Phase formation, magnetic properties, and phase stability in reducing atmosphere of M-type strontium hexaferrite nanoparticles synthesized via a modified citrate process
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Nanosize Sr-hexaferrite particles (SrM) were synthesized via a citrate-based sol–gel route, and the details of the calcination reaction conditions were investigated. Thermal decomposition of a citrate precursor proceeds in a two-step process: at low temperature T1 the precursor decomposes into maghemite and Sr carbonate, and transforms into hexaferrite upon a second treatment at another temperature T2. A synthesis protocol with T1 = 350 °C and T2 = 650 °C gives hexaferrite particles with size of below 100 nm. A systematic study of reaction conditions revealed that the formation of a hematite-free decomposition product at T1 is the prerequisite for the synthesis of single-phase hexaferrite nanosize particles. The hexaferrite particles exhibit a saturation magnetization at room temperature of Ms = 58 emu/g with a coercivity of Hc = 3.7 kOe. Further fine-milling of the as-synthesized ferrite in aqueous media gives particles below 50 nm in size with Ms = 48–54 emu/g and Hc = 4.2–5.5 kOe under preservation of the M-type structure. The thermal stability of SrM particles under reducing conditions at moderate temperature was also studied. Annealing of ferrite particles in Ar/5%H2 atmosphere at 350 °C results in magnetite formation; iron is formed at T ≥ 450 °C after complete hexaferrite decomposition; hence, SrM@Fe nanocomposites are not accessible via particle reduction of SrM particles.
The authors acknowledge financial support from the Federal Ministry of Research and Education, Germany, through Grant 03X3582D. We thank Dr. H.-J. Hempel for TEM microscopy (University Jena, Germany).
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