Magnetic, ferroelectric and structural phenomena in rare earth manganite and ferrite systems: Raman spectroscopy and neutron diffraction studies

摘要

The discovery of emergent phenomena arising from the interplay between magnetism, ferroelectricity and structure in multiferroic materials provides a myriad of potential capabilities in future solid state technologies. Despite extensive investigations in this field, many unsolved issues surround magnetoelectric coupling in several multiferroic systems, and in many cases there are competing approaches that attempt to describe their underlying mechanisms. Collectively, the projects in this thesis represent a two-pronged approach that utilises the complementary techniques of Raman light scattering and neutron diffraction as a means to investigate the contentious role of crystal structure in magnetoelectric coupling for the type-II multiferroic RMnO3 (R = Tb, Dy) and RMn2O5 systems (R = Tb, Ho, Y), and furthermore in potentially-multiferroic ErFeO3.The isotopic substitution of oxygen-18 significantly modifies the lattice dynamics of transition-metal oxide systems. In many materials, this can result in isotope-induced shifts in magnetic, ferroelectric, or superconducting phase transition temperatures that provide new understanding into the underlying physical processes that define their properties. Remarkably, oxygen-isotope substitution in RMnO3 (R = Tb, Dy) systems does not alter the onset of the multiferroic regime, indicating that magnetoelectric coupling is primarily a magnetically-driven phenomenon within these compounds. The combination of Raman scattering and neutron diffraction techniques allowed several key insights to be obtained. For both RMnO3 and RMn2O5 systems, Raman spectroscopy revealed that magnetically-driven ferroelectricity imposes significant effects that alter mode energies and lifetimes which indicate displacement-induced ferroelectricity. Furthermore, we observe remarkable structural behaviours in the form of altered bond lengths or faint Bragg reflections that indicate the existence of symmetry breaking. Our Raman study on ErFeO3 presents evidence of broken symmetry hitherto unobserved, as well as the interplay between magnetism and structure from strong interactions between different magnetic sublattices. Our combined approach provides crucial answers in understanding the emergent phenomena derived from the complex interplay between magnetic, ferroelectric and structural properties within these materials.