Superparamagnetism occurs when the material is composed of very small crystallites (1-10 nm). In this case, even though the temperature is below the Curie or Néel temperature and the thermal energy is not sufficient to overcome the coupling forces between neighboring atoms, the thermal energy is sufficient to change the direction of magnetization of the entire crystallite. The resulting fluctuations in the direction of magnetization cause the magnetic field to average to zero. The material behaves in a manner similar to paramagnetism, except that instead of each individual atom being independently influenced by an external magnetic field, the magnetic moment of the entire crystallite tends to align with the magnetic field.
In MRI superparamagnetic iron oxide is used as a contrast agent.

See also magnetism, ferromagnetism, paramagnetism, and diamagnetism.