In this paper, strontium hexaferrite nanoparticles were synthesized by the sol-gel auto-combustion method. Effect of annealing temperature on crystal structure, morphology and magnetic properties of nanoparticles was investigated by X-ray diffraction (XRD), Fourier transform infrared (FT-IR), transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM). Also, the thermal decomposition of as-synthesized powdered samples has been studied by thermogravimetric analysis (TGA). The XRD patterns confirmed the formation of single phase M -type hexagonal crystal structure for powders annealed above 950 • C, whereas the presence of hematite (α-Fe 2 O 3 ) as secondary phase was also observed for sample annealed at 900 • C. Furthermore, the crystallinity along with the crystallite size were augmented with annealing temperature. Comparison of the FT-IR spectra of the samples before and after annealing treatment showed the existence of metal-oxygen stretching modes after annealing. The thermogravimetric analysis confirmed the thermal decomposition of as-burnt powders happened in three-stage degradation process. The TEM images showed the nanoparticles like hexagonal-shaped platelets as the size of nanoparticles increases by increasing the annealing temperature. With increasing annealing temperature, the magnetic saturation and the coercivity were increased to the maximum value of 74.26 emu/g and 5.67 kOe for sample annealed at 1000 • C and then decreased. ‡ Corresponding author. 1550190-1 Int. J. Mod. Phys. B Downloaded from www.worldscientific.com by FLINDERS UNIVERSITY LIBRARY on 10/02/15. For personal use only. E. Roohani et al.

In this research, SrFe[Formula: see text]Ni[Formula: see text]O[Formula: see text] (x [Formula: see text] 0 − 1) hexagonal ferrites were prepared by sol-gel auto-combustion method. Effect of Ni substitution on structural, morphological and magnetic properties of nanoparticles was investigated by X-ray diffraction (XRD), Fourier transform infrared (FT-IR), Transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM), respectively. The XRD results confirmed that all samples with x [Formula: see text] 0.5 have single phase M-type strontium ferrite structure, whereas for the SrFe[Formula: see text]Ni[Formula: see text]O[Formula: see text] samples with x [Formula: see text] 0.5, the spinel NiFe2O4 phase has also appeared. The lattice parameters and crystallite sizes of the powders were concluded from the XRD data and Williamson–Hall method. Magnetic analyses showed that the coercivity of powders decreased from 5672 Oe to 639 Oe while the saturation magnetization increased from 74 emu/g to 81 emu/g with nickel substitution. The results of this study suggest that the strontium hexaferrites doped with Ni are suitable for applications in high density magnetic recording media as well as microwave devices because of their promising magnetic properties.

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