Ferrimagnetism

Ferrimagnetism is a form of magnetic ordering in which the magnetic moments of atoms or ions in a material are arranged in two or more sublattices that are aligned antiparallel, but with unequal magnitudes. As a result, the material exhibits a spontaneous net magnetization below a characteristic temperature, the Curie temperature, rather than canceling completely as in an antiferromagnet.

In many ferrimagnets, the sublattices correspond to distinct crystallographic sites or ion species. Exchange interactions favor antiparallel alignment between sublattices, but the moments on different sublattices have different sizes, so the vector sum is nonzero. A classic example is magnetite (Fe3O4), which contains Fe2+ and Fe3+ on alternating sublattices with opposite spins, yielding a measurable net moment. Ferrimagnetic order is common in ferrites and in many spinel and garnet materials.

Temperature dependence: The net magnetization decreases with increasing temperature and vanishes at the Curie temperature. Some ferrimagnets show a compensation point where the sublattice magnetizations cancel at a temperature below Tc, giving zero net magnetization while maintaining magnetic order. The magnetic structure can be predominantly collinear, but non-collinear arrangements and spin canting can occur, especially at elevated temperatures or under applied fields.

Materials and applications: Ferrites such as Fe3O4, NiFe2O4, and MnZn ferrites are technologically important for transformers, inductors, and magnetic storage. Rare-earth–transition-metal ferrimagnets (for example GdFe) are studied for high saturation moments and fast dynamics. Ferrimagnetism provides high net magnetization while often exhibiting favorable damping and frequency response for various magnetic devices.