【作者】 刘亚丕；

【导师】 葛世慧；

【作者基本信息】 兰州大学 ， 凝聚态物理， 2000， 博士

【摘要】 由铁磁金属纳米颗粒分散在氧化物绝缘介质中形成的颗粒膜近年来引起人们极大的研究兴趣,这不仅是因为在这种系统中发现了较大的隧道型巨磁电阻效应,这种巨磁电阻效应是继在金属超晶格、金属颗粒膜、平面型隧道结中发现巨磁电阻效应之后的又一新的进展,对理论研究提出了许多新的课题,也是因为这种薄膜还表现出了优异的高频软磁性能、巨霍尔效应及优异的机械性能等,从而使得这种薄膜有着广阔的应用前景。隧道颗粒膜的巨磁电阻效应与这种薄膜的微结构、电性及磁性都有着十分密切的关系,然而目前对该薄膜体系的各种物理性能尚缺乏系统的实验及理论研究。在本论文中,我们采用射频共溅射方法,在室温及77K 的衬底温度下首次成功地制备了性能优异的Co-Cr2O3及Fe-Cr2O3系列颗粒薄膜,并利用X-Ray 衍射、透射电镜(TEM)、电子衍射(ED)、X-Ray 光电子谱(XPS)及宏观电性及磁性(如VSM)测量等手段详细地研究了Co(Fe)-Cr2O3 颗粒膜的微结构、电性、磁性及它们与TMR 效应之间的关系,系统地研究了薄膜的电性和磁性随薄膜微结构的变化,通过实验数据点的理论拟合,得出了一些有价值的物理信息,探讨了磁电阻效应的温度依赖性及获得较大TMR 效应的途径,主要内容可概括如下:(一)、结构和磁性:(1)采用射频共溅射方法,成功地在室温及液氮衬底温度下制备了几个系列 的Co(Fe)-Cr2O3颗粒膜。(2)在以Cr2O3为绝缘介质的颗粒膜中发现,无论是对于Co 基还是Fe 基, 这种绝缘介质都处于晶态,而不是非晶态,并且铁磁金属颗粒的尺寸都 比较大,使薄膜在室温下都呈现出铁磁性而不是超顺磁性,这种现象即 使在衬底温度为77K 时也是如此,说明这种性质与这种绝缘介质本身的 性质有密切的关系。(3)运用XPS 技术对颗粒膜的结构进行了系统研究,证实所制备的Co-Cr2O3 和Fe-Cr2O3 系列颗粒薄膜在所研究的金属体积分数fv 范围内是由金属 Co(Fe)颗粒和Cr2O3 绝缘介质组成,说明我们所制备的样品是成功的。(4)我们发现,对于衬底温度在室温下制备的Co-Cr2O3颗粒薄膜,随着膜中更多还原

【Abstract】 Recently, insulating granular films coomposed of nanostructured ferromagnetic(FM) metallic granules dispersed in an oxide insulating matrix have receivedconsiderable interests due to the discovery of pronounced Tunnelingmagnetoresistance (TMR) effect in them and to superior high frequency softmagnetic properties, giant Hall effect and mechanical properties, etc., they hold.This is a new development after the discovery of Giant magnetoresistance (GMR) inmetallic superlattice, metallic granular films and in magnetic tunneling junctions,and raises interesting fundamental problems,all of which make them very promisingfor applications. It is known that the tunnel-type GMR effect in insulating granularfilms is closely related to their microstructure, electrical transport and magneticproperties, however, up to now no profound systematical experimental andtheoretical studies on the origins and physical properties in these systems have beenproceeded. In this dissertation, Co-Cr2O3 and Fe-Cr2O3 granular films with superiorproperties have been successfully fabricated by RF co-sputtering technique underroom and 77K substrate temperatures for the first time. The microstructure,electrical transport and magnetic properties as well as their relation to TMR effectare systematically investigated by various techniques such as X-Ray diffraction(XRD), transmission electron microscopy (TEM), electrical diffraction (ED), X-Rayphotoelectron spectroscopy (XPS) and VSM, etc., the change of electrical transportand magnetic properties in these films with their microstructure is also shown. Someinteresting and valuable results have been obtained after the theoretical fitting ofexperimental data with the existed theory. The temperature dependence of TMReffects in these films and the way to get larger TMR effect is also studied. Our mainprincipal results obtained are as follows:(1) Microstructure and Magnetism:I. Series Co-Cr2O3 and Fe-Cr2O3 granular films have been successfully fabricated by RF co-sputtering technical under room and 77K substrate temperatures.II. We have found that, in the Cr2O3-type insulating granular films, whether is Co-based or Fe-based, insulating media is polycrystal, and the size of FM granules are large, which made all the films exhibit ferromagnetic properties at RT, even to the films made under 77K substrate temperature. This implies that this phenomena is closely related to the properties of this insulating matrix.III. By using XPS technique, it is confirmed that, in the whole metallic volumefraction fv range we studied, Co-Cr2O3 and Fe-Cr2O3 granular films are all consisted of metallic Co(Fe) granules and Cr2O3 insulating media. This implies that the samples we have made is the same as we expected. IV. We have found that, for the Co-Cr2O3 granular films fabricated under room temperature (RT), as the contents of Co in the film increases, the fcc-Co granules in the film gradually grow and their size increases from 20 nm for fv=0.15 to 33nm for fv=0.35, then decrease as fv further increase, to 18nm for fv=0.55, that is, there is a maximum size for fcc-Co granules. At the meantime, some Co in the films exist in the form of hcp-Co. As fv increases, the ratio of fcc-Co to the total Co-phase in the films also exhibits a maximum, and its content is closely related to the size of fcc-Co granules. When the proportion of fcc-Co in the total Co-phase is large, the size of fcc-Co granules is also large, correspondingly, Hc of the film is small. V. We have found that in the Fe-Cr2O3 granular films fabricated under RT that, as fv increases, Fe granules in the films become larger, their average size increases from 15nm for fv=0.15 to 19nm for fv=0.39, and further increases to 24nm for fv=0.55, that is, the size of Fe granules increases monotonously with fv; while Hc of the film decreases with fv increase, and reaches a minimum at fv=0.39, with fv further increase, Hc also increases, despite the fact that Fe granules are very large at that case, which needs to be further studied.(2) Transport Properties: I. Measurements of temperature coefficient of resistivity (TCR) for Co-Cr2O3 granular films fabricated under RT indicates that their TCR change sign from negative to positive as fv increases. For fv≤0.5, TCR<0, these films are in the dielectric state; for fv>0.5, TCR>0, there is a normal metallic conductance. When fv=0.55, TCR changes its sign at about 65K: when T>65K, TCR>0, and metallic conductance exists in the films; but when T<65K, TCR<0, thermally activated tunneling becomes the main conductance mechanism. This implies the co-existing and competition of metallic and tunnel conductances. II. TCR measurements for Fe-Cr2O3 granular films fabricated under RT indicates that their TCR change sign from negative to positive as fv increases. For fv≤0.55, TCR<0, these films are in the dielectric state; for fv=0.28, TCR also changes its sign at low temperature: when T>105K, TCR>0, but when T<105K, TCR<0, implying the competition of conductance mechanisms. Increase in resistivity at low temperature may be related to the Coulomb Block effect.更多还原