Two perovskite cobaltites, ${\text{LaCoO}}_3$ and ${\text{DyCoO}}_3$, which are border compounds with respect to the Ln size, were investigated by the electric resistivity and thermopower measurements up to 800–1000 K. Special attention was given to effects of extra holes or electrons, introduced by light doping of Co sites by ${\text{Mg}}^{2+}$ or ${\text{Ti}}^{4+}$ ions. The experiments on the La-based compounds were complemented by magnetic measurements. The study shows that both kinds of charge carriers induce magnetic states on surrounding ${\text{Co}}^{3+}$ sites and form thus thermally stable polarons of large total spin. Their itinerancy is characterized by low-temperature resistivity, which is of Arrhenius type $\rho \sim \text{exp}\left(E_A/kT\right)$ for the hole $\left({\text{Co}}^{4+}\right)$ -doped samples, while an unusual dependence $\rho \sim 1/T^{\nu }$ $\left(n=8–10\right)$ is observed for the electron $\left({\text{Co}}^{2+}\right)$ -doped samples. At higher temperatures, additional hole carriers are massively populated in the ${\text{Co}}^{3+}$ background, leading to a resistivity drop. This transition become evident at $\sim 300\text{K}$ and 450 K and culminates at $T_{I-M}=540$ and 780 K for the La- and Dy-based samples, respectively. The electronic behaviors of the cobaltites in dependence on temperature are explained considering local excitations from the diamagnetic low-spin (LS) ${\text{Co}}^{3+}$ to close-lying paramagnetic high-spin (HS) ${\text{Co}}^{3+}$ states and subsequent formation of a metallic phase of the IS ${\text{Co}}^{3+}$ character through a charge transfer mechanism between LS/HS pairs. The magnetic polarons associated with doped carriers are interpreted as droplets of such intermediate (IS) phase.

Nanoparticles of manganese perovskite of the composition La(0.75)Sr(0.25)MnO(3) uniformly coated with silica were prepared by encapsulation of the magnetic cores (mean crystallite size 24 nm) using tetraethoxysilane followed by fractionation. The resulting hybrid particles form a stable suspension in an aqueous environment at physiological pH and possess a narrow hydrodynamic size distribution. Both calorimetric heating experiments and direct measurements of hysteresis loops in the alternating field revealed high specific power losses, further enhanced by the encapsulation procedure in the case of the coated particles. The corresponding results are discussed on the basis of complex characterization of the particles and especially detailed magnetic measurements. Moreover, the Curie temperature (335 K) of the selected magnetic cores resolves the risk of local overheating during hyperthermia treatment.

The structural and magnetic properties of Co(1-x)Zn(x)Fe2O4 ferrites (Co-Zn ferrites) are investigated in a narrow compositional range around x = 0.6, which is of interest because of applications in magnetic fluid hyperthermia. The study by x-ray and neutron diffraction, Mössbauer spectroscopy and magnetization measurements is done on nanoparticles prepared by the coprecipitation method and bulk samples sintered at high temperatures. In spite of the known preference of Zn2+ for tetrahedral (A) sites and Co2+ for octahedral [B] sites, the cations are distributed nearly evenly over the two sites of spinel structure and there is also a variable number of [B] site vacancies (see text), making cobalt ions trivalent. In particular for x = 0.6, the cationic distribution is refined to [Formula: see text] and [Formula: see text] for the 13 nm particles (T(C) = 335 K) and bulk sample (T(C) = 351 K), respectively.