Barium ferrite site occupancy analysis using the Rietveld method

Barium ferrite has attracted considerable interest in the fields of permanent magnets and perpendicular magnetic recording due to its strong uniaxial anisotropy and high Curie temperature. The magnetic properties, especially magnetic anisotropy, can be controlled by an appropriate doping element, such as Al. The Rietveld method is one of the few techniques including single crystal x-ray or neutron diffraction, EXAFS and solid state NMR that can analyze the occupancies of specific sites when the same element can be in multiple different sites in one crystal structure. Below is an example of Al and Fe site occupancy analysis done using the Rietveld method.

The sample was Al doped ferrite (BaAlxFe(1-x)O19) with a small amount of Ba0.79Al10.9O17.14, BaAl2O4 and Al2O3. Figure 1 shows the Rietveld refinement result. The data was collected on the SmartLab diffractometer and PDXL software was used for the data analysis.

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Figure 1: Rietveld refinement result

The ferrite has five Fe sites that can be replaced by Al as shown in Figure 2. Their site occupancies were refined during the Rietveld analysis. The results are show in Table 1.

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Figure 2: Al doped ferrite (BaAlxFe(1-x)O19) structure and five Fe sites
(The arrows indicate the spin direction.)

The refined Al and Fe site occupancies shown in Table 1 indicate that Al does not replace Fe in five different sites randomly but replaces the spin up sites preferably. This explains the nonlinear relationship between this material’s magnetic property and Al doping rate, and helps one understand the doping mechanism and ultimately control the material’s magnetic properties.

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Table 1: Refined Al and Fe site occupancies

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