You must have played with magnets when you were young. You must have also found that magnets can attract nails, iron pieces and other iron products, but they can not attract wooden tables or aluminum pots. Why is this?
To understand magnetism in solids, one must first understand the quantum properties of electrons -- spin. Like charge and mass, 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 quantification. Electron spins produce spin magnetic moments. Electrons move around the nucleus and also produce orbital magnetic moments. The direction of the magnetic moment is perpendicular to the loop plane. The magnetic moments can be seen as small magnetic needles. If the atomic magnetic moments arranged in an order are the same size and direction, the overall material will reflect magnetism.
Experiments have shown that any substance is more or less magnetized in the external magnetic field, but the degree of magnetization is different. Iron is a ferromagnetic material. There is a certain interaction(limitation) between the magnetic moments of adjacent atoms or ions in ferromagnetic materials, which will make the magnetic moments in some areas roughly arranged in the same direction, thus forming a magnetic domain, and the magnetism in the magnetic domain is 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 as a whole does not exhibit strong magnetism. The main reason why magnets attract iron is that the iron itself undergoes spontaneous magnetization and forms a spontaneous magnetization region, the magnetic domain. After the ferromagnetic material is magnetized, the internal magnetic domains will be arranged in a uniform manner, so that the overall magnetic properties can be attracted by the magnet. In addition to iron, transition metals such as cobalt, nickel, yttrium, and yttrium and rare earth metals have spontaneous magnetization properties. After removing the magnet, some ferromagnetic materials can still maintain an orderly arrangement of magnetic domains and retain magnetism, while others will quickly disappear magnetism. The magnetic properties of pure iron are not easy to maintain, and the magnetic properties of steel cast with a certain amount of carbon and other impurities are very good. At present, the strongest permanent magnet is neodymium iron boron material, which is called "magnetic King" because of its excellent magnetism and has important applications in modern industry and electronic technology.
There is no magnetic domain inside the wood, and the electron magnetic moments in the atoms cancel each other out. When affected by the external magnetic field, tiny magnetic moments that resist the external magnetic field are generated, resulting in inability to be attracted. Aluminum is a paramagnetic material. When it is not affected by a magnetic field, the magnetic moments are arranged in disorder. After adding an external magnetic field, the magnetic moments tend to be consistent with the external magnetic field of Yuhe, but the consistency is not as strong as the ferromagnetic material, so ordinary magnets can not attract aluminum. block.
The magnetism of a solid material is also related to the temperature of the material. For ferromagnetic materials, above a certain temperature, the thermal vibration of the atom is too intense and disrupts the previously uniformly arranged magnetic moment. The material will transition from the ferromagnet to the paramagnetic body, and the corresponding temperature point is called the Curie point. The household rice cooker uses the Curie point of the ferromagnet. When the rice in the pot is heated near 103 °C, the Curie point of the ferromagnetic material under the rice cooker is reached, and the magnetic steel plate after demagnetization is greatly reduced. This automatically disconnects the heat switch, allowing the rice cooker to enter the insulation state so that the rice can be cooked and not paste off.