Ferrimagnetism is a type of permanent magnetism that occurs in some crystals when the magnetic moments of nearby ions tend to align antiparallel: it is, therefore, a type of antiferromagnetism; this situation occurs mainly in compounds known as ferrites. The term ferrimagnetism was originally proposed by Néel to describe the magnetic ordering phenomena in ferrites, in which iron (Fe) ions appear in two different ionic states and hence bear different magnetic moments with mutual antiferromagnetic coupling.
The simplest magnetic ferrite variety may be represented by the chemical formula MOFe2O3, where M is a divalent metal ion. The crystalline structure shows two possible arrangements of metal ions: in one the metal ion is in the tetrahedral position, which is surrounded by four oxygen ions; in the other, the metal ion is in octahedral position, that is surrounded by six oxygen ions.
A ferrimagnetic material is one in which the magnetic moments of the atoms in different sub-lattices are antiparallel, as in the antiferromagnetic materials; however, in ferrimagnetic materials, since the antiparallel moments are not the same in modulus, a magnetic moment results which is not null and therefore the material has a spontaneous magnetization, high resistivity, and anisotropic properties.
All ferrimagnetic materials contain various types of magnetic ions, or various crystallographic positions for the magnetic ion, or a combination of the two cases. The magnetic behavior of single crystals of ferrimagnetic materials can be attributed to the parallel alignment; the magnetic force of these materials kept by the dilution effect of the atoms in the antiparallel arrangement is generally less than that of purely ferromagnetic solids such as metallic iron.
Ferrimagnetic materials, such as ferromagnetic ones, hold a spontaneous magnetization below the Curie temperature and show no magnetic order (are paramagnetic) above this temperature. However, there is sometimes a temperature below the Curie temperature, at which the two opposing moments are equal, resulting in a net magnetic moment of zero; this is called the magnetization compensation point. This compensation point is observed easily in garnets and rare-earth–transition-metal alloys (RE-TM). Furthermore, ferrimagnets may also have an angular momentum compensation point, at which the net angular momentum vanishes. This compensation point is a crucial point for achieving high-speed magnetization reversal in magnetic memory devices.
Ferrimagnetic materials are widely used in non-volatile memory devices such as hard drives, which utilize their ability to easily switch the spins of electrons and be magnetized. When a ferrimagnet inside a coil of conducting wire is rotated, the current is generated, so they are also widely used in power motors and generators. Because ferrimagnets are electrically insulating, they are also widely used in high-frequency devices because no eddy currents are induced under AC fields.