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Design Rules for Manganites with Novel Magnetic and Electronic Properties

100%
Dabrowski B., Chmaissem O., Mais J., Kolesnik S.
Acta Physica Polonica A
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2004
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vol. 105
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issue 1-2
45-56
EN
We have been systematically developing strategies for making new perovskite manganites with novel magnetic and electronic properties. This effort requires a two-fold approach: understanding the dependence of properties on chemical and structural factors and development of the ability to synthesize desired compounds. We show that, similar to other single valent 3d systems, the magnetic superexchange interactions in AMnO_3 manganites (A = rare or alkaline earth's) are dependent on the Mn-O-Mn bond angle which is a function of interatomic distances A-O and Mn-O. The local structural disorder on the A-site suppresses magnetic interactions. Recently, by comparing disordered (randomly mixed La/Ba) and ordered (forming Ba/La/Ba layers along the c-axes) perovskites we have demonstrated much more conspicuous effects of structural and charge disorder for mixed-valent manganites. We show that by stabilizing the ordered structure; i.e., by suppressing local structural and charge disorder on the A-site, a substantial increase in T_C from 340 to 365~K can be achieved. A similar control of order on the Mn-site was achieved for SrMn_{1-y}Ga_y O_{3-y/2} compounds near y=0.5, while SrMn_{1-y}Fe_yO_3 compounds are always randomly mixed. To achieve these compounds, we have been systematically developing special synthesis techniques for extending chemical composition ranges far beyond those previously achieved, developing rules for predicting which compositions should be possible to produce by these special techniques, and establishing methods for selectively ordering or disordering mixtures of metal atoms on the A- or Mn-sites.
2
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Reappearance of Antiferromagnetic Order in Nd_{0.9}Ca_{0.1}BaCo_{2}O_{5+δ} for δ > 0.51

100%
Piętosa J., Kolesnik S., Puźniak R., Dabrowski B.
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EN
The absence of the antiferromagnetic order in Nd_{0.9}Ca_{0.1}BaCo_2O_{5.5} has been found to remain unchanged by applying a hydrostatic pressure of 10 kbar. In magnetic studies of polycrystalline Nd_{0.9}Ca_{0.1}BaCo_2O_{5+δ} (δ=0.07-0.69), we have found a reappearance of the antiferromagnetic phase, caused by an increase in oxygen index δ above 0.51 related to extra oxygen ions addition into the NdO_{δ} plane. For the samples with δ=0.555 and 0.59, a coexistence of well developed antiferromagnetic phase with ferrimagnetic one is evidenced.
3
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Novel Properties of Atomically Arranged Perovskites

88%
Dabrowski B., Kolesnik S., Chmaissem O., Suescun L., Mais J.
Acta Physica Polonica A
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2007
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vol. 111
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issue 1
15-25
EN
Perovskites AMO_{3-δ} attain unique electronic, magnetic, ferroelectric, thermoelectric, mixed-conducting, and other functional properties through selection of the A- and M-site ions, their fractions, ionic sizes and valences, spin states, and orbital orderings, as well as the oxygen content and vacancy ordering. We illustrate here our systematic exploration of the effects of composition, temperature, pressure, and oxygen content on the thermodynamic stability and magnetic properties for La_{1-x-y}Sr_xBa_yMnO_3 manganites. Parameters controlling magnetic transitions are identified through examples of single-valent compounds of RMnO_3 (the Mn-O-Mn bond angles that can be equivalently described in terms of the tolerance factor) and Sr_{1-x}Ca_xMnO_3 (the tolerance factor and disorder described in terms of the variance of A-site ion sizes), and the mixed-valent La_{0.5}Sr_{0.5-y}Ba_yMnO_3 (the tolerance factor, variance of sizes, and the local strains described in terms of the elongated Mn-O bonds). By using an example of kinetically stable, atomically-ordered layered-perovskites we show that improved useful properties, such as the increase in the Curie temperature T_C and enhancement of colossal magneto-resistive effect at room temperature, can be achieved through decreases in structural distortions such as variance of sizes and local strains.
4
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Influence of Pressure on Magnetic Properties of La_{1-x} Sr_xCoO_{3-δ}(x=0.5, 0.67, 1)

76%
Piętosa J., Wiśniewski A., Puźniak R., Kolesnik S., Majjiga M., Dabrowski B.
Acta Physica Polonica A
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2006
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vol. 109
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issue 4-5
527-532
EN
Influence of pressure on the phase transition temperature T_C and on the spontaneous magnetisation M_{0} of highly Sr substituted and oxygen nonstoichiometric La_{1-x}Sr_xCoO_{3-δ} samples was investigated. An increase in T_C and M_0 with pressure in investigated samples was found, indicating strengthening of ferromagnetic double exchange interactions, and the expansion of volume of ferromagnetic phase as a~result of enhancement in ferromagnetic interactions in both cluster-glass and long range ferromagnetic states. We did not observe clear correlation between changes in pressure coefficient dT_C/dP and changes in the formal oxidation state of the Co ion.
5
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Influence of Hydrostatic Pressure on Magnetic Properties of (Sr,La)(Ru,Cr,Mn)O_3

76%
Piętosa J., Wiśniewski A., Puźniak R., Kolesnik S., Majjiga M., Dabrowski B.
Acta Physica Polonica A
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2007
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vol. 111
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issue 1
159-163
EN
An influence of hydrostatic pressure, P, on phase transition temperature, T_C, and spontaneous magnetisation, M_0, of selected perovskite ruthenates (SrRuO_3, La_{0.2}Sr_{0.8}RuO_3, SrRu_{0.9}Mn_{0.1}O_3, and SrRu_{0.9}Cr_{0.1}O_3) was determined to 12 kbar. A decrease in T_C with pressure was found for all of the studied samples. The M_0(P) remains unchanged for most of the samples, except for the La_{0.2}Sr_{0.8}RuO_3 sample. The weakening of ferromagnetic interactions with increasing pressure is consistent with complex band structure effects related to the modulation of the Ru-O hybridisation by the change of structural distortion.
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