Why do magnets attract iron?
Why do magnets attract iron but not other metals? Why are only iron, cobalt, and nickel magnetized? Why don't copper and aluminum have molecular magnets? Magnetism is one of the basic properties of matter, and all matter is a magnetic medium.
There are three types:
1. Paramagnetic substances, which generate the same additional magnetic field as the external magnetic field under the action of a magnetic field, and most substances belong to this category.
2, diamagnetic material, this material produces an additional magnetic field opposite to the external magnetic field under the action of a magnetic field, such as copper and inert gases.
3. Ferromagnetic material, this material produces the same strong additional magnetic field as the external magnetic field under the action of the magnetic field, such as iron, cobalt, nickel, etc.
According to the hypothesis first proposed by Ampere, there is a permanent molecular current with a certain magnetic moment in the paramagnetic molecules. In the absence of an external magnetic field, due to the thermal motion of the molecules, the current orientation of these molecules is irregular, so the magnetic field generated by them is equal to zero on average, showing no external magnetism. When there is an external magnetic field, these molecular currents are oriented by the external field, and their magnetic moment lattices turn 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 magnetization of paramagnetic substances.
Molecules that make up diamagnetic masses, in the absence of an external magnetic field, are structured such that their molecular current is equal to zero. When the external magnetic field is introduced, just as the induced current is generated when the magnetic field is introduced into the closed circular wire, the induced current is also generated in the molecules of these diamagnetic substances. Because there is no resistance in the molecule, as in the superconductor, the current will circulate forever once it is generated, until the reverse induced current cancels it out when the external magnetic field is withdrawn.
Under the action of an external magnetic field, all magnetic media will generate an induced molecular current, that is, diamagnetism is common to all substances, but in paramagnetic molecules, the magnetic moment of the molecular current is narrower than that of the induced current. The moment is much larger, so the diamagnetism of the material is masked and only paramagnetism appears.
As for the cause of ferromagnetism, there is a magnetic domain hypothesis:
The magnetic moment of a single atom of many substances is in an order of magnitude, so it is not the magnetic moment of the atom that is affected by the magnetic field that causes the difference between ferromagnets and other magnetic media. It's because the atoms of a ferromagnet are easier to align under the action of an external magnetic field. Why do the magnetic moments of atoms in ferromagnets align so easily? This is because in ferromagnets there is a molecular field due to strong interactions between atoms called exchange forces. The effect of the molecular field is the same as that of the magnetic field, so that the atomic moments are aligned, and the size of the molecular field is much stronger than the ordinary magnetic field. For example, at room temperature, iron has more than 95% of the original magnetic moment due to the molecular field. oriented according to the effect. But ferromagnets do not exhibit magnetism until they are magnetized, because each ferromagnet is actually divided into many small regions, which we call magnetic domains. The molecular field makes the original magnetic moments of each magnetic domain arranged in the same direction, but the magnetic moment directions of each magnetic domain are different from each other. Therefore, in the absence of an external magnetic field, although the original magnetic moments in each magnetic domain are almost all arranged. Now, the total magnetic short of the ferromagnet is still zero, and the whole ferromagnet does not exhibit magnetism. After adding the external magnetic field, the direction of the magnetic moment of each magnetic lug turns to the direction of the external magnetic field, and the total magnetic moment of the ferromagnet is not zero. In view of the fact that the original magnetic moment in each domain is oriented in the absence of an external magnetic field, the ferrite also exhibits strong magnetism in a small external magnetic field.
When the temperature exceeds a certain value (Curie point), the magnetic domain collapses and loses its ferromagnetic properties, which is the same as ordinary paramagnetic substances.
