Synthesis, Crystal Structure, Site Occupancy, and Magnetic Properties of Aluminum-Substituted M-Type Sr Hexaferrite SrFe_12-xAl_xO₁₉ Nanoparticles

The synthesis of aluminum-substituted strontium hexaferrite nanoparticles (SrFe_12-xAl_xO₁₉ with x = 0-3), via three different preparation methods, is investigated. The synthesis methods are hydrothermal autoclave (AC) synthesis, a citrate sol-gel (SG) synthesis, and a solid-salt matrix (SSM) synthesis. Evaluation of macroscopic magnetic properties and of lattice parameters obtained by Rietveld analysis of powder X-ray diffraction (PXRD) data indicate that successful substitution of Al into the crystal structure is only achieved for the SG method. For the SG sample with x = 3, the coercivity was found to increase by 73% to 830 kA/m, while the saturation magnetization was reduced by 68% to 22.6 Am²/kg compared to the nonsubstituted x = 0 SG sample. The SSM and AC samples did not show any significant changes in their magnetic properties. To examine the nature of the Al insertion in detail, neutron powder diffraction (NPD) data were collected on the SSM and SG samples. Combined Rietveld refinements of the PXRD and NPD data confirm that effective substitution of the Al ions is only achieved for the SG sample and reveal that Al occupies mainly the (2a)_Oh and (12k)_Oh sites and to a lesser extent the (4e)_BP, (4f)_Oh and (4f)_Td sites. Moreover, the relative degree of site occupation varies with increasing Al substitution. The intrinsic magnetization according to the refined atomic magnetic moments and Al site occupation fractions was extracted from the NPD data and compared with the measured macroscopic magnetization. A remarkable agreement exists between the two, confirming the robustness and accuracy of the Rietveld analysis.