Hydrothermal Synthesis And Property Study Of Single-phase Multiferroic Nanomaterials

Single-phase magnetoelectric multiferroic materials with broad application prospects in many fields, such as information storage and so on, have magnetism and ferroelectricity simultaneously, and these two orders can affect each other via magnetoelectric effect. Hence, multiferroic materials have become a hotspot for condensed-matter physics and materials physics during the past ten years, and many high-performanced multiferroics have been found in this upsurge. For example, BiFeO3is one of the best multiferroics, and its magnetic and ferroelectric transition temperatures are both above room temperature. Moreover, BiFeO3has a large remanent polarization at room temperature. But the G-type antiferromagnetism ground state makes it difficult for the magnetic control of the multiferroic in single phase bulk BiFeO3. In fact, most of the multiferroics are antiferromagnetic, and ferromagnetic ones are very rare so far. Fortunately, nanorization can be an effective way to enhance the magnetism in an antiferromagnetic material without a crystal structure alteration. Therefore, it will be useful for introducing nanotechnology to the multiferroic research. On one hand, it can enhance magnetism of the antiferromangetic multiferroics. On the other hand, nanotechnology could bring new physics, and allow people reach a whole new level for understanding and manipulating the mangetoeletric coupling effect. However, multiferroic materials are mostly multi-element complex oxides, so the high-quality nanocrystals are hard to controllably grow, and the correlative propery study is still in the initial stage.This dissertation focuses on the above challenges:the hydro thermal methods were used to grow high-quality BiFeO3nanocrystals and new type multiferroic nanobelts. The magnetic properties of them were systematically investigated, such as the dynamic properties of spin cluster glass and exchange bias effects. In addition, the magnetodielectric effect in an atypical magnetoelectric material was studied in-depth. The detailed experimental results and discussions are shown as follows:In chapter1, we introduced the history and research progresses of multiferroic materials. The main contents include:the basic property study of BiFeO3, magnetodielectric effects and the related multiferroic systems, the research progresses of multiferroic nanomaterials. According to the general analyses, several open questions have been proposed at the end.In chapter2, a hydrothermal method was developed for the fabrication of BiFeO3nanocrystals, and the dynamic properties of spin cluster glass in nanosized BiFeO3sample were systematically studied for the first time. Compared to conventional solid state method, the hydrothermal method can effectively reduce the formation temperature of BiFeO3, through which high-quality BiFeO3nanocrystals were produced. The BiFeO3nanocrystals were characterized with an average diameter of170nm, a Neel temperature of640K and a ferroelectric Curie temperature of1064K. We studied the dynamic properties of spin-glass-like state in BiFeO3nanocrystals by measuring the temperature dependence of ac susceptibility under different ac magnetic field frequencies or under different dc magnetic field values. By fitting the results with different dynamic laws, the spin cluster glass transition was confirmed, and it may be originated from a lot of the new uncompensated spins after the nanorization.In chapter3, exchange bias effect in BiFeO3nanocrystals was fully investigated. It was found that exchange bias exist in BiFeO3nanocrystals for the whole temperature range we measured (from2K to300K), and exchange bias field changed non-monotonically with increasing temperature. The exchange bias above the spin cluster glass transition temperature is very special, as the exchange bias would vanish for other systems in the same condition. To explain this new phenomenon, a Malozemoff’s random-field model and a2D-DAFF model were established, respectively. The evidence for the existence of2D-DAFF in the BiFeO3nanocrystals was also given.In chapter4, a simple hydrothermal method was developed for the synthesis of single-crystal Bi5Fe2O10.5nanobelts. The products were characterized to be a layered structure with a commensurate modulation wave vector q*=(0,0.25,1) and isostructural with the high-temperature superconductor Bi2Sr2CaCu2O8+δ. The regular stacking of the BiFeO3-like perovskite blocks and the rock salt [Bi2O2]2+slabs along the c axis of the crystal makes the Bi5Fe2O10.5nanobelts have a natural magnetoelectric-dielectric superlattice structure. The most important result was the room-temperature multiferroic behavior in this new compound, and the giant magnetodielectric effect indicated the magnetoelectric coupling interactions. This finding may be useful for developing single phase multiferroics.In chapter5, the dielectric properties of LaMn1-xFexO3(0.1≤x≤0.5) system under magnetic fields and electrical biases were studied. A low-field magnetodielectric effect is observed for x≤0.2samples, but it is suppressed by further Fe substitution. Besides the giant magnetodielectric effect, the dielectric constants can also be tuned by the dc electrical bias. These results have been explained by a contribution of the Maxwell-Wagner effect. It is notable that an intrinsic magnetodielectric effect in LaMno.8Fe0.2O3was observed, indicating the existence of a magnetoelectric coupling effect.