Magnetoelectric Effects And Related Properties In Multiferroic RFe₂O₄ Oxides

Magnetoelectric multiferroics with coexisting magnetic and ferroelectric orders enable strong magnetoelectric coupling in a single-phase material for many potential applications like sensors, memory, and spintronics. In recent years magnetically induced ferroelectrics have renewed interest in research on magnetoelectric correlations where ferroelectricity is induced by complex magnetic orders like spiral spin orders, and exhibit remarkable changes in electric polarization in response to external magnetic fields. However, direct coupling in these materials just shows a weak magnetoelectric effect, or operates far below room temperature, hindering their practical device applications.In this thesis, the author devoted his effort to reveal experimental evidence for magnetoelectric coupling in multiferroic RFe₂O₄ based on well-grown single crystals. The results revealed an electronic origin of the magnetic frozen state and a novel exchange bias effect around the freezing temperature. Upon magnetoelectric coupling, the author's work also presented multi-glass feature on the simultaneous occurrence of a polar glass and a spin glass state, which may open up new approaches towards applications using magnetoelectric effects. Moreover, other efforts were made to improve dielectric performance of ceramic multiferroic materials via doping and composite effects.The whole thesis consists of seven chapters.1. Brief overview of multiferroics and magnetoelectric effects.This chapter aimed at a brief overview of history, progress, and current status of multiferroics and magnetoelectric effects. By these illustrations, the author's work was urgent and highlighted for further investigations into magnetoelectric coupling.2. Experimental methods.This chapter listed the experimental methods applied in the author's studies.3. Introduction to recent studies in multiferroic RFe₂O₄ oxides.This chapter summarized basal concepts and recent advances in studies of the multiferroic RFe₂O₄ family oxides. In the author's work, high-quality YbFe₂O₄ single crystals were prepared by an optical floating zone technique, and were well characterized in the structure, magnetic, and electric properties, which confirmed the existence of an intrinsic anisotropy of the system. 4. Magnetic freezing and multi-glass state in multiferroic YbFe₂O₄ crystals.In this chapter, collective freezing in the c-axis magnetization of multiferroic YbFe₂O₄ single crystals had been observed. The freezing behavior coincided well with the remarkable enhancement of coercivity. Pyroelectric current measurements and dielectric response clearly evidenced magnetoelectric coupling effects associated with the magnetic freezing behavior, revealing an electronic origin of the magnetic frozen state. Thus, a plausible multi-glass model had been proposed.5. Exchange bias effects in YbFe₂O₄ crystals.In this chapter, exotic exchange bias effects in YbFe₂O₄ had been studied. In consideration of the charge-ordered double layer structure, glass-like scenario was well established on the short-range and frustrated interbilayer interactions responsible for the hypothetical FM/AFM interfaces. The exchange bias was discussed in terms of the collective freezing of magnetization other than the exchange coupling between FM/AFM interfaces.6. Doping and composite effects in LuFe₂O₄ ceramic materials.This chapter referred to the effort to improve dielectric performance of LuFe₂O₄ ceramic materials via doping and composite effects.7. Conclusion.This chapter made a brief conclusion and an outlook for future multiferroics and magnetoelectric effects.