Everyone must have played with magnets when they were young, and they must have also noticed this phenomenon: magnets can attract iron products such as nails and sheets, but cannot attract wooden tables or aluminum pots. Why is this?
To understand magnetism in solids, one must first understand the quantum property of electrons - spin. Like charge and mass, spin is also an intrinsic property of microscopic particles. It is not caused by the rotation of microscopic particles, but rather an inevitable characteristic of quantization. The electron spin will produce a Spin magnetic moment, and the electron movement around the atomic nucleus will also produce an orbital magnetic moment. The direction of the magnetic moment is perpendicular to the circuit plane, and the magnetic moment can be seen as a small magnetic needle. If the ordered arrangement of atoms has the same magnetic moment size and direction, then the material as a whole will exhibit magnetism.
Solenoid magnetic field
Experiments have shown that any substance is more or less magnetized in an external magnetic field, with varying degrees of magnetization. Iron is a ferromagnetic material. There is a certain interaction (limitation) between the magnetic moments of adjacent atoms or ions in ferromagnetic materials, which causes the magnetic moments in certain regions to be roughly arranged in the same direction, forming a magnetic domain with very strong magnetism. There are many such magnetic domains inside the material, but the magnetic moment orientation of different domains is random, and the material as a whole does not exhibit strong magnetism. The main reason why the magnet attracts iron is that the iron itself has Spontaneous magnetization and forms a Spontaneous magnetization region, namely magnetic domain. After magnetization of ferromagnetic materials, the internal magnetic domains will be arranged neatly and uniformly, making the whole show magnetism and can be attracted by the magnet. In addition to iron, transition metals such as cobalt, nickel, gadolinium, neodymium and rare earth metals all have Spontaneous magnetization properties. After removing the magnet, some ferromagnetic materials can still maintain the ordered arrangement of magnetic domains while retaining magnetism, while others will quickly lose magnetism. The magnetism of pure iron is difficult to maintain, while steel cast with a certain amount of carbon and other impurities has good magnetism. At present, the strongest permanent magnet discovered is neodymium iron boron material, known as the "magnetic king" due to its excellent magnetism, which has important applications in modern industry and electronic technology.
There are no magnetic domains inside wood, and the electronic magnetic moments in its atoms cancel out each other. When subjected to an external magnetic field, small magnetic moments that resist the external magnetic field are generated, resulting in being unable to be attracted. Aluminum is a Paramagnetism material. When it is not affected by the magnetic field, the magnetic moments are disorderly arranged. After the external magnetic field is applied, the magnetic moments tend to be consistent with the external magnetic field, but the consistency is not as strong as that of ferromagnetic materials, so ordinary magnets cannot attract aluminum blocks.
The magnetism of solid materials is also related to the temperature of the material. For ferromagnetic materials, above a certain temperature, the thermal vibration of atoms is too violent to disturb the original magnetic moment in uniform arrangement, and the material will transition from ferromagnet to paramagnet, and the corresponding temperature point is called the Curie temperature. The household Rice cooker uses the Curie temperature of the ferromagnet. When the rice in the cooker is heated to 103 ℃, it reaches the Curie temperature of the ferromagnetic material below the Rice cooker. The attraction of the demagnetized magnetic steel sheet is greatly reduced, so the heating switch is automatically turned off, allowing the Rice cooker to enter the temperature maintaining state, so that the rice can be cooked without being burnt.