Magnetic properties of BaNixFe12–xO19 (x=0.0–1.0) hexaferrites, synthesized by citrate–gel auto-combustion and sintered by conventional and spark plasma methods

Abstract

The influence of nickel substitutions on the local structure, dielectric and magnetic properties of Ba hexaferrites is investigated in this study for use as rare earth free permanent magnets and microwave devices, especially. For this purpose, BaNixFe12–xO19 (x = 0.0, 0.2, 0.5, 0.8 and 1.0) powders were prepared by auto–combustion of their citrate–nitrate gels precursor whilst spark plasma sintering and conventional sintering techniques were used in order to prepare BaNixFe12–xO19 sintered ceramics samples. Structure, microstructure, dielectric and magnetic properties of BaNixFe12–xO19 materials were investigated by X-ray diffraction, scanning electron microscopy (SEM), dielectric measurements, Mössbauer spectroscopy and SQUID magnetometry. X-ray diffraction analyses indicated that BaNixFe12–xO19, x = 0.0–1.0crystallized on the hexagonal BaFe12O19 structure and, the lattice parameters a and c, decrease as the Ni content increases. Raman spectroscopy analysis confirmed the formation of the BaFe12O19 phase. The presence of Fe in the +3 oxidation state (Fe3+) and of Ni2+ is confirmed by X-ray photoelectron spectroscopy (XPS). The SEM investigations on BaNixFe12–xO19 powders showed the change of the morphology from a granular-like structure to a platelet-like structure with increasing the nickel content which influences the dielectric and magnetic properties of these materials. Dielectric measurements performed on BaFe12−xNixO19 ceramics showed consistent differences between the two sintering methods. Impedance analysis shows a contribution of grains and grain boundaries in all samples. The Mössbauer spectroscopy results evidenced the five Wyckoff crystallographic iron positions and their site occupancy with respect to Ni substitutions content. The permanent magnet characteristics of the hexaferrites were confirmed by the high values of coercive field (up to ∼ 1.7 kOe) and remanence (∼28 emu/g), while the saturation magnetization values are found up to 95 emu/g at low temperatures and 70 emu/g at room temperature (for x = 0.8). The coercive field, remanence and maximum energy product decrease with nickel content, while the saturation magnetization behaves unmonotonically due to the specific variation of the site occupancy. More insight into the link between local structure and magnetic properties of Ni substituted Ba hexaferrite was given.