Ferrimagnetic materials commonly known as ferrites, can be generally represented by chemical formula XOFe2O3,
where X stands for Mn, Co, Ni, Cu, Mg, Zn, Cd etc.
If X is Fe, then compound is Fe3O4.
X-ray crystallography proves that ferrites are usually crystals and have a spine structure.
Reference: This article is referred from my book “electrical engineering materials” having ISBN 978-81-272-5069-0. In case of any doubt, comment it or post it.