Role of oxygen potential in dopant solubility in ferrite magnets: Enhanced Zn solubility and magnetization in La-Zn co-substituted magnetoplumbite-type strontium ferrite

To improve the performance of magnetoplumbite-type (M-type) AFe(12)O(19) (A = Sr, Ba, Pb,& mldr;) hard ferrite magnets, such as the remanent magnetization and the coercivity, it is effective to dope transition metal elements at the Fe site. Substitution of nonmagnetic Zn, which preferentially occupies a minority-spin site in the ferrimagnets, is expected to increase magnetization. However, the solubility of Zn is limited to approximately 2-3% of Fe when synthesized in air. The solubility of divalent transition-metal substituents achieved by simultaneous substitution with trivalent cations on the A-site is often limited due to the auto reduction of Fe3+ to Fe2+. In this study, La-Zn co-substituted M-type strontium ferrites, Sr1-xLaxFe12-yZnyO19, were synthesized under different oxygen pressures of p(O2) = 0.2, 1, and 10 atm, and the phase equilibrium and chemical composition were established by X-ray diffraction and wavelength dispersive X-ray spectroscopy. At p(O2) = 10 atm, Sr-free La-Zn co-substituted ferrite, LaFe11.25Zn0.75O19, and single phases with actual compositions (x, y) = (0.42, 0.33) and (0.50, 0.45) were obtained for the first time. The single-phase samples exhibited a 5-9% higher saturation magnetization compared to non-doped SrFe12O19 at room temperature. Together with the coercivity enhancement by Co2+ substitution, oxygen potential control is a promising strategy to tune both coercivity and magnetization in future ferrite magnets.