Superparamagnetic Behavior in the Two Polymorphous Lithium Ferrite Nanoparticles

摘要

The two polymorph modifications of LiFe₅O₈, fine particles were successfully prepared by ceramic technique at pre-sintering temperature of 500℃ and the pre-sintered material was crushed and sintered finally in the open air at 1000℃. The structural and microstructural evolutions of the nanophase were studied using powder X-ray diffraction (XRD) and the Rietveld method. The two polymorphous LiFe₅O₈ includes the ordered phase, termed α-LiFe₅O₈, with a primitive cubic unit cell (space group P4₃32, a=8.337 ?) in which the Fe³⁺ ions are at octahedral 12d and tetrahedral 8c sites, and Li⁺ ions occupy only the octahedral 4b positions in the cubic primitive unit cell and disordered face centered cubic structure, termed β-LiFe₅O₈ (space group Fd-3m, a = 8.3333 ?), The refinement of XRD results revealed the presence of mixed and nearly inverse spinel nanocrystalline Li-ferrite phases. Transmission electron microscopy (TEM) revealed that the particles of the prepared samples were spherical in shape, and were found to be ~20 nm in size, which increased up to ~ 60 nm on calcinations at 1000℃. TEM micrograph shows that the resultant nanospheres are solid like balls, the mechanism based on the lithium ferrite nanoparticles self-growth process is proposed in the formation of the nanospheres and TEM analysis confirmed the particle size obtained from X-ray results. Scherrer’s equation was used to calculate the particle size of lithium ferrite from XRD data. Magnetic measurements were assessed using a vibrating sample magnetometer. Our results for the two polymorphs LiFe₅O₈ prepared samples show that the H_C is very small and approaches zero compared to the applied field, the prepared samples for the two polymorphs exhibits no hysteresis, which may be attributed to superparamagnetic relaxation as confirmed by XRD.