标题: | 氧化铁奈米颗粒之电子传输与低磁场磁电阻研究 Electron Transport and Magnetoresistance of Magnetite Nanoparticles at Low Magnetic Field |
作者: | 巫俐莹 Wu, Li-Ying 简纹滨 Jian, Wen-Bin 电子物理系所 |
关键字: | 氧化铁;奈米颗粒;强局域;变程跳跃传输;磁电阻;magnetite;nanoparticles;strong localization;variable range hopping;magnetoresistance |
公开日期: | 2015 |
摘要: | 本论文主旨为研究氧化铁奈米颗粒(Fe3O4 nanoparticles (NPs))与二氧化矽包覆氧化铁奈米颗粒(Fe3O4@SiO2 NPs)在电荷传输上的分析与比较,以及在低磁场的作用下,两种材料在变温时的磁电阻表现。本实验用电子束微影及热蒸镀制作完整的奈米电子元件,以热退火减少接点电阻,并使用原子力显微镜测量样品的几何面积与高度,最后利用两点量测法,在不同温度下量测电流-电压曲线来观察导电行为。常温下Fe3O4@SiO2 NPs的电导率比Fe3O4 NPs低了一个数量级。根据三维Mott变程跳跃传输理论,对所有样品的电导率与温度关系图做拟合分析,得到Fe3O4@SiO2 NPs的特征温度约1 X 108 K,比氧化铁奈米颗粒的2 X 107 K大5倍左右。为瞭解材料的局域长度,本实验利用场效电晶体的操作模式,使用液态离子(ion liquid)当作闸极以施予偏压,测得两种氧化铁样品皆呈现电洞传输模式,且可以获得Fe3O4 NPs和Fe3O4@SiO2 NPs的迁移率分别是1 X 10-2 cm2 V-1 s-1和1 X 10-3 cm2 V-1 s-1,再由电导率可以估计电荷密度大小,最后从变程跳跃传输理论的特征温度求出局域长度与跳跃距离,其结果显示强局域化特性。 将磁场方向固定垂直于样品表面,量测磁场变动下的电阻变化,观察到特殊的正磁阻效应。由实验数据观察到高温呈现负磁阻效应,推测电阻变化与铁磁性样品的磁矩受到磁场影响产生偏转有关;低温呈现正磁阻效应,推论磁场对局域态之波函数产生缩减的影响,电子传输因所需的平均跳跃距离增加而变得困难。最后结合磁电阻效应与三维变程跳跃传输,观察到B.I. Sjklovskii提出的电阻变化率与磁场平方成正比的趋势。 In this work, electron transport in Fe3O4 and Fe3O4@SiO2 nanoparticles (NPs) are investigated and differentiated. The two kinds of nanoparticles, showing superparamagnetism, are confirmed by superconducting quantum interference device. The X-ray diffraction (XRD) data of both Fe3O4 and Fe3O4@SiO2 NPs exhibit the crystalline structure of Fe3O4. The two kinds of NPs were each dispersed in n-Hexane solution. The solution was dropped on a Si/SiO2 substrate so as to make two-probe devices for electrical property measurements. The temperature and electrical field dependent conductivity of Fe3O4 and Fe3O4@SiO2 NPs are systematically studied. The conductivity of Fe3O4 NPs can be fitted by the theory of three-dimensional Mott’s variable range hopping transport. The model can describe well the temperature behavior from 80 K to 300 K for Fe3O4 NPs but it can only fit to the data at high temperatures for Fe3O4@SiO2 NPs. The estimated temperature parameter ( ) of Fe3O4@SiO2 NPs is much higher than Fe3O4 NPs. On the other hand, from the field effect behavior, we obtained that the mobility of Fe3O4@SiO2 NPs is smaller than that of Fe3O4 NPs. The small mobility magnitudes confirm that both Fe3O4 and Fe3O4@SiO2 NPs are in the strong localization regime. Beside electron transport properties, we carried out magnetoresistance (MR) measurements. We discovered negative MR at high temperature and positive MR at low temperature for both Fe3O4 and Fe3O4@SiO2 NPs. The negative MR is due to the polarized magnetic moment of the magnetic NPs which could reduce spin scattering in electron transport. The positive MR could account for the shrinkage of localized electron wave functions under a magnetic field. |
URI: | http://140.113.39.130/cdrfb3/record/nctu/#GT070252038 http://hdl.handle.net/11536/126661 |
显示于类别: | Thesis |