纳米结构铁磁—非磁复合薄膜的制备、磁性和和磁电输运特性研究

发布时间：2018-07-15 22:46

【摘要】：磁电阻材料在磁感应、磁记录等领域都获得了广泛的应用。由磁性金属与非磁性基体材料组成的复合体系由于丰富的界面效应,具有特殊的磁性和电子输运特性。尽管磁性金属/非磁性基体薄膜的室温磁电阻效应已有大量研究,但是对其电子输运特性尚没有一个统一的认识,对于如何进一步提高磁电阻的数值和磁电阻效应的产生机制等有待进一步探究。研究表明,添加Fe、Co、Ni等磁性金属元素到碳基及硅基等非磁性基质中形成的纳米复合薄膜材料,不仅导电性质可以随添加量变化,而且表现出可以调控的大的磁电阻效应。因此,掌握复合膜中金属磁性元素对非磁性基薄膜材料结构与载流子迁移率等特性的影响规律,理解其磁电阻效应产生机理,有利于推动高性能、低成本的碳材料及硅材料等在电子信息领域的广泛应用,具有重要的科学意义和实用价值。本论文工作通过在多种非磁基体(窄禁带半导体SiN、宽禁带氧化物半导体ZnO、非晶C、绝缘体MgO)中掺杂过渡族元素(主要是高自旋极化率的FeCo),研究成分和结构对磁电性能的影响。论文采用溅射沉积技术制备了具有室温铁磁性的铁磁/非磁颗粒膜或多层膜的纳米复合材料。用X射线衍射、透射电镜、X射线光电子能谱、原子力与磁力显微镜等观察和分析其结构和物相。利用物理性能综合测试系统测量薄膜的磁性、电性和磁电输运性质。主要研究内容与结论如下:首先,用反应磁控溅射沉积制备了FeCo-Si-N复合薄膜,在室温沉积的条件下成功获得FeCo颗粒分布在Si-N基体中的纳米颗粒膜;用电子显微分析和磁力显微分析清晰表征出其结构特征和磁性特征;系统研究了FeCo-Si-N薄膜的磁电输运特性,在室温获得了大的正磁电阻,磁电阻在宽的磁场区间内随磁场呈线性变化。通过磁性测量和磁电阻随成分的变化关系分析,间接判断出颗粒内的磁畴结构,得到了获得最大磁电阻所需的结构特征和规律;研究了温度对薄膜磁电阻的影响,发现低温下样品的电子输运机制为隧穿导电,表现为隧道负磁电阻。在磁性FeCo纳米颗粒截止温度附近,正负磁电阻效应发生转变,表明该磁电阻与磁矩的排列有关。结果表明,薄膜大的室温正磁电阻既有轨道效应的影响,也有自旋效应的影响。其次,制备了FeCo-ZnO纳米颗粒薄膜并研究了磁电性能。发现溅射功率对FeCo-ZnO薄膜的影响很大。对于较低溅射功率制备的薄膜,FeCo不以零价的金属态出现,没有FeCo团簇,表现为正的磁电阻效应,并且磁电阻几乎不受成分和厚度的影响,表明该正磁电阻与磁性的相关性小,极有可能来源于轨道效应而不是自旋效应。薄膜在220 K的磁电阻高达350%,这种正磁电阻值随温度急剧变化的表现说明该磁电阻效应与声子的散射有关。而对于较高溅射功率下制备的薄膜,Fe、Co以零价的金属态存在,薄膜具有超顺磁性。在较厚的样品中出现负磁电阻效应,可能来源于FeCo团簇产生的隧穿磁电阻。此外,制备了FeCo与非晶碳复合的不同周期数的[C/FeCo]_n多层复合薄膜,研究了结构和磁电性能。结果表明,随着周期数增加,磁电阻MR由正值变为负值。正磁电阻的大小与磁场成线性关系,而且在高场下也没有饱和。通过薄膜电阻率随温度的变化行为研究了薄膜的传导机制。对周期数不同的样品的变程跃迁ln R--1/2与ln R--1/4曲线的拟合显示,低温下都属于Efros变程跃迁,因此磁电阻为负,但在高温下,周期数少的薄膜由于热激活不再是变程跃迁导电机制,磁电阻也转变为正值;而周期数多的样品由于界面和碳化物等缺陷的增加导致其局域化程度较强,即使温度升高也依然是变程跃迁,磁电阻保持为负值。研究表明,在2-300 K范围内传导机制的转变导致了磁电阻的转变,证实传导机制确实对磁电阻起着关键作用。最后,论文探索了FePt/C和FePt/MgO铁磁-非磁复合膜的磁电输运性能及其在垂直磁记录介质方面的潜力。采用共溅射沉积的方法制备了FePt-C和MgO/[FePt-MgO]薄膜。利用C、MgO与FePt不互溶特性,在FePt颗粒间获得C和MgO隔离物。研究了成分、退火温度对FePt-C和FePt-MgO颗粒膜结构和磁性能的影响。发现需要合适的隔离物含量以获得较大的矫顽力。提高退火温度有助于薄膜内部FePt相的形成,从而获得较大的矫顽力;但是,温度过高会使薄膜的质量遭到破坏,反而降低了矫顽力。此外,在FePt-C和FePt-MgO颗粒膜中还获得了一定的磁电阻效应,分析了磁电阻的产生机制。综上所述,本文采用磁控溅射沉积技术制备了铁磁/非磁纳米复合薄膜,在FeCo-Si-N、FeCo-ZnO和FeCo/C薄膜中获得了较大的室温磁电阻效应,并且通过结合其结构、磁性、正负磁电阻的转变和电阻的温度变化关系等的分析,研究了薄膜的输运机制和磁电阻机制,对铁磁-半导体复合膜中的自旋相关输运有进一步的认识。针对FePt-C和FePt-MgO颗粒膜,探索了其磁电阻效应,同时研究了成分、退火工艺等对其结构和磁性能的影响,这些工作对自旋电子学的发展和自旋电子学器件的开发以及新的磁记录介质材料开发具有一定的借鉴意义。
[Abstract]:Magnetoresistance materials have been widely used in magnetic induction, magnetic recording and other fields. The composite systems composed of magnetic and nonmagnetic materials have special magnetic and electronic transport properties because of their rich interfacial effects. There is no unified understanding of the electronic transport characteristics. It is necessary to further explore how to further improve the value of magnetoresistance and the mechanism of the magnetoresistance effect. The study shows that the addition of magnetic metal elements such as Fe, Co, Ni and other non-magnetic substrates, such as carbon and silicon, is not only conductive. The effects of the magnetoresistance on the structure and carrier mobility of the nonmagnetic base film in the composite membrane, and the mechanism of the magnetoresistance effect are understood, and it is beneficial to push high performance, low cost carbon materials and silicon materials. Extensive applications in the field of electronic information have important scientific significance and practical value. This paper studies the doping of transition elements (mainly FeCo of high spin polarity) in a variety of nonmagnetic matrix (narrow band gap semiconductor SiN, wide band gap oxide semiconductor ZnO, amorphous C, insulator MgO), and studies the composition and structure of the magnetoelectric properties. The ferromagnetic / nonmagnetic particle film or multilayer nanocomposite with room temperature and ferromagnetism was prepared by sputtering deposition technique. The structure and phase of the film were observed and analyzed by X ray diffraction, transmission electron microscopy, X ray photoelectron spectroscopy, atomic force and magnetic force microscopy. The main contents and conclusions are as follows: first, FeCo-Si-N composite films were prepared by reactive magnetron sputtering. The nano particles of FeCo particles distributed in the Si-N matrix were successfully obtained under the condition of room temperature deposition. The structural characteristics and magnetic properties of the particles were clearly characterized by electron microanalysis and magnetic microanalysis. The magnetic and electrical transport characteristics of the FeCo-Si-N film are systematically studied. A large positive magnetoresistance is obtained at room temperature. The magnetoresistance varies linearly with the magnetic field in a wide range of magnetic fields. The magnetic domain structure in the particles is indirectly judged by the magnetic measurement and the relationship analysis of the magnetoresistance with the composition, and the necessary junction to obtain the maximum magnetic resistance is obtained. The influence of temperature on the magnetoresistance of thin film is studied. It is found that the electron transport mechanism of the sample at low temperature is tunneling conduction, which shows the tunnel negative magnetoresistance. The positive and negative magnetoresistance effect occurs near the cut-off temperature of the magnetic FeCo nanoparticles, indicating that the magnetoresistance is related to the arrangement of the magnetic moment. The temperature positive magnetoresistance has both the influence of the orbital effect and the influence of the spin effect. Secondly, the FeCo-ZnO nanocrystalline film is prepared and the magnetoelectric properties are studied. It is found that the sputtering power has a great influence on the FeCo-ZnO film. For the thin films prepared by the lower sputtering power, FeCo does not appear in the zero valence metal state and does not have a FeCo cluster, showing a positive magnetic field. The resistance effect, and the magnetoresistance is almost unaffected by the composition and the thickness, shows that the correlation between the positive magnetoresistance and the magnetism is small, and it may be derived from the orbital effect rather than the spin effect. The magnetoresistance at 220 K is as high as 350%, and the positive magnetoresistance values with the temperature change sharply indicate that the magnetoresistance effect and the scattering of the phonons are shown. The thin films of Fe and Co are superparamagnetic in the presence of zero valence metal states. The negative magnetoresistance effect in the thicker samples may come from the tunneling magnetoresistance produced by the FeCo clusters. In addition, the [C/FeCo]_n