K. Knížek

Electrical resistivity and thermopower measurements of the hole- and electron-doped cobaltites Ln CoO 3

Two extreme members of the cobaltite series, LaCoO3 and DyCoO3, were investigated by the electrical 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 Mg2+ or Ti4+ ions. The experiments on the La based compounds were complemented with magnetic measurements. The study shows that both kinds of charge carriers induce magnetic states on surrounding CoIII sites and form thus thermally stable polarons of large total spin. Their itinerancy is characterized by low temperature resistivity, which is of Arrhenius type r~exp(EA/kT) for the hole (CoIV) doped samples, while an unusual dependence r~1/Tn (n=8-10) is observed for the electron (CoII) doped samples. At higher temperatures, additional hole carriers are massively populated in the CoIII background, leading to a resistivity drop. This transition become evident at ~300 K and 450 K and culminates at TI-M=540 and 780 K for the La and Dy based samples, respectively. The electronic behaviours of the cobaltites are explained considering two excitation processes in parent compounds. The first one is related to a local excitation from the diamagnetic LS CoIII to close-lying paramagnetic HS CoIII state. Secondarily, a metallic phase of the IS CoIII character is formed through a charge transfer mechanism between LS/HS pairs. The magnetic polarons associated with doped carriers are interpreted as droplets of such IS phase.

Dielectric, magnetic, and lattice dynamics properties of Y-type hexaferrite Ba0.5Sr1.5Zn2Fe12O22: Comparison of ceramics and single crystals

We prepared multiferroic Y-type hexaferrite Ba0.5Sr1.5Zn2Fe12O22 ceramics and compared their magnetic and dielectric properties with single crystal. Magnetic susceptibility and microwave resonance measurement revealed magnetic phase transition at TC=312u2002K, similar as in single crystal. Ferroelectric (FE) phase can be induced by external magnetic field in all investigated samples and the phase diagram in ceramics qualitatively resembles that of the single crystal. The range of magnetic fields, where the FE phase is induced, broadens after annealing of single crystal. Ceramics quenched after sintering exhibit several orders of magnitude lower conductivity than the single crystal. Heavily damped magnetic resonance was discovered in terahertz spectra at 10 K and its frequency softens below 5 GHz near TC. Number and symmetry of observed infrared (IR) and Raman active phonons correspond to paraelectric phase with D3d5 hexagonal structure. No evidence for a structural phase transition was found in the IR and Ram...

Relaxor-like behavior of lead-free Sr2LaTi2Nb3O15 ceramics with tetragonal tungsten bronze structure

Lead-free tetragonal tungsten bronze Sr2LaTi2Nb3O15 ceramics were prepared and its crystal structure determined. Dielectric response of Sr2LaTi2Nb3O15 was investigated in a broad frequency (100 Hz–90 THz) and temperature range. The observed relaxor-like behavior can be generally described in the framework of polar nanocluster dynamics concept. However, some peculiarities in the dielectric response, relaxational, and crystal lattice dynamics (e.g., hardly resolved Vogel–Fulcher part of the relaxation, reduced dielectric strength of the cluster flipping mechanism, absence of ferroelectric phase under electric field, absence of the polar soft phonon, etc.) were revealed. Therefore, we claim that Sr2LaTi2Nb3O15 is more alike to dipolar glasses than lead-based perovskite relaxor ferroelectrics.

Stabilization of the high-spin state of Co3+in LaCo1−xRhxO3

The rhodium doping in the LaCo

Structure and properties of novel cobaltates Ln0.3CoO2 (Ln = La, Pr, and Nd)

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Valence and spin states in perovskites LaCo0.95M0.05O3 (M=Mg, Ga, Ti)

Rh

Glassy ferromagnetism and phase separation in Pr0.5Ca0.5CoO3

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Magnetism and transport properties of layered rare-earth cobaltates Ln0.3CoO2

O

EXAFS study of LaMn1-xCoxO3 compounds

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Terbium induced glassy magnetism in La,Ca-based cobaltites

perovskite series (