The Investigation Of Multiferroic Mn_1-2xIr_xMnO₄ And Ca₃(Ti_1-xMn_x)₂O₇

Multiferroic physics is an important branch of condensed matter physics.It maintains a high degree of openness due to the time-dependent definition of multiferroicity.Originally,multiferroics define a class of materials that combine ferroelectricity and magnetism.Now,the widely accepted definition refers to those materials that exhibit two or more kinds of ferroic orders and these orders are coupled.Magnetic frustration is a phenomenon in a magnetic system where the magnetic moments can be aligned in more than one configuration due to geometrical reason or multifold competing interactions.Consequently,a plenty of degenerate ground states may be allowed.A ferroelectric polarization?P?may be generated in a multiferroic with proper spin spiral structure,while spin spiral as one of the most famous non-collinear spin structures is certainly one representative of frustrated configurations.In this thesis,we address the multiferroic behaviors of MnWO₄,a highly frustrated magnetic oxide with the above-mentioned spiral magnetic structure and thus magnetism-induced ferroelectricity.This thesis intends to introduce stronger spin-orbital coupling into MnWO₄ by Ir⁴⁺-substitution of Mn²⁺,and then to explore whether such a substitution can enhance the ferroelectricity and magnetoelectric coupling or not.The whole thesis is organized as the following chapters:The first chapter first introduces some basic concepts and background of multiferroicity.Subsequently,the structure and multiferroic properties of MrnWO₄ will be summarized,including the effects of various substitutions.It seems that the non-magnetic substitution always suppresses the magnetism of MnWO₄.The consequence of magnetic substitution seems to be more complicated and the effect is substitution-dependence and no consistent conclusion is available.The second chapter describes the sample preparation procedure and techniques for characterizations.The techniques used for this study include the X-ray diffraction?XRD?,the scanning electron microscopy?SEM?,the X-ray photoelectron spectroscopy?XPS?and the physical property measurement system?PPMS?.In the end,we outline the techniques for dielectric and ferroelectric properties.The main body of third chapter is the results of the microstructure and multiferroic properties of a series of Mn_1-2xIr_xWO₄?MIWO?samples.The XRD refinement results show that Ir⁴⁺-substitution reduces the unit cell volume.The XPS measurement shows that the valence states of Mn ions and Ir ions remained stable at the divalent and tetravalent values.The characterizations of magnetic,specific heat,pyroelectric current and ferroelectric properties of Mn_1-2xIr_x WO₄?MIWO?samples reveal that Ir⁴⁺-substitution significantly reduces the stability of non-ferroelectric collinear AF1 phase,but Ir⁴⁺-substitution does not completely suppress the AF1 phase.Chapter four briefly describes the background and preliminary exploration results of ferroelectric polarization and structures of Ca₃(Ti_1-xMn_x)₂O₇.The fifth chapter is the summary and outlook of this thesis.