Phase stability and magnetic properties of SrFe18O27 W‐type hexagonal ferrite

W‐type SrFe22+Fe163+O27 ferrite is a member of the hexagonal ferrite family and a potential permanent magnet material. However, its synthesis conditions are not fully understood yet. Samples were sintered either at 1400°C in air and quenched, or at 1300°C at reduced oxygen partial pressure. The precise stability conditions of this W‐type ferrite were investigated in the temperature range of 1200°C‐1400°C using thermogravimetry, XRD, and electron microscopy. At 1300°C, the ferrite is stable at oxygen partial pressures of -3.1≤logpO2≤-1.6. At more oxidizing conditions, the ferrite decomposes into M‐type ferrite and hematite, while at more reducing atmospheres Sr4Fe6O13 and magnetite are formed. The nonstoichiometry δ of SrFe18−δO27 was derived from thermal analysis data at 1300°C as function of oxygen partial pressure and was found to be mainly due to cation vacancies. Magnetization measurements show that this W‐type ferrite exhibits Ms = 103 emu/g at T = 4 K, which agrees well with a ferrimagnetic spin arrangement according to Gorter's model. As alternative, Zn‐substituted W‐ferrite was found to be stable in air at 1200°C with a large Ms = 123 emu/g at 4 K.