Explanation

If the magnetic materials is a collection of single domain particles, for each particle, due to the strong exchange interaction between magnetic atoms or ions, the magnetic moment will be parallel to the orientation, and the magnetic moment orientation is in the easy magnetize direction determined by the magnetocrystalline anisotropy. However, due to the different easy magnetize directions between particles, the orientation of the magnetic moment will also be different. Nowadays, if the particle size is further reduced, that is, the volume, because the total magnetocrystalline anisotropy energy is proportional to K1V, and the thermal disturbance kinetic energy is proportional to kT (K1 is the magnetocrystalline anisotropy constant, V is the particle volume, k is the Boltzmann constant, and T is the absolute temperature of the sample), when the particle volume is reduced to a certain value, the thermal disturbance kinetic energy will be equivalent to the total magnetocrystalline anisotropy performance, so that the direction of the magnetic moment in the particle may be as time goes by, Keep the whole parallel and repeatedly change between one easy magnetize direction and the other easy magnetize direction. From the perspective of single domain particle aggregates, the magnetic moment orientation of different particles is changing every moment. The magnetic characteristics are similar to the normal paramagnetism, but also different. Because in a normal paramagnet, each atom or ion has only a few magnetic moment of Bohr magneton, but for a specific spherical particle aggregate with a diameter of 5nm, each particle may contain more than 5000 atoms, and the total magnetic moment of particles may be greater than 10000 Bohr magneton. So the magnetism of single domain particle aggregates is called superparamagnetism.

Classification