Co/ZnO纳米颗粒薄膜的磁特性研究

【作者】 崔福良；

【导师】 薛成山；

【作者基本信息】 山东师范大学 ， 微电子学与固体电子学， 2000， 硕士

【摘要】 纳米磁性颗粒膜是将磁性微颗粒嵌在互不固溶的基质材料中而形成的复合薄膜。金属颗粒悬浮于不固溶的基质中，其结构和特性比较稳定。膜中的颗粒是介于宏观和微观之间的物质的一种新凝聚态，其尺寸范围一般在1个纳米～100个纳米之间。同大块材料相比，由于颗粒尺寸的减小，颗粒具有明显的小尺寸效应与表面效应。在一定的组成和微结构条件下，颗粒膜常呈现出独特的磁性行为。 本文第一章主要介绍了颗粒膜的磁特性。颗粒膜的磁性与磁性金属在膜中所占的体积百分比有密切关系。颗粒所占体积比很小时，颗粒嵌在介质中，互相绝缘。随着体积比的增大，颗粒膜的矫顽力也随之增大。当体积比达到逾渗值时，膜中的颗粒开始互相交错，形成网络状，导致矫顽力的急剧下降。颗粒膜中的磁性主要是受颗粒大小的影响，当颗粒很小时，以至于颗粒的磁能小于热能，在热扰动的作用下，整个颗粒可视为一个顺磁原子，其磁特性可用Langevin函数来描述。颗粒尺寸变小，比表面积相应增大，此时，表面效应成为影响颗粒磁性的一个重要机制。由于颗粒表面处原子排列的不完整以及不对称造成表面原子的自旋结构不同与体内，是混乱分布的，存在表面各向异性。表面原子与颗粒内部原子形成了一种core/shell结构。颗粒内部的铁磁性物质为core，外层包围的原子为shell。 门圳内’o仰凹＊en比en亡入颗粒的农厂备向 异性自后，推导出 了卜＊－Sin。颗粒腆的矫顽力 jL与颗粒直径 d之间的关系。合理地解释了H col松的规律。颗粒尺寸的变小，意味着颗粒内自由电于数减少。根据KUbO理论，这将使颗粒中的电子能级由连续变为分立，呈现量于尺寸效应。微观粒于还有宏观量子隧道效应，一些宏观龄，如磁化强度，磁通量，l七荷等可以通过隧道效应，穿越宏观系统的势垒而产牛变化。木章的最后还介绍了制备磁性颗粒吸的方法。最广泛攸用的就是肘频（RF）磁控溅射技术。采川这种技术可以摆脱溅射时靶材导电性的限制，井且可以提高溅射诬率，改善）吠的质旦。这种技术最适宜制备金属／纶缘体颗粒腆。另一种常用的方法就是采用蒸发法。这种技术制备的顺粒尺寸与真空室中充入的惰性气体的气压有关。 第二章介绍我们制备Co／Zno 4W粒膜所用的溅射系统、制备过稚以及颗粒膜特性的测量。我们采用配有SY型500w射频电源的JCK－500W型磁控溅射仪来制备Co／Zno ＃粒膜。靶材分别．采用分立的h靶和b心靶。溅射时，载有衬底的基片架转动。结构特性采川X射线衍射仪（川m）测量，磁特性和电特性分别采用振动样，宛磁强计（VSM）和四探针方法测量。颗粒大小利用CO衍射峪的半 if宽址行计算得到。 第三章给出了我们的实验结果及其分析情况。溅射时采用＊气作为工作人（体，在低气压1～3Pa范围内，冰气压对腆的淀 ．4． W刎山芥摘业积浊率没有明显的影lllN。固定靴的溅射功率在100w，C。靶的溅射I’D压在350V，改变C。靶的溅射电流，膜中＊。的体积百分比近似与其溅射电流成正比。X射线衍射谱表明：ZnO的c－轴垂直膜面择优生长明显，而Co在Co／ZnO颗粒膜中以伙结构存在，在我们制备的所有样品中没有出现 hCp结构。在 TS习7℃～150‘〔范田 内退火萝7＞350’C时JI二夕出现明 显的COO的行射峰沪在整个退火温度范围内，没有发现C。的其他氧化物。C。的体积百分比为扣％左右时，在C刀℃～的0℃范围内退火，可明显改变 Co／ZnO颗粒膜的磁性。在乙＜200℃时，＃s随C的升高而增大，这是对o的体积百分比为49％的样品而言，当体积百分比增大为52％时，这一退火温度有下降的趋势。随后C升高，人s又随之降低。而矫顽力凤在入＜200”C时，随C的升高而减小，之后，又随入的升坏而抑大至引3A。在继个退火范围内，剩磁比小于或近似等于0．5。CO的含量较少时（约为36％人颗粒腆呈现超顺磁性。尽管在7：l＝27℃～800℃之间的大范围内退火，颗粒腆仍保持超顺磁性。用 Langevin函数对实验结果进行拟和，符合的很好。常温下，Co／ZnO颗粒膜的颗粒直径在17urn～46urn 之问 变化时，Hccol／d。Co／ZnO颗粒膜经 TS＝450”C谁火处划的样品在T＝ 278K时，磁滞M线发生了较大的偏移。更多还原

