Synthesis, Transport and Magnetic Properties of Emergent Transition Metal Oxides and Sulfides

• Main Author: Yang, Wencao
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2019
• Subjects: Condensed matter physics; Materials science; Nanotechnology; Physics
• ISBN: 9781088394489; 1088394485

Heavy transition metal compounds possess both strong electron correlation and spin-orbit coupling (SOC). The competition and interplay of the two interactions are predicted to promote emergent topological phases. Among transition metal compounds, complex metal oxides are well known for their versatile physical properties. In the family of transition metal oxides, pyrochlore iridates have attracted considerable interest because of their potential for realizing new topological electronic and magnetic states. In this work, we have synthesized two representative pyrochlore iridates, namely Bi2Ir2O7 and Lu2Ir2O7. The former was in a thin film form intended for electrical transport studies, while the latter was in bulk polycrystals for the study of magnetic properties.The Bi2Ir2O7 thin films were grown epitaxially on YSZ substrates by pulsed laser deposition. The as-grown films are highly off-stoichiometry while exhibiting a variety of interesting transport behaviors. A post-annealing method was developed to improve the stoichiometry of the films. The annealed film shows a metallic to weakly-insulating transition at ~ 150 K, accompanied by a low-temperature positive magnetoresistance. Polycrystalline Lu2Ir2O7 samples were synthesized via a solid-state reaction method. Weak ferromagnetism was detected and was attributed to the existence of Ir defects. A cooling-field-induced shift of the magnetic hysteresis loop was also observed, which can be understood using an exchange-bias-like model.The study of magnetism in two-dimensional (2D) transition metal sulfides was missing until the recent discovery of intrinsic ferromagnetism in 2D van der Waals crystals. Since then, there is an increasing effort in the search for new 2D magnets. We synthesized quasi-2D chromium sulfide (Cr2S3) nanoplates by sulfurizing Cr metal layers that were deposited on sapphire substrates. Raman scattering suggests the nanoplates have a Rhombohedral crystal structure. Depending on their strain state, Cr2S3 nanoplates exhibit distinct electrical transport properties.