Effect of oxygen potential on Co solubility limit in La–Co co-substituted magnetoplumbite-type strontium ferrite
Highlights: • Oxygen pressure control dramatically affect the substitution range of La–Co co-substituted Sr M-type hexaferrite. • Successful synthesis of LaFe{sub 11}CoO{sub 19} under high oxygen pressure. • LaFe{sub 11}CoO{sub 19} achieves the highest anisotropic field. - Abstract: La–Co co-substituted magnetoplumbite-type (M-type) strontium ferrite is typically used as a base material for high-performance hard ferrite magnets. Generally, the unquenched orbital moment of Co{sup 2+}, substituted for Fe{sup 3+}, is thought to enhance magnetic anisotropy and then coercivity. La{sup 3+} is substituted for Sr{sup 2+} to compensate for the charge unbalance associated with Co{sup 2+} substitution. Even if the synthesis starts with equal amounts of Co and La, the Co contents of the resulting samples are generally lower than the La contents because of the partial reduction of Fe{sup 3+} to Fe{sup 2+}. We studied how the oxygen potential suppresses the formation of Fe{sup 2+} and expands the Co solubility range in this system. Polycrystalline samples, synthesized under oxygen partial pressures of 0.2, 1.0, and 387 atm, were characterized by powder X-ray diffraction, wavelength-dispersive X-ray analysis, and magnetization measurements. We reveal that the application of higher oxygen pressure dramatically expands the Co solubility range, resulting in the enhancement in magnetic anisotropy. We successfully synthesized Sr-free LaFe{sub 11}CoO{sub 19} with high magnetic anisotropy using the hot isostatic pressure (HIP) technique.