Preparation And Electromagnetic Properties Control Of Multiferroic Materials

The first definition of multiferroic materials was given by Hans Schmid in 1994. He stated that a material is called multiferroic, if there are two or more primary ferroic order parameters(for example: ferroelectricity, ferromagnetism, ferroelasticity and ferrotoroidicity) present in one phase of the material. In recent years, the studies of multiferroic materials mainly focus on two issues: How to enhance the essential properties(ferroelectricity, ferromagnetism, etc.)? How to improve the electric and magnetic response in multiferroic materials? Based on above issues, in this thesis, the ferromagnetism of BFO was enhanced significantly at room temperature, by means of controlling crystallinity, element doping and reducing the dimension. In addition, we constructed an electromagnetic performance test system, the voltage-controlled resistive switching behaviors in multiferroic materials(BFO thin film and LCMO/PMN-PT heterostructure) were investigated. The main results were as follows:1. Pure and doped BFO nanoparticles were synthesized by the hydrothermal method and the co-precipitation method, respectively. The microstructure and ferromagnetic behaviors have been studied. Enhanced ferromagnetism was obtained in sub-crystallized BFO nanoparticles, which are made up of the crystalline particles and the amorphous parts, and the crystalline particles were embedded in the amorphous form. Besides, the enhancement of ferromagnetism was also realized in doped BFO. The effect of the manganese substitution on ferromagnetism had been analyzed through the first-principle calculation, the effective magnetic moments of Mn and Fe are different, resulting in net magnetic moments in the unit cell, eventually leading to the enhancement of ferromagnetism.2. One-dimensional BFO nanotubes are successfully synthesized by sol-gel-based electrospinning process. By compared with BFO nanoparticles, enhanced room temperature ferromagnetism has been successfully realized in BFO nanotubes at room temperature. The results of electron spin resonance measurement further confirm that ferromagnetic resonances are detected in BFO nanotubes at different temperature. X-ray photoelectron spectroscopy study proves the existence of plentiful oxygen vacancies in BFO nanotubes, which will play a key role in terms of enhanced ferromagnetism.3. BFO thin film has been fabricated on Nb:SrTiO3(NSTO) substrate by sol-gel spin coating method. The electric field controlled resistive switching behaviors were investigated in BFO thin film. By the means of doping elements or inserting ferrite thin film, the stability of resistive switching behavior was improved in BFO thin film. Besides, enhanced polarization is obtained in BFO thin film by inserting a layer of ferrite thin film. Good stability, excellent retention, and anti-fatigue characteristics are observed simultaneously.4. Using magnetron sputtering method, complex multiferroic heterostructures were fabricated by growing LCMO films on ferroelectric PMN-PT(011) single-crystal substrates. The nonvolatile or pulsed resistive switching behaviors induced by an electric field are achieved simultaneously. Further analyses indicate that the different resistive switching behaviors are resulted from co-control of piezostrain and polarization current effects. The results further enhance the application of complex oxides in multifunctional memory devices.