| 1. |
- Pashchenko, V. P., et al.
(författare)
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Structural and magnetic inhomogeneity and the NMR of Mn-55 and La-139 in the magnetoresistive ceramics La0.7Ba0.3-xSnxMnO3 -> La0.7-xBa0.3-xMnO3+0.5xLa(2)Sn(2)O(7)
- 2003
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Ingår i: Low temperature physics (Woodbury, N.Y., Print). - : AIP Publishing. - 1063-777X .- 1090-6517. ; 29:11, s. 910-916
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Tidskriftsartikel (refereegranskat)abstract
- The effects of substitution of barium by tin on the phase composition, structural imperfection, and properties of lanthanum manganite perovskites La0.7Ba0.3-xSnxMnO3 (X=0, 0.1, 0.15, 0.2, 0.3) are established by comprehensive studies done by x-ray diffraction, resistive, and magnetic (including NMR on Mn-55 and La-139) methods. It is shown that the introduction of Sn leads to the formation of a pyrochloric phase La2Sn2O7, to an increase in the density of lattice defects of the manganese-enriched main lanthanum manganite phase, and to a substantial decrease of the magnetoresistive effect. The smearing of the metal-semiconductor phase transition temperature is explained by an increase in the inhomogeneity and imperfection of the perovskite structure. The low activation energy is confirmed by a high degree of inhomogeneity and imperfection of the crystal lattice of the samples studied. The broad, asymmetric NMR spectra of Mn-55 and La-139 attest to high-frequency electron exchange between Mn3+ and Mn4+ and nonequivalence of the environment of those ions and La3+ due both to heterovalent ions and to vacancies and clusters.
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| 2. |
- Pashchenko, V. P., et al.
(författare)
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Structure and properties of La0.6Sr0.4-xBaxMnO3 (0 <= x <= 0.4) magnetoresistive ceramics
- 2002
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Ingår i: Inorganic Materials (Neorganicheskie materialy). - 0020-1685 .- 1608-3172. ; 38:3, s. 302-307
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Tidskriftsartikel (refereegranskat)abstract
- The effect of Ba content on the phase composition, lattice parameter, Mn-55 NMR spectrum, resistance, and magnetoresistance of La0.6Sr0.4-xBaxMnO3 perovskites was studied. The lattice parameter of the single-phase ceramic samples was found to increase with increasing x. The tolerance factor increases and the ionic field strength decreases as x increases from 0 to 0.4. The Mn-55 NMR results obtained at 77 K indicate a high-frequency electron exchange between Mn3+ and Mn4+ in the ferromagnetic phase. The peak-magnetoresistance temperature tends to decrease with increasing x. The magnetoresistance of the ceramics is correlated with the bond covalence. It is inferred that the ceramics contain inhomogeneities differing in nature and length scale (macroscopic, mesoscopic, and microscopic), which influence their magnetoresistance.
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| 3. |
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| 4. |
- Migas, Dmitry B., et al.
(författare)
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Temperature induced structural and polarization features in BaFe12O19
- 2023
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Ingår i: Journal of Materials Chemistry C. - : Royal Society of Chemistry (RSC). - 2050-7526 .- 2050-7534. ; 11:36, s. 12406-12414
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Tidskriftsartikel (refereegranskat)abstract
- We report the observation of a peculiar polarization behavior of BaFe12O19 in electric field where the linear polarization is detected at temperatures below 150 K whereas at higher temperatures a hysteresis-like polarization response is observed. At the same time, the performed neutron diffraction analysis shows no variations in crystal or magnetic structures with temperature. Based on the results of ab initio calculations we suggest the mechanism able to explain the experimentally observed behavior. We show that specific Fe atoms do not occupy the positions formally assigned to them by the conventional centrosymmetric P6(3)/mmc (#194) space group (z = 0.25; 0.75) as these positions correspond to local energy maxima. Instead, these Fe atoms are shifted along the z-axis to positions z = 0.259 (0.241) and z = 0.759 (0.741), which correspond to local energy minima. To an inversion center move between these minima Fe atoms need to overcome an energy barrier. This barrier is rather insignificant for smaller volumes but it becomes larger for expanded volumes due to coupling between the displacements of these Fe atoms. Additionally, our analysis suggests that the non-centrosymmetric and polar P6(3)mc (#186) space group could be appropriate for the description of the BaFe12O19 structure.
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