Company News

Friday 06 January, 2023 | RSS Feed

Will a magnet still be magnetic when it is ground into powder?

by AOMag | post a comment

The magnets we refer to are generally understood as permanent magnets, and the magnetism in the title is understood as whether the permanent magnet powder has permanent magnetic properties. The magnetic properties of permanent magnet powder are specifically related to the size of the powder.

To explain it phenomenologically, there are different scales from atomic magnetic moments, to magnetic domains, to macroscopic magnetism. Below the Curie temperature, the Weiss molecular field makes the atomic magnetic moments aligned, that is, spontaneous magnetization. Oriented arrangement of atomic magnetic moments will increase the magnetostatic energy. In order to reduce the magnetostatic energy, there are magnetic domains. Within a magnetic domain, the atomic magnetic moments are in the same direction. However, different magnetic domains have different directions of magnetic moments. An unmagnetized magnet consists of many magnetic domains. The directions of the magnetic moments in each magnetic domain are different, cancel each other out, and do not show magnetism to the outside. When the external magnetic field is sufficient, the magnet is magnetized, and the directions of the magnetic moments in all magnetic domains are the same, and the magnet has external magnetism.
Grind the magnet into powder. The size of the powder is critical.

If the powder size is larger than the single domain size, a magnetic powder particle is still composed of many magnetic domains, which is the same as the bulk. As long as the powder is magnetized, the powder still has permanent magnetism.

If the size of the powder is smaller than the size of a single magnetic domain, then a powder particle is a single domain. The atomic magnetic moments are oriented and arranged, and there is no need for magnetization to have permanent magnetism.

If the particle size continues to be small, it is smaller than the size where thermal disturbance and magnetocrystalline anisotropy can balance. Then the direction of the magnetic moment of the particle is always changing, and the particle shows superparamagnetism at this time. There is no permanent magnet.

News for Friday 30 December, 2022

View all news for Friday 30 December, 2022 on one page

Recent News

News archive