Effects of synthesis route on the structural and magnetic properties of Sr1-xRExFe12O19 nanocrystalline hexaferrites

The present study is concerned with the synthesis and characterization of rare-earth-doped SrM (Sr 1- x RE x Fe 12 O 19 ) hexaferrites (RE = La, Ce and Tb; x = 0, and 0.1). The hexaferrite precursor powders were prepared by high-energy ball-milling and sol–gel auto-combustion methods, and the powders were sintered at temperatures ≥ 900 °C. The effects of RE substitution and synthesis route on the structural and magnetic properties of SrM hexaferrites were investigated by X-ray diffraction, scanning electron microscopy (SEM) and vibrating sample magnetometry (VSM). Single SrM phase was obtained in the sample with x = 0, and the sample with La substitution was prepared by ball-milling method. The rest of the samples, however, contained small amounts of minor phases of rare-earth oxides and α-Fe 2 O 3 in addition to the major SrM phase. SEM imaging revealed significant decrease in particle size with RE substitution. Also, VSM measurements revealed small decrease in the saturation magnetization, and a significant increase in the coercivity with RE substitution. Comparable saturation magnetization was observed for the samples prepared by sol–gel method, whereas the coercivity increased significantly. The coercivity of samples prepared by sol–gel method exhibited a large increase with the decrease in sintering temperature, reaching ~ 6.2 kOe for the samples with Ce- and Tb-substituted samples sintered at 900 °C. The relatively high remnant magnetization (~ 30–35 emu/g) and high coercivity make these materials important candidates for permanent magnet applications.