Within any magnetic material, there is a microscopic region where the magnetic dipole moments are aligned in the same direction in each microscopic region. This phenomenon is called spontaneous magnetization of a magnetic material, and such microscopic region is called a magnetic domain . The magnetic domain and the magnetic domain are separated by the domain walls, and the direction of the magnetic polarization in the magnetic domains on both sides of the domain wall is 90 ° or 180 °. When the magnetic material is in a thermally demagnetized state, the directions of the magnetic polarization in different magnetic domains are different. Therefore, the macroscopic magnetic polarization J of the magnet is 0 and does not show magnetism to the outside. When the magnetic material is in a magnetized state, due to the magnetostatic energy , Some of the magnetic domains are swallowed by the domain wall to swell the magnetic domain so that the volume of the magnetic domain in the magnetization direction is larger than the volume of the reverse domain so that the macroscopic magnetic polarization J of the magnet is larger than 0 and the magnetic property is shown externally.
The magnetic domain is a unique "microstructure" inside the magnetic material. The macroscopic magnetic polarization J of the magnet is a vector sum of the magnetic polarization of the magnetic domains in the magnets. All macroscopic magnetic behaviors of the magnet are related to the domain structure of the magnet. Magnetic domains can be observed using the Bitter method or the Faraday method.