Multiferroicity In Spin Frustration Systems

With the development of science and technology, more and more ferroelectric and ferromagnetic materials, such as ferroelectric sensors and ferroelectric memories, such as magnetic memories, sensors, transformers, and recording equipment, are applied to our lives. It has been an inevitable issue for a long time to synthesize a class of materials which have both spontaneous electric polarization and spontaneous magnetization, i.e. the so called multiferroics. Recently, such materials have got specific attentions due to a series of promising potentials of applications.In the spin ice systems, pyrochlore oxides have attracted much attention in recent years due to their interesting magnetic structures and spin dynamics. Although several groups over the world in addition to us have carried out preliminary researches on the possible multiferroic existence in these materials, no widely accepted results have been available. In order to determine whether the ferroelectric polarization exists in these spin ice systems or not, we improve and experimental techniques based on previous works and study carefully single crystals of Ho2Ti2O7 via various characterizations.In addition, the mechanisms of the polarization generation for most type-II multiferroics are roughly understood, the mechanism of polarization in delafossite oxides is still under debates and the experimental results are somehow authors-dependent. In order to find a clue for working out the mechanism, intensive experimental study on CuFeO2 via the self-doping approach is performed in this thesis. The main contents are highlighted as follows:In chapter 1, an overview of multiferroic materials, including the theoretical background and challenges in the research of multiferroics is briefly introduced, then followed by the description on spin frustration systems. We mainly focus on the experimental works on Ho2Ti2O7 and CuFeO2.In chapter 2, the motivation of investigations on Ho2Ti2O7 is given first. Then the synthesis method and measurement results are described, and some discussions are made. It can be concluded that the ferroelectric polarization actually exists in Ho2Ti2O7 and it couples with the magnetic structure, verifying the theory about magnetic monopoles offered by Khomskii.In chapter 3, the oxidation state of Cu and Fe ions and also its magnetic structure in CuFeO2 are modulated by the self-doping. It is found that the ferroelectric polarization appears as an after-effect according to the experimental results. The self-doping samples are characterized by XPS in order to eliminate the possible impact of the oxygen vacancy on polarization. Through intensive study, we reveal a connection between the polarization and the oxidation state of self-doping materials.In chapter 4, the conclusion of this thesis is made and perspectives based on it are highlighted, the acknowledgement and a list of publications are presented at last.