What is the chemical formula of a spinel ferrite?

The chemical formula for a spinel ferrite is M(Fe₂O₄), where M typically represents a divalent metal cation such as manganese (Mn²⁺), nickel (Ni²⁺), cobalt (Co²⁺), zinc (Zn²⁺), copper (Cu²⁺), or magnesium (Mg²⁺).12

In this structure, the metal M can also be a monovalent lithium cation (Li⁺) or represent vacancies, provided that the absence of positive charge is compensated for by additional trivalent iron cations (Fe³⁺). The most familiar example of a spinel ferrite is magnetite (Fe₃O₄), which can be expressed as Fe(Fe₂O₄). These materials are characterized by a close-packed cubic crystal structure in which the metal cations occupy tetrahedral and octahedral interstitial sites.132