Magnetic Properties of Lithium Ferrite Nanoparticles with a Core/Shell Structure

We present a magnetic study of lithium ferrite nanoparticles of composition Li0.5Fe2.5O4, synthesized by a citrate gel decomposition method. The as prepared sample was composed of nearly spherical nanoparticles with an average particle size < d >(TEM)similar to 12 nm. Further annealing at 573 K and 673 K for 4 hours did not increase particle size noticeably, while annealing at 973 K led to morphology changes and significant increase in size ranging from 40 to above 200 nm. The magnetic properties of samples have been studied using Mossbauer specroscopy, and static magnetic measurements. The hyperfine parameters obtained from Mossbauer data at T = 10 K are in agreement to the bulk lithium ferrite phase. Annealed samples showed an evolution from monodomain structure to polycrystalline behaviour, what is evident from TEM imagines, as well as the evolution of the coercive filed, H-C, and the saturation magnetization, M-S, with particle size increase. The exchange interactions have been observed in the single-domain nanoparticles, which probably originate from their core-shell structure. At low temperatures and in high enough magnetic fields, the cubic magnetic anisotropy stays preserved and the magnetic moments in the particle core are aligned along < 111 > directions of the spinel structure.