Site-selective Rh substitution and magnetic property of magnetoplumbite-structured BaFe_12−xRh_xO₁₉

Magnetoplumbite-type ferrites AFe12O19 (A = Sr, Ba, La, Pb) are widely utilized as practical magnetic materials. Site-selective chemical substitutions for five distinct crystallographic Fe sites in AFe12O19 are desired for precise adjustment of magnetic properties but only a few examples have been reported. In this study, we attempt to synthesize BaFe12−xRhxO19 (x = 1–6), where the Fe3+ ions at octahedral sites are partially replaced by non-magnetic Rh3+ ions with octahedral site preference, in high-pressure and high-temperature conditions of 8 GPa and 1373–1523 K. BaFe12−xRhxO19 samples are successfully obtained as the primary phase for all the compositions. The lattice constants a and c of BaFe12−xRhxO19 increase and decrease, respectively, with x up to the Rh solubility limit of x = 5. Rietveld refinement reveals that the octahedrally coordinated Fe3+ ions are preferentially replaced by the Rh3+ ions. Saturation magnetization decreases monotonically with increasing x, and its x dependence is reasonably explained by considering the octahedral site preference of the Rh3+ ions. These results show that the high-pressure synthesis is effective for site-selective substitution of the magnetoplumbite-type ferrites over a wide composition range, which may allow precise control of their magnetic properties.