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The Properties and Fabrication of the Nanostructured Multiferroic Materials

Author: LanLiNing
Tutor: LiJiangYu
School: Xiangtan University
Course: Materials Physics and Chemistry
Keywords: multiferroic materials Sol-gel composite nano-lithography soft lithography nanostructure
CLC: TB383.1
Type: Master's thesis
Year: 2009
Downloads: 280
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Abstract


Multiferroic materials with two or more types of ferroic orderings have attracted a great deal of attention in the past a few years. The magnetoelectric coupling makes the multiferroic materials promising for a wide range of applications including multiple-state memory devices, sensors, tranformers, gyrators, drivers, optical devices and Micro-electromechanical systems (MEMS), among others. In this dissertation, the nanostructured multiferroic materials have been systematically investigated. The current state of multiferroic materials and their potential applications were reviewed in the Introduction first, including: fabrication and properties of ferroelectric and multiferroic nanostructures by soft lithography. The main research findings are summarized as follows:The fabrication of ferroelectric and multiferroic nanostructures was explored. Lead zirconnate titanate(PZT), cobalt ferrite(CFO), nickel ferrite(NFO) and bismuth ferrite(BFO) were synthesized by Sol-gel firstly. Then, the PZT, two-phase random composite PZT-CFO and PZT-NFO, laminated composite PZT-CFO and PZT-NFO, and BFO nanostructure were synthesized by soft lithography. In this process, different kinds of patterned moulds by microfabrication were used as original moulds, and soft PDMS was used as elastomeric stamps. Ordinary optical microscopy, x-ray diffraction(XRD), atom force microscopy(AFM), piezoresponse force microscopy (PFM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM) were used to characterize the microstructure and macroscopic properties of the fabricated nanostructures. The preliminary experimental results indicated that:The patterned structure of soft PDMS stamps were the same as the initial moulds. Straight and regular PZT nanostructures with uniform stripe gap space were fabricated successful by nanolithography. The width of the stripe was about 2 um, and the corresponding depth was around 200 nm. Crystalline structure of PZT is characterized by x-ray diffraction and transmission electron microscopy. The piezoelectric behavior is confirmed by piezoresponse force microscopy. The value of the effective piezoelectric coefficient d33 for PZT nanopatterns is about 200 pm/V. Due to the similar fabrication process, the pattern morphology of random composite and laminated composite PZT-CFO and PZT-NFO were almost identical, and some nano-scale particles were distributed homogeneously on the stripes of random composite patterns. The remanent magnetization of the random composite PZT-CFO pattern was 0.66 emu/cm3, and the corresponding coercive magnetic fields was 180 Oe. Because the solution concentration of ferromagnetic precursor was rather low, the morphology of laminated PZT-CFO and PZT-NFO composite patterns, in which the ferromagnetic thin films were synthesized by spinning coating on top of patterned PZT film, were similar to those of random composite patterns. When the ferromagnetic phases were coated with the help of flat PDMS, the resulted laminated PZT-CFO and PZT-NFO patterns were regular wavy framework with uniform space between the straight stripes, since the ferromagnetic solution is deposited at the bottom of the stripes of PZT pattern. The width of the stripe was about 2 um, and the depth of filling was around 100 nm. The remanent magnetization of the laminated composite PZT-CFO pattern was 8.9 emu/cm3, and the corresponding coercive magnetic fields Hc was 1000 Oe. Single-phase multiferroic BFO patterns were also manufactured by nanolithography, and the patterns were the stripes with uniform space gap and regular straight shape. The PFM test results of the BFO microstructured show that the existence of piezoelectric response. However, some problems also existed in the samples, such as the collapse of the existence of stripes, abnormal grains grew up, regional uneven printing and so on. This shows that our preparation process need to be further explored and improved.

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