English abstract
Motivated by the reported possibility of modifying the physical properties of
ferrite materials by cation-doping and/or reduction of the particle size to the nanometer
scale, we report on the main experimental results of our studies to modify the structural,
magnetic, and optical properties of four ferrite systems synthesized using nonconventional routes. A host of diffraction, spectroscopic and magnetic measurements
techniques were employed. In the 1st system of mechanosynthesized antiferromagnetic
LaFeO3 perovskite-related nanoparticles, we show that doping with a 4d transition metal
(TM) ion, Ru3+, results in structural distortions, where a weak Jahn-Teller-like effect and
a monotonic decrease in the optical band were observed and were associated with the
particle-size surface-induced disorder in the Ru3+- doped mechanosynthesized LaFeO3
nanoparticles. In the second system, a comparative study of the effect of single-cation
doping of LaFeO3 nanoparticles with the rare-earth cation Eu3+ and their double doping
with both Eu3+ and the transition metal Cr3+ ions was performed. In both cases, a structural
transition from the O՝-type to the O-type was observed. Minor traces of reactants,
intermediate phases, and a small amount of Eu2+ ions were detected on the surfaces of the
nanoparticles. A novel aspect of this research is determining the significant influence of
these trace metal oxide impurities on the antiferromagnetic LaFeO3 ground state. This has
resulted in the nanoparticles having antiferromagnetic cores flanked by ferromagnetic
shells, leading to novel magnetic properties, including temperature-dependent magnetic
hardening, spin glass, spin reversal, and a huge exchange bias effect. A key focus of this
study is the calculations related to the prominent role of Eu3+ and Eu2+ ions in the
magnetism of the materials. For the third system of mechanosynthesized GdFeO3
nanoparticles co-doped with the multivalent rare earth Tb3.5+ cation and the TM Mn4+
cations, we show the emergence of intriguing physical properties, including spin
reorientations at two temperatures, a decrease in the optical bandgap, and an increase in
both the absorption and Urbach energies with increasing dopant concentrations. For the
fourth system of the ferrimagnetic garnet-type solid solution of Dy3-xYxFe5O12 (x = 0.0,
1.5, 3.0) nanocrystalline particles synthesized via a modified sol-gel method, we report on
the structural and novel magnetic behaviour that has hitherto not been reported. The latter
is exclusively a size-induced large negative magnetization that is suppressed by Y3+
substitution. This behaviour is associated with the complex anisotropic exchange
interaction between the three magnetic Fe3+ and Dy3+ sublattices and the presence of
magnetic anisotropy that becomes significant at the surface (shell) layers of the
nanoparticles at low temperatures.
