part 1 - Why are things magnetic ?   magnetic exchange
Fig. 1.7 Some elements, especially transition elements, have  microscopic magnetic moments that interact strongly with neighboring atoms. The nature of these interactions is of quantum mechanical origin and depends on the crystal structure of the material.

In ferromagnetic materials (e.g., iron) all microscopic magnetic moments are aligned parallel to each other, resulting in a very strong net-magnetic moment μnet
(Fig. 1.7.a)

Antiferromagnetic materials (e.g., goethite - FeOOH) show antiparallel alignment of neighboring magnetic moments, leading to a zero net magnetic moment (Fig. 1.7.b).

In ferrimagnetic materials (e.g., magnetite - Fe3O4, maghemite - γ-Fe2O3, pyrrhotite - ) the magnetic moments of neighboring atoms are aligned antiparallel, but neighboring moments have different magnitudes, resulting in a net magnetic moment as indicated in fig. 1.7.c.

In some antiferrimagnetic materials (e.g., hematite - α-Fe2O3)the antiparallel alignment is not perfect, resulting in a very small net-magnetic moment (Fig. 1.7.d)

As shown later, ferro- and ferrimagnetic materials are characterized by strong positive magnetic susceptibility values.

 
part 1 - index 1.1 currents 1.2 solenoids 1.3 electromagnets 1.4 mag. moment 1.5 diamagnets 1.6 paramagnets 1.7 mag. ordering