Magnetic Properties of La/Ni-Substituted Strontium Hexaferrite Nanoparticles Prepared by Coprecipitation at Optimal Conditions

La/Ni-substituted strontium hexaferrite Sr0.8La0.2NixFe12-xO19 (x = 0.2 to 1.0 in steps of 0.2) nanoparticles have been produced by a coprecipitation method at reaction and calcination temperature of 80°C and 1200°C, respectively. X-ray diffraction (XRD) analysis confirmed formation of single-phase M-type hexaferrite structure. The average crystallite size and morphology of the nanoparticles were obtained from XRD analysis and transmission electron microscopy (TEM), respectively. The magnetic properties in magnetic field of 12 kOe obtained from room-temperature hysteresis loops revealed minimum and maximum magnetization for x = 0.4 and 0.8, respectively, a behavior attributed to the ionic radii of the substituted cations, canted spin structure, electron hopping between cations, and the substitutional sites of the ions. It was also confirmed that the magnetization of nanoscale particles was lower than that of bulk material, which can be explained based on the different behavior of surface versus bulk atoms. The coercivity varied with x in a similar way to the magnetization, being related to sample anisotropy. In the M-type hexaferrite structure, substitution of Fe3+ by Ni2+ occurred at octahedral sites, making the greatest contribution to the anisotropy.