Date of Completion

5-5-2016

Embargo Period

5-2-2016

Keywords

Transition Metal Oxides, Phase Separation, BaCrO3

Major Advisor

Barrett O. Wells

Associate Advisor

Joseph I. Budnick

Associate Advisor

Boris Sinkovic

Field of Study

Physics

Degree

Doctor of Philosophy

Open Access

Campus Access

Abstract

In this work, we explore from various perspectives the electronic structures in several typical perovskite oxides. In the first part, we study magnetic and electronic structures of the film-stabilized Mott insulator BaCrO3, a missing member of perovskite ACrO3 family compounds with A being Ca, Sr divalent ions. We have achieved a unique method to synthesize the film BaCrO3 samples and performed some basic characterizations. BaCrO3 films are insulating, and exhibit a weak ferromagnetic response. Employing the first-principle calculations we attribute such weak ferromagnetic behavior to a canted C-type antiferromagnetic spin order. Additionally, comparison with the other sister compounds CaCrO3 and SrCrO3 suggests an anomalous Mott transition where magnetism is independent of whether the compound is metallic or insulating.

In the second part of this dissertation we study the local structure in magnetically phase separated SrCoO3-y by using the muon spin rotation (μ+SR) and neutron scattering experiments. It is interesting that the sample consists of two magnetic phases of a specific oxygen concentration while remains a single crystalline phase confirmed by high resolution synchrotron X-ray diffraction. It implies that there may exist microscopically separated electronic phases. Our study has established the presence of two physically separated magnetic regions as we postulated above. The two phases most likely represent areas of the sample with different effective valence charge density. Further, the phases exist over regions with a length scale intermediate between nanoscale charge inhomogeneity and systems such as manganites or super-oxygenated cuprates with large length scale phase separation; thus indicating a key role for length scale in the variety of reported phenomena associated with electronic phase separation.

Lastly, we investigate systematically the periodicity of charge patterns in Pr2NiO4.25 and Pr2NiO4.22 samples using soft resonant X-ray scattering (RXS). Recently, the observation of the stripe-like charge ordering in the high TC superconductor yttrium barium copper oxide triggers great interest of interrelationship between the local charge pattern and superconductivity. Studying the behavior of the periodic charge pattern in non-superconductors would be also useful and complementary to understand the role in superconductivity. In the Pr2NiO4.25 sample we have observed resonant peaks at (0.2758, 0, 0) and (0.294, 0.365, 0), respectively, with photon energy tuned to be Ni L3 edge. The former peak is temperature independent and remains robust from room temperature to 22 K, while the peak at (0.294, 0.365, 0) is temperature dependent and exhibits two transitions at 240 K and 100 K, respectively. We tend to interpret the peak at (0.294, 0.365, 0) as a magnetic peak by comparing with the compound La1.8Sr0.2NiO4. In the Pr2NiO4.22 sample, we have observed two resonant peaks at (0.338, 0, 1) and (0, 0, 1), respectively. The former peak is temperature independent, while the later peak at (0, 0, 1) is a forbidden Bragg peak. It is temperature dependent and exhibits two transitions at 240 K and 100 K, respectively.

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