A ferrimagnetic material is material that has populations of atoms with opposing magnetic moments, as in antiferromagnetism. For ferrimagnetic materials these moments are unequal in magnitude so a spontaneous magnetization remains. This can for example happen when the populations consist out of different atoms or ions (such as Fe2+ and Fe3+).
Ferrimagnetism has often been confused with ferromagnetism. The oldest known magnetic substance magnetite (Fe3O4) was classified as a ferromagnet before Louis Néel discovered ferrimagnetism in 1948. Since the discovery, numerous uses have been found for ferrimagnetic materials, such as hard drive platters and biomedical applications.