Site preference and magnetic properties for a perpendicular recording material: BaFe₁₂₋ₓZnx/2Zrx/2O₁₉ nanoparticles

The site preference and magnetic properties of Zn-Zr substituted BaM ferrite BaFe_12-xZnx/2Zrx/2O₁₉ nanoparticles with $x=0--1.6$ have been studied using M\"ossbauer spectra and magnetic measurement. The results show that the magnetizations and magnetocrystalline anisotropies are closely related to the distributions of Zn-Zr ions on the five sites. The BaFe_12-xZnx/2Zrx/2O₁₉ nanoparticles exhibit unusual saturation magnetization, which increase at low substitutions, reach a maximum and then decrease. M\"ossbauer spectra show that the Zn-Zr ions preferentially occupy the $2b$ and 4fVI sites. The preference for the 4fVI site is responsible for the anomalous increase in the magnetization at low Zn-Zr substitutions. At $x>~1.2$ the rapid decrease in the magnetizations is mainly attributed to a noncollinear magnetic structure. In addition, a monotonic decrease in the magnetocrystalline anisotropy with Zn-Zr substitutions has its origin in the preference of the Zn-Zr ions for the $2b$ and 4fVI sites.