We must have played with magnets when we were young. We must have also discovered this phenomenon: Magnets can attract iron nails, iron pieces and the like, but they cannot attract wooden tables or aluminum pans. Why is this?
To understand the magnetic properties of solids, we must first understand the quantum properties of electrons—spin. Like charges and masses, spin is also the intrinsic property of microscopic particles. It is not caused by the rotation of microscopic particles but is an inevitable characteristic of quantization. Electron spins generate spin magnetic moments, and electrons orbiting the nuclei also generate orbital magnetic moments. The direction of the magnetic moment is perpendicular to the circuit plane. The magnetic moment can be seen as a small magnetic needle. If the ordered atomic magnetic moments have the same magnitude and the same direction, then the material as a whole will exhibit magnetic properties.
Solenoid magnetic field
Experiments have shown that any substance is magnetized more or less in an external magnetic field, but the degree of magnetization is different. Iron is a ferromagnetic material. There is a certain interaction (restriction) between the magnetic moments of adjacent atoms or ions in a ferromagnetic material, which causes the magnetic moments in some regions to be roughly aligned in the same direction. Thus, a magnetic domain is formed, and the magnetic domain is magnetic. Very strong. There are many such magnetic domains inside the material, but the magnetic moment orientations of different magnetic domains are random and the material does not exhibit strong magnetism as a whole. The main reason why a magnet attracts iron is that the iron itself spontaneously magnetizes and forms a spontaneous magnetization zone, ie, a magnetic domain. After the ferromagnetic material is magnetized, the internal magnetic domains are arranged in a uniform manner, so that the entire body exhibits magnetism and can be attracted by the magnet. In addition to iron, transition metals such as cobalt, nickel, ruthenium, rhodium, and rare earth metals all have spontaneous magnetization properties. After the magnets are removed, some ferromagnetic materials can retain magnetic domains in order to retain their magnetic properties, while others can quickly disappear. The magnetic properties of pure iron are not easy to maintain, and the steel cast with a certain amount of impurities such as carbon is very good. At present, the strongest permanent magnet has been found to be neodymium-iron-boron material. Because of its excellent magnetic properties, it is called “magnetic king” and has important applications in modern industrial and electronic technologies.
There are no magnetic domains in the interior of the wood. The magnetic moments of the electrons in the atom cancel each other out. When they are subjected to an external magnetic field, they will produce tiny magnetic moments that resist the external magnetic field, resulting in being unable to be attracted. Aluminum is a paramagnetic material. When it is not affected by a magnetic field, the magnetic moment is disorderly arranged. After adding an external magnetic field, the magnetic moment tends to be consistent with the external magnetic field, but the consistency is not strong with ferromagnetic materials, so ordinary magnets cannot attract Aluminum block.
The magnetic properties of solid materials are also related to the temperature of the material. For ferromagnetic materials, at a certain temperature or more, the thermal vibration of atoms is too severe and disturbs the original uniformly arranged magnetic moments. The material will transition from a ferromagnetic body to a paramagnetic body. The corresponding temperature point is called the Curie point. The domestic rice cooker uses the Curie point of the ferromagnetic body. When the rice in the pot is heated to around 103°C, it reaches the Curie point of the ferromagnetic material under the rice cooker. The attraction of the magnetic steel sheet after demagnetization is greatly reduced, and the automatic Disconnect the heating switch and allow the rice cooker to enter the keep-warm state so that the rice can be cooked and not pounded.
Keyword resource from http://www.magnet-forever.com
Xiamen Magforever 22th March 2018