Fe/Ni多层膜的结构与磁性研究

【作者】 唐佳；

【导师】 马斌；

【作者基本信息】 复旦大学 ， 光学， 2010， 硕士

【摘要】 随着信息存储需求的不断提高,磁存储技术作为最主要的存储方式,其研究与发展受到学术界与工业界的广泛关注。磁记录密度已从1957年的2 Kbit/in2提高到了如今的329 Gbit/in2。当硬盘记录颗粒尺寸降低至纳米量级时,为了防止超顺磁效应(即保证记录的热稳定性),硬盘介质的矫顽力很大。一般而言,磁头的写入场要达到记录介质矫顽力的两倍,才能有效写入。因此,记录密度的继续提高,很大程度上受限于磁头的写入能力。写磁头材料应具有良好的软磁性能,包括高磁化饱和强度、低矫顽力、高磁导率等。通常,铁基材料的饱和磁化强度很高；然而,它的磁晶各向异性能也很高,按照传统方法难以降低其矫顽力,因此无法作为理想的磁头材料投入应用。但是,纳米结构介质的发展开创了磁头软磁材料研究的新时期。通过减小晶粒尺寸,可使有效各向异性常数<K>相比局部的磁晶各向异性常数K1小几个数量级,有利于提高磁导率。多层膜溅射是一种方便有效的制备纳米结构软磁材料的方法,IBM和Hitachi都曾采用Fe基多层膜来制备高效能的磁头。依照不同类型的中间层,铁基软磁多层膜大体可分为两类：磁性中间层与非磁性中间层。后者的性能往往更好,磁性中间层能贡献磁矩,提高材料的饱和磁化强度。在本文中,我们采用Fe/Ni多层膜结构来制备软磁纳米结构材料介质。Fe,Ni单层厚度远小于畴壁厚度时,由于层间耦合作用,磁矩的翻转不再受局部高磁晶各向异性的控制,而是被平均化后大为降低的各向异性所影响,因此材料的矫顽力可显著降低。在论文的第一部分,我们分析了Fe/Ni的结构,以及在此结构下的多层膜的磁性,主要讨论了Fe(或Ni)层厚度变化对矫顽力的影响及其原因,还通过掺Cu层进一步验证了层间耦合作用在Fe/Ni多层膜体系中的重要性。在论文的第二部分,鉴于以往的研究对软磁多层膜的粗糙度关注有限,而随着极薄薄膜的发展,界面情况对多层膜的结构与磁性的影响愈发重要,我们研究了粗糙度对Fe/Ni多层膜的结构与磁性的影响。我们使用两种方法调控粗糙度,一是改变样品生长的温度,二是在MgO基板预先溅射Ag作为底层。我们探讨了Fe/Ni多层膜的磁化过程的影响因素,最终解释了界面粗糙度对矫顽力的影响的原因,提出改善Fe/Ni多层膜软磁性能的方法。更多还原

【Abstract】 As the demand for information storage continuously increases, magnetic recording has been widely studied and developed as the chief data-storage technique, gaining intense interest both from academe and industry. The areal recording density has increased from2 Kbit/in2 in 1957 to 329 Gbit/in2 recently. When the grain size of recording media is reduced to several nanometers, the magneto-crystalline anisotropy needs to be large enough to avoid superparamagnetic effect, in other words, to stabilize the recorded bit. However, high coercivity due to large anisotropy gives rise to the difficulty of magnetic writing, for the writing field has to be approximately twice the coercivity of the recording media for effective writing. Therefore, further increase of magnetic recording density is largely confined by the writing ability of the magnetic head.The material for magnetic writing head needs to be reasonably soft, its permeability must be sufficiently high and its coercivity sufficiently small so that the field intensity within the head pole and yoke becomes negligible. In early research, Fe-based material has high saturation magnetization, but also a high magneto-crystalline anisotropy, its coercivity then can hardly be lowered by traditional methods. However, the development of nanocrystallite materials ushered a new era for the study of soft magnetic materials. By reducing the grain size, the effective anisotropy can be decreased to extremely low value, usually several orders less than the local anisotropy. Multilayers preparation by sputtering is an effective way to fabricate nanocrystalline soft materials; IBM and Hitachi both have adopted Fe-based multilayers in the production of magnetic heads. In general, two kinds of multilayers are classified based on different spacer materials in between:one is nonmagnetic and the other ferromagnetic. The latter kind is more favorable, for ferromagnetic spacers can help to increase the saturation magnetization of multilayers.In our research, we prepare Fe/Ni multilayers to study their structural and magnetic properties. As the Fe (or Ni) layer thickness is far less than the domain wall width, due to exchange coupling, the magnetization of Fe and Ni layers cannot follow each easy axis, and then the anisotropies are averaged out, the coercivity of multilayers is largely decreased. In the first part of this paper, the structure and orientation of Fe/Ni multilayers is analyzed, and the influence of layer thickness on the coercivity is discussed in detail. Cu insertion confirms the dominant role of exchange coupling in the multilayer system. In the second part, the effects of interface roughness on the structural and magnetic properties of Fe/Ni multilayers are investigated. Interface roughness is introduced in two ways, one is to tune the growth temperature, and the other is to prefabricate Ag on MgO substrate as underlayer. The mechanism of Fe/Ni multilayers’magnetization is discussed, smoother interfaces are more advantageous to optimize the exchange coupling, and the coercivity can be decreased more effectively.更多还原