multilayer composite films with different periodic numbers of FeCo and amorphous carbon have been prepared. The results show that the magnetoresistance MR changes from positive to negative with the increase of the number of cycles. The size of the magnetoresistance has a linear relationship with the magnetic field and is not saturated at the high field. The conduction mechanism of the film is studied by the change of the resistivity with the temperature. The transition transition of LN R--1/ to the sample with different periodicity is changed. 2 and the fitting of the LN R--1/4 curve show that at low temperature, it belongs to the Efros transition transition, so the magnetoresistance is negative, but at high temperature, the thin film with less periodic number is no longer the transition conduction mechanism, and the magnetoresistance turns to positive value, and the number of periodic samples is localized due to the increase of the interface and carbide and so on. It is strong that the magnetoresistance remains a negative value even if the temperature rises. The study shows that the transformation of the conduction mechanism in the range of 2-300 K leads to the transition of the magnetoresistance. It is proved that the conduction mechanism does play a key role in the magnetoresistance. Finally, the magnetic and electrical transport properties of the FePt/C and FePt/MgO ferromagnetic nonmagnetic composite membranes are explored. The potential of the vertical magnetic recording medium. FePt-C and MgO/[FePt-MgO] films were prepared by CO sputtering. Using C, MgO and FePt insolubility, the separation of C and MgO between FePt particles was obtained. The effects of composition, annealing temperature on the structure and magnetic properties of FePt-C and FePt-MgO particles and magnetic properties were investigated. The increase of the annealing temperature can help the formation of the FePt phase inside the film and obtain a larger coercive force. However, the high temperature will destroy the quality of the film, but reduce the coercive force. In addition, a certain magnetoresistance effect has been obtained in the FePt-C and FePt-MgO granular films, and the production of the magnetoresistance is analyzed. In summary, the ferromagnetic / nonmagnetic nanocomposite films have been prepared by magnetron sputtering. The magnetoresistance effects in FeCo-Si-N, FeCo-ZnO and FeCo/C films have been obtained, and the film is studied by the analysis of the structure, magnetic properties, the change of the positive and negative magnetoresistance and the temperature change of the resistance. The transport mechanism and magnetoresistance mechanism have a further understanding of the spin dependent transport in the ferromagnetic semiconductor composite membrane. The magnetoresistance effect is explored for the FePt-C and FePt-MgO granular films, and the effects of the composition and annealing process on its structure and magnetic properties are also studied. The development of spintronics and the Spintronics are carried out. The development of parts and the development of new magnetic recording medium materials are of some reference significance.
【学位授予单位】：华南理工大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TB383.2

【参考文献】