An effect observed in certain solids, notably garnets and ferrites, in which the magnetic properties change at a certain critical temperature known as the Néel temperature. Ferrimagnetism occurs in materials that have a permanent molecular magnetic moment associated with unpaired electron spins. At temperatures above the Néel temperature thermal agitation causes the spins to be randomly orientated throughout the material, which becomes paramagnetic: it obeys the Curie-Weiss law approximately but is characterized by a negative Weiss constant (see paramagnetism).

At temperatures below the Néel temperature ferrimagnetic materials behave in a similar manner to ferromagnetic materials (see ferromagnetism): they show spontaneous magnetization within a domain structure and magnetic hysteresis, but the spontaneous magnetization observed is less than that of ferromagnetic materials and does not correspond to full parallel alignment of the individual magnetic moments.

Néel explained this behaviour by suggesting that the interatomic exchange forces are antiferromagnetic (see antiferromagnetism) in nature but that the magnetic moments of sublattices containing the antiparallel spins are unequal, causing a net magnetization. Some possible arrangements are shown in the diagram: (i) unequal numbers of identical moments; (ii) unequal moments; (iii) two identical moments on each sublattice plus one unequal moment on one. The net small spontaneous magnetization causes the material to behave as a weak ferromagnetic material.

Ferrimagnetic materials are technically important because they are usually insulators and hence have low eddy-current losses in radiofrequency applications, while exhibiting substantial magnetic moments at room temperature, although less than those of ferromagnetics.

Possible arrangements of magnetic moments on two sublattices