Charge and spin correlations in transition metal oxides

Abstract

Strongly correlated electron systems such as the transition metal oxides have provided important results in condensed matter physics over the past few years. They display intriguing phenomena such as superconductivity, colossal magnetoresistance and stripe ordering due to charge and spin correlations at low temperatures. This thesis presents the study of hole doped nickelate and manganite materials using X-ray scattering over an extended energy range. The development of X-ray scattering at energies ranging from 600 eV up to 130 keV is reported. High energy scattering results, are presented on the La(_2-x)Sr(_x)NiO(_4+6) system with x = 0.33. The sensitivity of X-ray scattering to the charge distribution combined with the use of high energy X-rays has allowed the bulk of the sample to be probed, and have resulted in the discovery of a 'charge stripe glass' within the bulk of the material. We compare these results to previous measurements in the near surface region using ~ 12 keV X-rays. Jahn - Teller, charge and orbital ordering has been studied in the Bi(_1-x)Ca(_x)MnO(_3) system with x = 0.74 and the bilayer manganite La(_2-2x)Sr(_1+2x)Mn(_2)O(_7) with x = 0.475 using resonant X-ray scattering techniques at the manganese K edge and high energy X-ray scattering. These results have confirmed the wavevector of ordering along with their associated correlations. The design, and commissioning of a novel diffractometer using focused X-rays from graded & spacing parabolic mirrors is presented. Results show an increase of 10 fold in intensity, together with a simultaneous increase in resolution of 7.5 over previous systems employing graphite monochromating crystals. The first single-crystal X-ray diffraction results are presented which display huge resonances at the L edges of manganese. The resonant enhancement is sufficiently large to allow the observation of magnetic scattering from a correlated antiferromagnetic alignment of spins. Resonant soft X-ray scattering will become a important technique for the study of 3d magnetic materials in the future such as the superconducting, striped cuprate systems.