The crystallographic and magnetic properties of manganese oxides belonging to the Ruddlesden Popper (RP) series, described by the formula A<sub>n+1</sub>B<sub>n</sub>O<sub>3n+1</sub>, are presented. Compounds having n = 2, 3 and ∞ are discussed in Chapters 3, 4 and 5, respectively, their properties being studied by powder X-ray and neutron diffraction, SQUID magnetometry and magnetotransport measurements. In Chapter 3 (A<sub>3</sub>B<sub>2</sub>O<sub>7</sub>) it is shown that Sr<sub>2-x</sub>Nd<sub>1+x</sub>Mn<sub>2</sub>O<sub>7</sub> (0 ≤ x ≤ 0.5) and Sr<sub>2</sub>PrMn<sub>2</sub>O<sub>7</sub> exhibit colossal, negative magnetoresistance (CMR) below 150K. The zero field resistivity signal is reduced by 2-3 orders of magnitude in a field of 14T, at ~4.2K. The compounds Sr<sub>2</sub>HoMn<sub>2</sub>O<sub>7</sub> and Sr<sub>2</sub>YMn<sub>2</sub>O<sub>7</sub> show no significant magnetoresistance. In contrast to the tetragonal Sr,Nd and Sr,Pr compositions, these compounds show a symmetry lowering to space group P4<sub>2</sub>/mnm, and spin glass freezing on the Mn sublattice at ~20K. Chapter 4 (A<sub>4</sub>B<sub>3</sub>O<sub>10</sub>) focuses on A- and B-cation substitutions in the parent compound Ca<sub>4</sub>Mn<sub>3</sub>O<sub>10-δ</sub>. Substitution of Ca by Sr yields Sr<sub>4</sub>Mn<sub>3</sub>O<sub>10-δ</sub>, an orthorhombic (Cmca) compound, composed of trimers of face sharing octahedra. The magnetic susceptibility of this sample is interpreted in terms of direct and indirect antiferromagnetic (AFM) Mn-Mn exchange interactions. The orthorhombic (Pbca) RP phase Ca<sub>3.95</sub>La<sub>0.05</sub>Mn<sub>3</sub>O<sub>10-δ</sub> has a magnetic transition at ~114K, suggesting that the antiferromagnetic groundstate, with associated weak ferromagnetism arising through the Dzyaloshinsky-Moriya interaction observed in Ca<sub>4</sub>Mn<sub>3</sub>O<sub>10-δ</sub>, is preserved on introduction of a fraction of La<sup>3+</sup> dopant cations. Ca<sub>3.95</sub>La<sub>0.05</sub>Mn<sub>3</sub>O<sub>10-δ</sub> displays CMR at 4K, with the resistivity signal reduced to 18% of the zero field value, in 14T. B-cation substitution yields Ca<sub>4</sub>Mn<sub>2</sub>TiO<sub>9.93</sub>, an orthorhombic (Pbca) RP phase in which the Mn:Ti cation distribution, deduced from the combined results of anomalous dispersion X-ray experiments and neutron diffraction studies, is 59.8%(2.6):40.2%(2.6) and 70.1%(1.3):29.9%(1.3) across the 4b and 8c octahedral sites, respectively. Neutron diffraction studies at 5K show the presence of only short range magnetic interactions in this insulating material, the resistivity of which is reduced by just 10% at 75K in 14T. Chapter 5 (ABO<sub>3</sub> or A<sub>2</sub>BB'O<sub>6</sub>) describes the mixed B-cation phases, La<sub>2</sub>GaMnO<sub>6</sub> and Nd<sub>2</sub>GaMnO<sub>6</sub>, containing Mn<sup>3+</sup>, 3d<sup>4</sup> cations. Both are cation disordered, orthorhombic (Pnma) materials, and Nd<sub>2</sub>GaMnO<sub>6</sub> exhibits a static, cooperative Jahn-Teller (JT) distortion. La<sub>2</sub>GaMnO<sub>6</sub> contains a relatively higher proportion of dynamic, cooperative JT distortions, and as such exhibits isotropic ferromagnetism at 5OK and 5K (2.80(5)μ<sub>B</sub> per Mn aligned along y at 5K), predicted by the 'quasistatic hypothesis', which describes the correlation between electron spin configurations of neighbouring JT cations. The magnetic structure of Nd<sub>2</sub>GaMnO<sub>6</sub> at 5K and 1.7K is modelled as 'A<sub>X</sub> F<sub>Y</sub> G<sub>Z</sub>' for the Mn sublattice, and 'F<sub>γ</sub>' for the Nd sublattice, using Bertaut's notation, resulting in competing ferromagnetic and antiferromagnetic superexchange interactions on the Mn sublattice along the [010] direction of the unit cell. Both materials are highly insulating and neither displays CMR, with resistivity values in each being reduced to just 96% of the zero field value at 200K, in a field of 14T.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:312588 |
| Date | January 1999 |
| Creators | Spring, Lauren E. |
| Publisher | University of Oxford |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://ora.ox.ac.uk/objects/uuid:dbb9fd94-9030-4e8e-b372-96e60b628001 |
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