Magnetic and microstructural properties on Mn- and Ru-doped hexagonal barium ferrites

Magnetic and microstructural properties for Mn- and Ru-doped hexagonal barium ferrites have been investigated in terms of magnetization and scanning electron microscope (SEM) measurements. The field dependent magnetization data of BaFe_12-xMn_xO₁₉ (0 ≤ x ≤ 6), sintered at 1100°C, reveal that with increasing the Mn concentration, the coercivity (H_c) gradually increases whereas the saturated magnetization (M_s) decreases. Remarkably, the H_c value at x = 6 is close to 1 tesla (T). Any noticeable difference in SEM images of the Mn-doped samples was not found, implying that the coercivity change in the Mn-doped ferrites is sensitive to the Mn content rather than the surface morphology. On the other hand, the Ru-doped BaFe₈Ru₄O₁₉ sample, in which the Ru ions are partially substituted into the Fe sites, did not show any substantial enhancement of H_c. These substitution effects are presumably interpreted in terms of competing super-exchange interactions between magnetic ions in the nearest neighboring sites. The effect of grain size on coercivity and the magnetic coupling between the magnetic metal ions are discussed.