Preparation And Properties Of Multiferroic Nanomaterials

The coming of information age leads to the exponential growth of information. Seeking a memory device which has a high density, low-power, portable and multi-functional features is of great significance. Multiferroic material has two kinds of ferroic properties including ferroelectric and(anti-)ferromagnetism, which can be utilized to achieve multi-state memory. The existing ferroelectric and ferromagnetic storage memory have their shortcomings, but the new multiferroic memory which has magnetoelectric coupling effect and can solve the problems of the existing ferromagnetic and ferroelectric memory attracts everyone’s attention. However, due to the difficulty to present single-phase multiferroic materials, which have a strong magnetoelectric coupling effect at room temperature, the current academic focuses on magnetoelectric composites. In the terms of the existing magnetoelectric composite compound,there are mainly two forms: multilayered eptaxial the films(2-2 structure) and self-assembly vertical structure(1-3 structure). Because 2-2 structure has clamping effect, only a smaller magnetoelectric coupling effect exists. 1-3 structure with a small aspect ratio can solve the substrate clamping effect.However, the growth of 1-3 structure belongs to growth of self-assembly, so the preparation of films are still have many problems. Aiming at these problems, we used two different research ideas based on the design of materials structural to prepare a new type of magneticoelectric coupling structure and its properties.(1) for the 2-2 structure problems, we fabricated single crystal freestanding multiferroic CFO-BTO nanocomposite films on the Si via tranfer, which reduced the substrate clamping effect.Through comparative researching of the epitaxial growth MgO film on c-plane sapphire substrates and STO substrates, we discovered that(001) plane MgO film surface on STO substrates was relatively flat. So we chose epitaxial(001) plane MgO film as a buffer layer for subsequent films growth. Then we deposited the(004) plane CFO film on the MgO film, which whose RMS was only 0.19 nm. then a single crystal CFO was transferred to the Si substrate by etching MgO film. The freestanding CFO was as low as 2nm and had a flat surface. So it could provide an effective experimental reference for preparring freestanding quasi-two-dimensiona functional oxide film. Later we also found that the substrate clamp affect was decreased for the single crystal freestanding(004) plane CFO film. Growing(200) plane BTO film on the CFO/Si, we transformed the crystal structure of BTO and found that the CFO magnetic propertie was changed, indicating freestanding BTO/CFO/Si structure had magneticoelectric coupling characteristics. In this thesis, we prepared a single crystal of magneticoelectric composite material on Si via a simple and effective method.(2) for the 1-3 composite structure, AAO stencil assisted PLD technique was used and we prepared highly ordered core-shell CFO-BFO nanocomposite arrays. By changing AAO stencil dimension, we researched the growth process and magneticoelectric properties of the CFO-BFO nanocomposite arrays.By processing the original(002) plane STO substrates, we prepared an atomically smooth surface teminated by the latter atomic layer. Also we prepared different dimension of Anodic aluminum oxide stencil(AAO).Combined with AAO transfer and PLD technique, we got CFO-BFO nanocomposite arrays. Under different AAO dimension constraints, the morphology of the composite structure was completely different. When using Ph-AAO stencil, there were some nanopillars on nanoislands similar to 1-3 structure. When the AAO stencil dimension was reduced to 70 nm, the previous CFO nanopillar gradually fused into a single nanopillar, and formed a core-shell CFO-BFO nanocomposite arrays. We also researched ferroelectric properties of the CFO-BFO nanocomposite arrays by PFM technique and discovered that the center parts of these islands were CFO nanopillar and the edge parts were BFO phase. And there were some stress-induced magneticoelectric coupling effect between CFO-BFO nanocomposite arrays.In this thesis, we prepared the new magneticoelectric composite structure, which could solve the traditional magneticelectric coupling structure problems partly and help to improve magnetic coupling effect. It may provide a experiment reference to develop new multiferroic memory.