【Abstract】 Nanometer magnetic granular films are small metal pafticlesembedded in an insulating matrix that hardly solve each other. Thegrain size is in the range one to one hundred nanometers. Becausethe particles are embeddcd in an insulating matrix, their structureand properties are more stable. On the otlier hand,they offer someunique properties due to finite size effects and surface effectscompared with bulk materials.Chapter 1 mainly deals with the magnetic properties ofgranular fllms. For granular films, the magnetic properties arecIoseIy reIated to magnetic metal volume fraction (P) .When thevolume fraction is very small,the magnetic metal pariicles dispersein the form of isolated particles in the insulator continuum. Thecoercivity increases gradually with increasing volume fraction ofmagnetic metal, while decreases rapidly when the volume fraction islarger than the percolation threshold. One of the principal factorsdetermining the coercivity of magnetic granular films is themagnetic granular size. WI1en the granular size is so small that thegranular magnetic energy is lower than the thermal energy,thewhole magnetic particle acts as a paramagnetic atom due to thethermal agitation, thus its magnetic properties can be described bythe Langevin function. For nanoparticles, a large fraction of theatoms are surface atoms, which significantly influence the magneticproperties. The existence of broken bounds and the asymmetry ofarrangement at the surface causes electron spin disorder whichdistinctly differs from that in the bulk, and results in the surfaceanisotropy. Thus,a type of structure with the ferromagnetic core andsurface shell is well defined. Considering surface anisotropy energy,Chen chen deduced the particle diameter(d)dependence of thecoercivity (H) for the Fe-SiO2 system, and reasonably explained therelation: H. oc l/d .According to the Kubo theory, the decrease ofgranular size will lead to the energy band discrete, and quantum sizecffects appeaL Macroscopic quantul11 tunneling effects are alsofOund for slna1l size particles. In addition, chaPter l includesmethods fOr making granular films. R.f magnetron sputteringsystem is widely used fOr preparing metal-insul8tor granular films,because r.f magnetron sputtering both frees from the limitation ofconductivity of target and enhances the spwtering rate. Anothermethod adopt evaPoration-deposition technique, by Which theparticle size is related to the inter gas pressure in vacuum chamber.Chapter 2 describes the method for making Co/ZnO granulartilms as well as the test. We prepared the samples by an r.fmagnetron sputtering system, which involves a rotating substrateagainst the separated Co target and ZnO target. The microstructureswere checked by X-ray diffractOmetry(Xan), and particle size wascalculated from the full width at half maximum(FWHM) using theScherrer relation. Magnetic and electric properties were measured bya vibrating sample magnetometer(VSM) and dc-four terminalmethod.Chapter 3 treats our experimental results. Spultering wasperfOrmed in Ar gas whose pressure hardly influence the depositionrate in the range l to 3Pa. The rf sPuttering power for ZnO targetwas conirolled at l00W and the voltage loaded on Co target wasfixed at 350K we find tl1at the volume fraction of Co is proportionalto the current flowing through Co target. XRD spectrum indicatesthat ZnO appears with a hexagonal structure and a preferredorientatio11 with the c-axis perPendicuIar to the substr8te, whi1e Coappears with fcc structure. Annealing temperature from 27’C to 450C, CoO begin to crystalize until Ta=350’C, while other Co oxidesdon’t aPpear. The magnetic properties change with increasingannealing tempefature. The saturation momellt increases with theannealing temperature until Ta=200’C, and then decrease (p=0.49),while the coercivity shows a opposite tendency. The remanence ratioincreases with t更多还原