Magnetic-field- and pressure-tuned phases in lanthanum praseodymium calcium manganese oxide and manganese(II,III) oxide: Inelastic light scattering studies and single crystal growth

In this dissertation, I present our studies on two distinct strongly correlated materials: the manganese perovskites LaxPr yCa1-x-yMnO3; and the manganese-based spinel Mn3O4. For this study, Raman (inelastic light) scattering spectroscopy was used as a main experimental technique because Raman scattering can provide detailed information---such as energy, symmetry, and lifetime information---about all the important electronic, magnetic, and lattice degrees of freedom involved in the complex phases and phenomena observed in strongly correlated materials. Also, for the studies presented in this thesis on the magnetodielectric material Mn3O4, the single crystal sample measured was grown using a floating zone technique. High quality single crystals play a key role in the detailed studies on the mechanisms responsible for the complex phases and intriguing phenomena---which, particularly, show the strong dependence on the crystallographic orientation---of strongly correlated materials Therefore, the development of a single crystal growth capability to get good single crystals was a major component of this thesis work.;In LaxPryCa 1-x-yMnO3 studies, I present field- and temperature-dependent Raman scattering studies of La0.5Ca 0.5MnO3 and La0.25Pr0.375Ca0.375 MnO3, which provide the microscopic details about the thermal and field-induced melting of charge- and orbital-order (COO) in the (La,Pr,Ca)MnO 3 system---including the role of disorder in the COO melting---and, furthermore, enable us to map out the structural and COO phases of these systems as functions of temperature and magnetic field. These studies demonstrate how magnetic-field-dependent Raman scattering can be used to identify the structural phases of materials under conditions---such as high magnetic field and high pressure---that aren't easily accessible using other techniques.;In Mn4O4 studies, I present our temperature-, field-, and pressure-dependent Raman scattering studies on single-crystal Mn 3O4, which reveal the rich and novel magnetostructural phases of this material. Our results provide microscopic details regarding the magnetostructural changes responsible for the diverse phase behavior and interesting phenomena observed in the spinels, which are essential to understanding the complex interplay between strong spin-orbital coupling, geometric frustration, and applied magnetic field in these materials.;In the thesis study, not only do I elucidate the rich phases and phase transitions sensitively tunable with doping, magnetic field, and/or pressure, but also demonstrate that Raman scattering spectroscopy is a powerful method to microscopically investigate the complex phases in the strongly correlated materials as functions of temperature, magnetic field, and pressure---in fact, I demonstrate the capabilities of the simultaneous low temperature, high magnetic field, and high pressure measurements using Raman scattering. Also, I demonstrate the importance of using good single crystal samples for more detailed studies on the material properties.