Superparamagnetism

 

Superparamagnetism is a property found mainly in small, single-domain magnetic particles without magnetic memory. It is more related to ferromagnetism than paramagnetism.

For example, when a ferromagnetic multi-domain Fe3O4 sample is reduced to less than about 40 nm, a single-domain magnetic particle will eventually form. When placed in an external magnetic field, this molecule develops strong internal magnetization as a result of the interchangeable coupling of electrons in the domain, and thus becomes superparamagnetic.

Since only one domain is involved, the susceptibility of a superparamagnetic substance is not as great as that of a ferromagnetic substance. Additionally, since each domain is in a separate particle, there must be no interactions or the order of the domains in the sample. Thus, unlike ferromagnetic materials, superparamagnetic substances do not retain net magnetization when the external field is removed. In other words, they don’t have magnetic memory.