Magnetism can be defined as an interaction of the forces existing between magnetizable objects. It is expressed in the ability of these objects to attract or repel. Underlying this action are the movements in electrical charges. These generate a magnetic field that acts in turn on those charges, thus creating the phenomenon of magnetism.
Magnetism is almost imperceptible to human beings and it therefore took a long time before the correct explanation was discovered, situating it in the context of electrical charges. Until that time, it was very much a thing of concealed mystery. In most cases, magnetic phenomena are depicted by a magnetic field with field lines. This also makes it possible to depict the force exerted, known as the Lorentz Force, which acts in a plane perpendicular to the lines of the magnetic field.
The occurrence of magnetism is associated with the movement of electrical charges. This encompasses the movement of electrons around atomic nuclei as well as the 'spin' of electrons (i.e. their innate rotational movement), both of which generate magnetic 'moments' that add together vectorially to yield 'atomic moment'. If the total amounts to zero, the material is referred to as 'diamagnetic'. With paramagnetic, ferromagnetic, antiferromagnetic and ferrimagnetic materials, the total of moments does not add up to zero.
Paramagnetism occurs in materials whose atoms have at least one electron shell that is not completely filled. Examples are O, Al, Pt, Ti, various transition metals, rare earth metals, and actinides. These atoms possess a permanent magnetic moment. Neighbouring atoms are not coupled to each other. In an external magnetic field, the atoms align their magnetic moments in the direction of the external field. Here 1+4·10-4<µr<1+10-8.
Ferromagnetism occurs in materials whose atoms have a particular electron shell occupation ans also a particular relationship between zheir interatomic spacing distance and their atomic radii. Examples are Fe, Co, Ni and compounds such as Alnico. Neighbouring atomic magnetic moments couple paralell to each other and from domains with a total magnetic moment of a certain size and orientation. Here5·105>µr>100.
Antiferromagnetic materials also from domains. However, they have two different sublattices, whose magnetic moments are antiparalell. That is, they are of equal magnitude and opposite direction. These materials behave like paramagnetic substances. Examples are a-Mn, (a-Mn, FeO, Fe2O3, FeS, CoO).
Domains with magnetic moments from different sublattices, pointing in opposite directions, characterise ferrimagnetism. The magnetic moments are of different magnitude, so the material behaves like a ferromagnet. (Cubic ferrites, such as MnO⋅FeO, are soft magnetic materials, whereas hexagonal ferrites such as BaO⋅6Fe2O3, are hard magnetic materials).