresistance materials, magnetic bubble materials, magneto-optical materials and magnetic film materials.