The magnetism like a magnet is one of the basic properties of matter, and all matter is a magnetic medium. Zh
Divided into three types:
1. Paramagnetic substances, which produce the same additional magnetic field as an external magnetic field under the action of a magnetic field. Most substances fall into this category.
2. A diamagnetic substance, such as copper and inert gas, which generates an additional magnetic field opposite to the external magnetic field under the action of a magnetic field.
3. Ferromagnetic substances, which produce the same strong additional magnetic field as the external magnetic field under the action of a magnetic field, for example, iron, cobalt, nickel, etc.
According to the hypothesis first proposed by Ampere, there is a permanent molecular current with a certain magnetic moment in paramagnetic molecules. In the absence of an external magnetic field, due to the thermal movement of molecules, the orientation of these molecular currents is irregular, so The magnetic field generated by them is equal to zero on average, and does not show magnetism to the outside. When an external magnetic field exists, these molecular currents are subject to the orientation of the external field, and their magnetic moments are turned to the direction of the external magnetic field, which generates an additional magnetic field in the direction of the external magnetic field. This is the reason for paramagnetic magnetization. In the absence of an external magnetic field, the molecules that make up a diamagnetic substance have a molecular structure that makes their molecular current equal to zero. When an external magnetic field is introduced, it is just like when a magnetic field is introduced into a closed circular wire. Induced currents are produced in these diamagnetic molecules as well as induced currents. Because there is no resistance in the molecules, as in superconductors, once the current is generated, it will circulate forever, until the external magnetic field withdraws and causes reverse induction. The current cancels it. Under the effect of the external magnetic field, all magnetic media must generate induced molecular currents, that is, It is common to cut matter, but in paramagnetic molecules, the magnetic moment of the molecular current is much larger than the moment of the induced current, so the diamagnetism of the substance is masked and only paramagnetism appears.
The cause of ferromagnetism has a magnetic domain hypothesis:
The magnetic moment of a single atom in many substances is on the order of magnitude, so it is not the magnetic moment of the atom that is affected by the magnetic field, which causes the difference between ferromagnets and other magnetic media. It is because the atoms of the ferromagnet are more easily aligned under the influence of an external magnetic field. Why are the magnetic moments of atoms arranged so easily in ferromagnets? This is because there is a molecular field in ferromagnets due to strong interactions between atoms (called exchange forces). The role of molecular fields is the same as that of magnetic fields. 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, more than 95% of the original magnetic moments are aligned due to the molecular field. But Ferromagnets do not exhibit magnetic properties before being magnetized. This is because each ferromagnet is actually divided into many small regions. We call such small regions magnetic domains. The molecular field makes each magnetic domain in each magnetic domain The moments are arranged in the same direction, but the directions of the magnetic moments of each magnetic domain are different from each other. Therefore, in the absence of an external magnetic field, although the original magnetic shortness in each magnetic domain is almost all aligned, the total magnetic shortness of the iron body is still zero. The entire ferromagnet does not exhibit magnetism. After the external magnetic field is added, the magnetic moment direction of each magnetic field turns to the direction of the external magnetic field, and the total magnetic moment of the ferromagnet is not zero. Orientation occurs when there is no external magnetic field, so the iron nutrient body also shows strong magnetism in a small external magnetic field. When the temperature is higher than a value (Curie point), the magnetic domains disintegrate and lose the ferromagnetic properties, which is the same as that of ordinary paramagnetic substances.