In physics, a ferrimagnetic material is one that has populations of atoms with opposing magnetic moments, as in antiferromagnetism; however, in ferrimagnetic materials, the opposing moments are unequal and a spontaneous magnetization remains.  This happens when the populations consist of different materials or ions (such as Fe2+ and Fe3+).

Ferrimagnetic ordering. image: wikipedia

Ferrimagnetism is exhibited by ferrites and magnetic garnets. The oldest known magnetic substance, magnetite (iron(II,III) oxide; Fe3O4), is a ferrimagnet; it was originally classified as a ferromagnet before Néel's discovery of ferrimagnetism and antiferromagnetism in 1948.

Known ferrimagnetic materials include YIG (yttrium iron garnet), cubic ferrites composed of iron oxides and other elements such as aluminum, cobalt, nickel, manganese and zinc, hexagonal ferrites such as PbFe12O19 and BaFe12O19, and pyrrhotite, Fe1−xS.

source: adapted from: Ferrimagnetism. (2017, March 5). In Wikipedia, The Free Encyclopedia. Retrieved 23:53, March 5, 2017, from https://en.wikipedia.org/w/index.php?title=Ferrimagnetism&oldid=768757246