It is not possible to set up such a convention for ferromagnetic, antiferromagnetic and ferrimagnetic materials. All three of these classes of materials can be considered a special case of paramagnetic behavior. The description of paramagnetic behavior is based on the assumption that every molecule behaves independently. The materials discussed here result from a situation in which the direction of the magnetic field produced by one molecule is affected by the direction of the magnetic field produced by an adjacent molecule, in other words their behavior is coupled. If this occurs in a way in which the magnetic fields all tend to align in the same direction, a ferromagnetic material results and the phenomenon is called ferromagnetic coupling.
Antiferromagnetic coupling gives an equal number of magnetic fields in opposite directions.
Ferrimagnetic coupling gives magnetic fields in two opposite orientations with more in one direction than in the other. With a few exceptions, the magnetic moments are not aligned through out the entire material. Typically regions, called domains, will form with different orientations. The existence of domains of coupled molecules gives rise to a number of types of behavior as described in the following paragraphs. The tendency of molecules to align themselves to one another enhances the magnetization of the material due to the presence of an external magnetic field.