Phase formation, magnetic properties, and phase stability in reducing atmosphere of M-type strontium hexaferrite nanoparticles synthesized via a modified citrate process

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 T-1 the precursor decomposes into maghemite and Sr carbonate, and transforms into hexaferrite upon a second treatment at another temperature T-2. A synthesis protocol with T-1=350 degrees C and T-2=650 degrees C gives hexaferrite particles with size of below 100nm. A systematic study of reaction conditions revealed that the formation of a hematite-free decomposition product at T-1 is the prerequisite for the synthesis of single-phase hexaferrite nanosize particles. The hexaferrite particles exhibit a saturation magnetization at room temperature of M-s=58emu/g with a coercivity of H-c=3.7kOe. Further fine-milling of the as-synthesized ferrite in aqueous media gives particles below 50nm in size with M-s=48-54emu/g and H-c=4.2-5.5kOe 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%H-2 atmosphere at 350 degrees C results in magnetite formation; iron is formed at T450 degrees C after complete hexaferrite decomposition; hence, SrM@Fe nanocomposites are not accessible via particle reduction of SrM particles.