Enhancement of the physico-mechanical properties of La-doped Mg-Ni-Co tri-ferrites

This report investigates the effect of La doping on the structural, microstructural, magnetic, thermodynamic, and mechanical properties of Mg1/3Ni1/3Co1/3Fe2-xLaxO4 (x = 0.00, 0.02, and 0.08) spinel ferrites prepared via the wet co-precipitation method. The prepared samples attained a single cubic ferrite phase; except for the pure sample, additional minor traces of hematite secondary phase were formed. The La3+ dopants influenced the expansion of the lattice from 8.351 to 8.372 A and the reduction of the crystallite size from 30.21 to 14.02 nm, as the dopant concentration increased from zero to x = 0.08. The morphology of the samples depicted highly agglomerated spherical nanoparticles, in which the agglomeration increased with the doping concentration due to the magnetic interaction and size effects. The local physical properties were investigated using Mossbauer spectrometry analysis at 300 K and 77 K by measuring the isomer shift, the quadrupolar shift, and the hyperfine field. The appearance of sextet peaks was registered for all the samples, accompanied by a paramagnetic doublet for x = 0.08 sample at 300 K. Moreover, the La3+ dopants enhance the lattice distortion due to the mismatch of their ionic radii with Fe3+ ions, causing a decrease in the hyperfine field from 46.9 to 26.0 T, at 300 K. The elemental composition, valence, and cationic distribution were investigated using X-ray photoelectric spectroscopy (XPS). The results showed a migration of Fe3+ ions from octahedral to tetrahedral sites upon La doping, which was matched by Mossbauer and XPS measurements. Additionally, Fourier transform infrared spectroscopy was employed to determine the force constants, elasticity, energy, thermodynamic, and mechanical parameters. The La dopants have enhanced the elastic moduli by 11% and the Debye temperature by 4% with x = 0.08 concentration. The Vickers microhardness measurements showed a reverse indentation size effect of the prepared samples, assured by Meyer's rule. Also, it is revealed that La3+ dopants have boosted the Vickers microhardness, as it increased from 6.951 to 7.520 GPa, as La3+ concentration increased from zero to x = 0.08. This enhancement was correlated to the grain size reduction, following the Hall-Petch relation. These findings suggest the usage of the synthesized La-doped tri-ferrites for industrial applications.