Effect of Sm3+ ions doping on structure and magnetic properties of nanocrystalline NiFe2O4 fibers

The nanocrystalline NiFe2−xSmxO4 (x = 0, 0.05, 0.10, 0.15) fibers with diameters 0.5–2 μm have been prepared using the organic gel-thermal decomposition method. The composition, morphology and structure of the gel precursors and NiFe2−xSmxO4 fibers derived from calcination of these precursors were characterized by FTIR, XRD and SEM. The thermal decomposition of the gel precursors was investigated by TG–DSC and the magnetic properties of the as-prepared fibers were examined using vibrating sample magnetometer (VSM). The results show that the Sm3+ ions doping has not resulted in crystal structural changes of the Ni ferrite spinel structure and all the ferrite fibers obtained are characterized with a simple spinel phase. The crystalline grain sizes of the NiFe2−xSmxO4 fibers are influenced by the calcination temperature and Sm contents. The crystalline grain sizes of the simple NiFe2O4 fibers increase from about 18.9 nm to 45.1 nm corresponding the calcination temperature from 550 °C to 800 °C, while the crystalline grain sizes of the NiFe2−xSmxO4 fibers obtained at 800 °C reduce from 45.1 nm (x = 0) to 14.0 nm (x = 0.15). The lattice parameters of the grains building the NiFe2−xSmxO4 fibers become large with the Sm3+ ions doping owing to larger Sm3+ ions substitution of Fe3+ ions. The saturation magnetization is decreased by addition of Sm3+ ions while increased with the crystalline grain sizes. The coercivity is very small for the NiFe2−xSmxO4 fibers with a high Sm3+ ions doping level (x = 0.15) at calcination temperature of 550 °C and increased with the reduction of Sm contents and increase of the calcination temperature.