Study On The Dielectric Tunability With External Fields In Multiferroic Materials

Recently, with the development of the information storage industry, it is necessary to achieve multi-functionality in one device. Therefore, the mutual manipulation of the magnetic and electric properties in multiferroics, which shows both magnetic and ferroelectric properties, is of great interest. Although many efforts have been made in this system, there are lots of problems demanding prompt solution, such as how to achieve the room-temperature magnetic/electric tunability with low external fields. Moreover, the mechanism of magnetic and electric coupling in most of multiferroics is not so clear. So the further study on multiferroics becomes an important issue both for fundamental physics and technical applications.In this dissertation, the dielectric tunability with external fields is studied in some multiferroic materials. The possible routes to achieve the room-temperature tunability with low fields are proposed. In addition, the relationship among the structural, magnetic, ferroelectric properties and the origin of the tunability is discussed.In chapter one, the research history and relevant mechanisms of multiferroic materials are concisely reviewed. The multiferroic properties of"electronic ferroelectric"LuFe₂O₄ are particularly introduced, and the previous results show lots of interesting properties closely related to the charge ordering in this system.In chapter two, the phonon spectra and structural characteristics of the LuFe₂O₄ system are investigated using X-ray diffraction as well as Raman and infrared (IR) spectroscopic techniques. Two step-like anomalies of the wavenumbers of the peak in Raman and peak in IR spectra, as well as some weak anomalies of the lattice parameters, are found around the ferrimagnetic and ferroelectric transition temperatures. These results illustrate that the structural fluctuations could effectively influence the phonon modes, and a possible interplay among the structural, magnetic, and charge-ordering properties exists in this multiferroic system. On the other hand, the dielectric properties and tunability with external magnetic and electric fields for LuFe_2-xMn_xO₄ (0≤x≤1) are systematically studied. It is found that the dielectric loss, the ferrimagnetic Curie temperature, as well as the conductivity reduces with increasing Mn doping. One of the most important results is that the room-temperature dielectric tunability with low magnetic and electric fields can be achieved in these samples. The analysis demonstrates that the electron transfer between Fe²⁺ and Fe³⁺is efficiently suppressed with Mn doping and thus results in the decreases of the leaky conductivity as well as the dielectric loss.In chapter three, the properties of garnet Lu₃Fe₅O₁₂ bulks and films are investigated. The dielectric loss in bulks changes with different annealing processes is reduced effectively with decreasing oxygen deficiencies through annealing in O₂. Furthermore, the epitaxial Lu₃Fe₅O₁₂ films are grown on (111) Y₃Al₅O₁₂ substrates by rf magnetron sputtering technique and subsequently annealed in O2 atmosphere. The magnetization loops and the ESR results provide that the magnetic properties can be tuned by atmospheres and temperatures during sputtering and annealing processes.In chapter four, the dielectric tunability with external electric fields in charge ordering La_1-xCa_xMnO₃ is studied. The tunability is up to 80% under a bias field as low as the order of 10 V/cm. Through the dielectric and impedance analyses, the tunability is believed to be related to the Maxwell-Wagner effect associated with Schottky barriers and grain boundary. Moreover, the tunability strongly depends on the stability of the charge-ordered state of the compounds.In chapter five, the magnetodielectric effect in (1-x) La2NiMnO₆-(x) La_2/3Sr_1/3MnO₃ composites is studied. A large dielectric response with magnetic fields is found in wide temperature and frequency ranges. The results indicate that variation of dielectric constants with magnetic fields depends non-monotonously on the molar ratio of two compounds and reaches a maximum up to 20% for x = 0.4 at the frequency of 1 MHz and field of 1 kOe at room temperature.In chapter six, the crystal growth process for multiferroic Y/DyMnO3 by optical floating-zone single crystal furnace is introduced. The structural characteristics show the quality of these single crystals. The magnetic and diaelectric properties were also investigated briefly.