Ferrimagnetism occurs in polyatomic compounds that contain ions with different
core magnetic moments. The core magnetic moments order in an antiferromagnetic
pattern, but, since alternate moments differ in magnitude, they do not cancel. The
material has a net ferromagnetic moment.
The most important of the ferrimagnetic materials are the ferrites , which have the composition MFe2O4, where M is a divalent metal ion. The most familiar of these is magnetite, Fe3O4, which is the historic lodestone that was the first known ferromagnetic material. The ferrites crystallize in the spinel structure. While I shall not attempt to draw it here, it is a relatively simple structure that has a close-packed, FCC arrangement of oxygen atoms with metal ions in 1/2 of the octahedral interstitial sites and 1/8 of the tetrahedral sites. Since there are 2 tetrahedral and 1 octahedral void per oxygen ion, there are 2 metal ions in octahedral sites for each ion in a tetrahedral site. The magnetic order is an antiferromagnetic order in which the moments of all of the octahedral ions are oriented in one direction, while those of all the tetrahedral ions are oriented in the opposite direction.
The most important of the ferrimagnetic materials are the ferrites , which have the composition MFe2O4, where M is a divalent metal ion. The most familiar of these is magnetite, Fe3O4, which is the historic lodestone that was the first known ferromagnetic material. The ferrites crystallize in the spinel structure. While I shall not attempt to draw it here, it is a relatively simple structure that has a close-packed, FCC arrangement of oxygen atoms with metal ions in 1/2 of the octahedral interstitial sites and 1/8 of the tetrahedral sites. Since there are 2 tetrahedral and 1 octahedral void per oxygen ion, there are 2 metal ions in octahedral sites for each ion in a tetrahedral site. The magnetic order is an antiferromagnetic order in which the moments of all of the octahedral ions are oriented in one direction, while those of all the tetrahedral ions are oriented in the opposite direction.
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