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Griffiths-Like Phase in Nanocrystalline Manganite La_{0.85}Ca_{0.15}MnO_3

100%
Pękała M., Pękała K., Szydłowska J., Drozd V.
Acta Physica Polonica A
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2017
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vol. 131
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issue 4
774-776
EN
The ferromagnetic Curie temperatures T_{C} derived from a temperature derivative of AC susceptibility are equal to 106 K and 161 K for the nanocrystalline and polycrystalline manganites, respectively. The magnetic susceptibility and electron spin resonance confirm that the Griffiths-like phase exists above the Curie temperature in paramagnetic matrix of the nanocrystalline manganite. An analysis of electron spin resonance spectra allows to detect the upper temperature limit for an existence of Griffiths-like phase at temperature T_{GI}=290 K, which is somewhat higher than the T_{G} of the magnetic susceptibility.
2
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XANES Study of La_{0.75-x}Gd_xCa_{0.25}MnO_{3-δ} Solid Solutions

88%
Drozd V., Pęka̶a M., Liu R., Lee J., Chen J.
Acta Physica Polonica A
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2006
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vol. 109
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issue 4-5
583-589
EN
A series of solid solutions La_{0. 75-x}Gd_xCa_{0.25}MnO_{3-δ} with 0.0≤x≤0.75 was prepared via carbonate precursor precipitation method. Final sintering was performed at 1250ºC in oxygen flow atmosphere. The samples obtained were characterized by scanning electron microscope, X-ray diffraction measurements. Oxygen stoichiometry was analyzed by iodometric titration method. X-ray absorption spectroscopic methods of Mn L-edge and Mn K-edge X-ray absorption near edge structure were used to study oxidation state of manganese in the solid solutions and elucidate features of their local crystal structure. Orthorhombic crystal structure characteristics of the solid solutions were refined by Rietveld method. An increase in oxygen deficiency and average manganese oxidation state were found to accompany Gd concentration increase in La_{0.75-x}Gd_xCa_{0.25}MnO_{3-δ}. These results are consistent with Mn L-edge X-ray absorption near edge structure spectra, where a gradual change of Mn oxidation state with Gd concentration increase was detected. Origins of oxygen deficiency La_{0.75- x}Gd_xCa_{0.25}MnO_{3-δ} are discussed in terms of structural disorder caused by Gd substitution for La.
3
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Structure and Transport Properties of La_{0.75-x}RE_xCa_{0.25}MnO_3 (RE = Gd, Dy, Ho; 0≤ x≤0.75) Manganites

76%
Drozd V., Pękała M., Kovac J., Skorvanek I., Szymański M., Nedilko S.
Acta Physica Polonica A
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2004
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vol. 106
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issue 5
751-758
EN
Structure and transport properties have been studied for a series of La_{0.75-x}RE_xCa_{0.25}MnO_3 manganites with heavy rare earth ions of Gd, Dy, Ho substituting La with x=0, 0.10, 0.25, 0.50, and 0.75. Polycrystalline samples were prepared by the carbonate precipitation route. The oxygen content was determined by the iodometric titration. The X-ray investigations carried out by the powder method show that the unit cell volume gradually decreases and orthorhombic distortion of the lattice increases with rising RE content. Below the room temperature the electrical resistivity is of the semiconducting type for all the samples studied. Electrical resistivity vs. temperature dependences were analyzed within different models: simple thermal activation, Mott's variable range hopping, adiabatic, nonadiabatic, and bipolaron. The Curie temperatures of Gd, Dy, and Ho substituted manganites determined from magnetization measurements show that at 280 K all the samples are in the paramagnetic phase. The increasing RE fraction reduces magnetization at 4 K as compared to La_{0.75}Ca_{0.25}MnO_3.
4
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Local Structure of Mn in (La_{1-x}Ho_{x})_{2/3}Ca_{1/3}MnO_{3} Studied by X-ray Absorption Fine Structure

64%
Pietnoczka A., Pękała M., Bacewicz R., Drozd V., Fagnard J., Vanderbemden P., Antonowicz J., Zalewski W.
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issue 2
281-285
EN
Results of X-ray absorption fine structure measurements in manganites (La_{1-x}Ho_{x})_{2/3}Ca_{1/3}MnO_{3} with 0.15 < x < 0.50 are presented. When LaMnO_{3} is doped with a divalent element such as Ca^{2+}, substituting for La^{3+}, holes are induced in the filled Mn d orbitals. This leads to a strong ferromagnetic coupling between Mn sites. Ca ions in La_{1-x}Ca_{x}MnO_{3} introduce a distortion of the crystal lattice and mixed valence Mn ions (Mn^{3+} and Mn^{4+}). On the other hand, in manganites (La_{1-x}Ho_{x})_{2/3}Ca_{1/3}MnO_{3} the substitution of La for Ho causes a lattice distortion and induces a disorder, which reduces a magnetic interaction. The ferromagnetic transition temperature and conductivity decrease very quickly with increasing x. The magnetic and transport properties of compounds depend on the local atomic structure around Mn ions. The information on the bond lengths and Debye-Waller factor are obtained from the extended X-ray absorption fine structure (EXAFS) data analysis. The charge state of Mn is determined from the position of the absorption edge in X-ray absorption near edge structure (XANES) data. XAFS results are in good agreement with magnetic characteristics of the studied materials.
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