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Two perovskite cobaltites, LaCoO 3 and 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 Mg 2+ or 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 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 ϳ exp͑E A / kT͒ for the hole ͑Co 4+ ͒ -doped samples, while an unusual dependence ϳ 1 / T ͑n =8-10͒ is observed for the electron ͑Co 2+ ͒ -doped samples. At higher temperatures, additional hole carriers are massively populated in the Co 3+ background, leading to a resistivity drop. This transition become evident at ϳ300 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 ͑LS͒ Co 3+ to close-lying paramagnetic high-spin ͑HS͒ Co 3+ states and subsequent formation of a metallic phase of the IS 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 ͑IS͒ phase.

In the quest for suitable materials for hyperthermia we explored the preparation and properties of nanoparticles of Co ferrite. The material was produced by coprecipitation from water solution of Co and Fe chlorides and afterwards annealed at 400, 600 and 800 °C. The resulting particles were characterized by XRD, TEM, Mössbauer spectroscopy, and dc and ac magnetometry. The heating experiments in ac magnetic fields of various amplitudes were performed with diluted systems of particles suspended in agarose gel and the results were interpreted on the basis of the ac magnetic losses measured at various temperatures. The increase of magnetic losses and consequently of the heating efficiency with increasing temperature is explained by the strong dependence of the constant of magnetocrystalline anisotropy of Co ferrite on temperature.

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.

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