Publikationsserver der RWTH Aachen University
Synthesis of CMR manganites and ordering phenomena in complex transition metal oxides
Abstract
dc:descriptionThis dissertation covers: the instrumentation development; the optimization synthesis of polycrystalline materials; the single crystal growth of perovskite manganites; the determination of crystal and magnetic structures from x-ray and neutron powder diffraction data of polycrystalline and powdered single-crystalline CMR La1-xSrxMnO3 manganites as well as its application in quantitatively correlating structural and magnetic properties; the application of resonant soft x-ray scattering in the investigation of complex ordering phenomena in La7/8Sr1/8MnO3 and La1Sr2Mn2O7 at the transition metal LII, III-edges and the rare earth MIV, V-edges. Two new devices have been developed: one is the rotating specimen holder for laboratory x-ray powder diffraction, which not only can effectively minimize the problem of preferred orientation but also produces a ‘zero-background’ contribution to the collected data; another is the miniature goniometer for resonant soft x-ray scattering, which makes it feasible to adjust samples along chi (±2.5°) and psi (360°) and dramatically improves the efficiency of soft x-ray scattering chambers and can be utilized to unravel the electronic ordering phenomena by resonant soft x-ray scattering. The materials synthesis is the first and critical step for any research in condensed matter physics. One good route has been acquired for preparing high-quality polycrystalline La7/8Sr1/8MnO3 samples, i.e. two times calcining and two times sintering, through exploring various preparation parameters. This synthesis route can be also used to prepare high-quality La1Sr2Mn2O7 and La0.83Ca0.17MnO3. Thus it is a universal one for the present testing. The route for optimizing the synthesis of polycrystalline manganites has been established. One method for preparing straight and very uniformly densified feed rods with a homogenous composition distribution has been acquired, which makes growing large high-quality single crystals possible. Three nominal La1-xSrxMnO3 (x = 1/8) polycrystals prepared under different conditions air, Ar and O2 at 1 atm show a huge contrast in the Curie temperature Tc. A combined Rietveld refinement of the room-temperature x-ray and neutron powder diffraction data has been performed for these three samples. This study not only can determine the relative content of the various elements (especially the oxygen) with higher precision as compared to one probe alone, but also makes it possible to correlate the drastic change in Tc with the detailed structural parameters. In addition, the individual effect of the number of nearest magnetic neighbors on the structural and magnetic properties has been investigated in detail using a set of La7/8Sr1/8MnyO3 samples with initial y = 0.97, 1.00 and 1.03. Moreover, a systematic neutron powder diffraction study of the Ar annealed and air sintered La7/8Sr1/8Mn1-gammaO3+delta; samples between 10 and 900 K has been performed, where the structural and magnetic phases and their respective transitions for two samples are completely different. For the air sintered sample, a structural transition from rhombohedral to orthorhombic upon cooling from 180 to 65 K has been observed for the first time. From the refined parameters the cooperative Jahn-Teller distortion parameter Delta, the tolerance factor t based on the equilibrium La(Sr)-O and Mn-O bond lengths, and the one-electron bandwidth W of the eg-band have been determined. A neutron powder diffraction study on the crystal and magnetic structure of a powdered La1-xSrxMnO3 (x approximate 1/8) single crystal from 5 to 300 K has been performed systematically. The structural information obtained from this study provides the basis for establishing the correct charge- and orbital- order model from single crystal anomalous x-ray scattering. Especially, the structural information in the Jahn-Teller regime has been correctly established for the first time. The SQUID measurements show very interesting magnetic properties, e.g. the existence of magnetic polarons. Based on the size of Jahn-Teller distortion and the Bond-valence sums, the charge/orbital ordering phenomena are briefly discussed. The first direct evidence of the hole ordering in a single-crystalline La1-xSrxMnO3 (x approximate 1/8) has been observed, i.e. strong soft x-ray resonances of the superstructural reflection (0 0 ½) in Pbnm symmetry have been observed at the Mn LIII-edge and the oxygen K-edge but not at the Mn K-edge. A double-layered La1Sr2Mn2O7 has been systematically investigated via the resonant soft x-ray scattering. Two reflections were observed for La1Sr2Mn2O7, (0 0 1) at the Mn LII, III-edges and (0 0 2) at the La MIV, V-edges. The (0 0 1) reflection can be clearly assigned to the A-type antiferromagnetism, whereas the unexpected dramatic enhancement of the Bragg (0 0 2) reflection was observed for the first time and the possible driving forces are briefly discussed. In addition, the six obvious features displayed in the energy dependence of (0 0 1) are confirmed to purely originate from the spin ordering. The temperature dependence of (0 0 2) shows a strong interplay with that of the orbital ordering reflection.
Degree
thesis:*- Grantor dc:publisher
- Publikationsserver der RWTH Aachen University
- Year dc:date
- 2008
Author and committee
dc:creator, dc:contributor.*- Author dc:creator
-
- Li, Haifeng
- Contributors dc:contributor
-
- Brückel, Thomas
Subjects
dc:subject × 8Rights
dc:rights- Statement dc:rights
-
- info:eu-repo/semantics/openAccess
- Language dc:language
- eng