Studies on the synthesis and characterisation of compounds showing colossal magnetoresistance
The crystallographic and magnetic properties of manganese oxides belonging to the Ruddlesden Popper (RP) series, described by the formula An+1BnO3n+1, 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 (A3B2O7) it is shown that Sr2-xNd1+xMn2O7 (0 ≤ x ≤ 0.5) and Sr2PrMn2O7 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 Sr2HoMn2O7 and Sr2YMn2O7 show no significant magnetoresistance. In contrast to the tetragonal Sr,Nd and Sr,Pr compositions, these compounds show a symmetry lowering to space group P42/mnm, and spin glass freezing on the Mn sublattice at ~20K. Chapter 4 (A4B3O10) focuses on A- and B-cation substitutions in the parent compound Ca4Mn3O10-δ. Substitution of Ca by Sr yields Sr4Mn3O10-δ, 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 Ca3.95La0.05Mn3O10-δ has a magnetic transition at ~114K, suggesting that the antiferromagnetic groundstate, with associated weak ferromagnetism arising through the Dzyaloshinsky-Moriya interaction observed in Ca4Mn3O10-δ, is preserved on introduction of a fraction of La3+ dopant cations. Ca3.95La0.05Mn3O10-δ displays CMR at 4K, with the resistivity signal reduced to 18% of the zero field value, in 14T. B-cation substitution yields Ca4Mn2TiO9.93, 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 (ABO3 or A2BB'O6) describes the mixed B-cation phases, La2GaMnO6 and Nd2GaMnO6, containing Mn3+, 3d4 cations. Both are cation disordered, orthorhombic (Pnma) materials, and Nd2GaMnO6 exhibits a static, cooperative Jahn-Teller (JT) distortion. La2GaMnO6 contains a relatively higher proportion of dynamic, cooperative JT distortions, and as such exhibits isotropic ferromagnetism at 5OK and 5K (2.80(5)μB 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 Nd2GaMnO6 at 5K and 1.7K is modelled as 'AX FY GZ' for the Mn sublattice, and 'Fγ' for the Nd sublattice, using Bertaut's notation, resulting in competing ferromagnetic and antiferromagnetic superexchange interactions on the Mn sublattice along the  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.