Papers by Author: A.N. Taldenkov

Abstract: The oxygen isotope effect in ordered half-doped manganite PrBaMn₂^16-18O₆ has been investigated. Real and imaginary parts of ac magnetic susceptibility, dc magnetization and magnetoresistance were measured at temperatures from 4.2 to 320 K in applied magnetic field up to 35 kOe. Substantial increase of charge ordering (CO) transition temperature T_CO under oxygen isotope substitution ¹⁶O ¹⁸O and small decrease of ferromagnetic (FM) transition temperature T_FM were found. Small systematic shift of ferromagnetic transition temperature in oxygen reduced manganite PrBaMn₂^{16 18}O₅ is also considered. The obtained results are compared with those observed in other manganites demonstrating the large isotope effect.

Abstract: Effect of the partial oxygen isotope substitution on (Pr_1-yEu_y)_0.7Ca_0.3CoO₃ cobaltites was studied in detail for the crossover region (0.12<у<0.26) of the phase diagram. We prepared a series of the cobaltite samples with the different degrees of enrichment by the heavy oxygen isotope ¹⁸O, namely, containing 90%, 67%, 43%, 17%, and 0% of ¹⁸O. The Eu doping of the samples was chosen to be near and on the both sides of the crossover doping level y_cr. Based on measurements of the ac magnetic susceptibility χ(Т) and electrical resistivity ρ(Т), we were able to analyze the evolution the sample properties with the change Eu and ¹⁸O content.

Abstract: The magnetic/spin-state phase diagram of the (Pr1-yEuy)0.7Ca0.3CoO3 series was obtained on the basis of measurements of the specific heat, thermal expansion, magnetization and resistivity. The phase diagram reveals three different states depending on the static distortions (Eu content), the oxygen-isotope mass, and the temperature. The samples with the lower Eu concentrations are ferromagnetically ordered up to moderate temperatures (about 50 K),, most probably, due to the low-spin Co4+ – intermediate-spin Co3+ interaction of the double-exchange type. As the Eu doping increases, the Co3+ LS (S = 0) state becomes stabilized and the magnetic ordering of the Co4+ ions is suppressed up to temperatures well below 5 K, resulting in a low-temperature anomaly in Cp. At higher temperatures, we observe a first-order spin-state transition from the LS to the IS state of Co3+, which is accompanied by a decrease in the electrical resistivity, an increase in the magnetization, and a strong lattice expansion.

Abstract: The effect of 16О → 18О isotope substitution on the properties of Pr0.5Ca0.5MnO3 manganites doped by Cr and Ru is studied. In these compounds, chromium and ruthenium favor (i) the suppression of a charge-ordered state and (ii) the formation of a ferromagnetic metallic phase. The 16О → 18О isotope substitution leads to the growth of the charge-ordering transition temperature (TCO), and to the lowering of ferromagnetic transition temperature (TFM) accompanied by a decrease in the content of ferromagnetic phase. The difference in the behavior of the Cr- and Ru-substituted samples is analyzed.

Abstract: The oxygen isotope effect in ceramic bilayer manganite (La1 zPrz)1.2Sr1.8Mn216 18O7 (z = 0, 0.3, 0.4, 0.6) has been investigated. Real and imaginary parts of ac magnetic susceptibility, dc magnetization and magnetoresistance were measured at temperatures from 4.2 to 320 K in applied magnetic field up to 40 kOe. Substantial decrease of ferromagnetic (FM) transition temperature TFM under oxygen isotope substitution 16O → 18O was found. This positive isotope effect amounts to more than 20 K. A number of additional transitions of ferromagnetic type were observed at T >170 K, which also depend on oxygen overall mass. The obtained results are compared with those observed in other manganites exhibiting the large isotope effect.