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Principle of magnet

Why do magnets attract iron? What is the principle? Magnetism is one of the basic properties of substances, and all substances are magnetic media.

There are three types:

1. Paramagnetic materials, which produce the same additional magnetic field as the external magnetic field under the action of magnetic field, most of which belong to this category.

2. Antimagnetic material, which generates additional magnetic field opposite to external magnetic field under the action of magnetic field, such as copper and inert gas.

3. Ferromagnetic material, which produces strong additional magnetic field with the same intensity as external magnetic field under the action of magnetic field, such as iron, cobalt and nickel.

According to the hypothesis first proposed by Ampere, there are permanent molecular currents with a certain magnetic moment in paramagnetic molecules. When there is no external magnetic field, the orientation of these molecular currents is irregular due to the thermal motion of molecules, so the magnetic field generated by them is equal to zero on average, and does not show magnetism to the outside. When there is an external magnetic field, these molecular currents are subject to the orientation of the external field, and their magnetic moment lattice turns to the direction of the external magnetic field, resulting in an additional magnetic field along the direction of the external magnetic field. This is the reason for the paramagnetic magnetization. When there is no external magnetic field, the intramolecular structure makes their molecular current equal to zero. When the external magnetic field is introduced, it is just like the induction current generated when the magnetic field is introduced in a closed circular wire, In these antimagnetic molecules, induced current is also produced. Because there is no resistance in the molecule, as in superconductors, the current will circulate forever once it is generated, until the reverse induced current is offset when the external magnetic field is old. Under the action of external magnetic field, all magnetic media will produce induced molecular current, that is, antimagnetism is common to all substances, but in paramagnetic molecules, the magnetic moment of molecular current is much larger than the narrow moment of induced current, so the antimagnetism of substances is covered up and only paramagnetism appears.

As for the origin of ferromagnetism, there was a magnetic domain hypothesis: the magnetic moment of a single atom of many substances is on the same order of magnitude, so it is not that the magnetic moment of the atom is affected by the magnetic field, resulting in the difference between ferromagnetism and other magnetic media. It is because the atoms of ferromagnets are easier to arrange under the action of external magnetic field.

Why is it so easy to arrange atomic magnetic moments in ferromagnets? This is because there is a molecular field generated by the strong interaction between atoms (called exchange force) in ferromagnets. The role of the molecular field is the same as that of the magnetic field, which makes the atomic z moments aligned. The size of the molecular field is much stronger than that of the ordinary magnetic field. For example, at room temperature, more than 95% of the original magnetic moments are aligned due to the role of the molecular field. However, ferromagnets do not show magnetism before being magnetized, because each ferromagnet is actually divided into many small areas, which we call magnetic domains. The molecular field makes the original magnetic moments in each domain align in the same direction, but the directions of the magnetic moments in each domain are different from each other. Therefore, in the absence of an external magnetic field, although the original magnetic moments in each domain have almost all been aligned, the total magnetic short of the ferromagnet is still zero, and the entire ferromagnet does not show magnetism. With the addition of an external magnetic field, the direction of the magnetic moments of each magnetic field turns to the direction of the external magnetic field, The total magnetic moment of a ferromagnet is not zero. Since the original magnetic moment in each domain is oriented without an external magnetic field, the ferrite also shows strong magnetism in a small external magnetic field. When the temperature is higher than a value (Curie point), the magnetic domain will collapse and lose its ferromagnetic properties, which is the same as that of ordinary paramagnetic materials.