Feri-magnetism is a form of antiferromagnetism in which some magnetization remains below a critical temperature (the Neel temperature). This type of materials contains two different types of ions. Moments of the ions remain antiparallel but since their values are not same, so there exists a net moment in then (fig: b). Fe3O4 is a material of this type. It is a type of permanent magnetism that occurs in solids in which the magnetic fields associated with individual atoms spontaneously align themselves, some parallel, or in the same direction, and others generally antiparallel, or paired off in opposite directions.
In a different field, there are extensive uses of these materials. If heated above a fixed temperature, these materials also acquire paramagnetism. That means exchange integral disappears.
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.