Aspects of Phase Separation in Transition Metal Oxides

Date of Completion

January 2011

Keywords

Physics, Condensed Matter

Degree

Ph.D.

Abstract

This work details the studies of the microscopic and macroscopic properties of transition metal oxide materials that exhibit an array of fascinating physical phenomena. These materials are related in that they exhibit electronic, magnetic and/or structural phase separation; the high-Tc superconductor La2CuO 4+δ is such a material. The magnetic properties of La2CuO 4+δ were probed using neutron diffraction and muon spin rotation (µSR) to determine the effects of oxygen disorder on the magnetic and superconducting phases. Neutron diffraction demonstrated that the disordered state exhibits a decrease in background scattering, a narrowing of the associated magnetic peaks, and an enhancement of the magnetic peaks as a function of applied field. µSR spectroscopy provided a measure of the magnetic volume fraction, indicating that in the disordered state there is a local increase in magnetism due to solely to oxygen disorder. We quantify these effects, and propose a model that describes the co-existence of both the superconducting and magnetic phases in the presence of disorder.^ We have also investigated films of Sm-doped BiFeO3 which exhibits a structural phase separation between two structures; this is described as having a morphotropic phase boundary. High-resolution synchrotron X-ray diffraction reveals substantial phase coexistence as one changes temperature to crossover from a low-temperature PbZrO3-like phase to a high-temperature orthorhombic phase. We also examine changes due to strain for films greater or less than the critical thickness for misfit dislocation formation. Particularly, we note that thicker films exhibit a substantial volume collapse associated with the structural transition that is suppressed in strained thin films. ^ Single crystals of (V1−xCrx)2O 3 were studied using µSR spectroscopy where a new muon precession frequency was discovered. We propose that this signal is results from fractures occurring, resulting in changes to the local magnetic environment. Muon measurements over a range of temperatures below the Néel temperature reveal that ordered anti-ferromagnetic order is reduced near the transition temperature as pure V2O3 regions experience a reduction in precession frequency, but the muons stopping in fractured regions precess at the same frequency until the ordered magnetism vanishes and the signal vanishes. ^

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