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1、华中科技大学 博士学位论文 磁性纳米颗粒膜的阻尼因子和微波磁谱微磁学计算 姓名：袁林 申请学位级别：博士 专业：微电子学与固体电子学 指导教师：江建军 20080515 I 摘 要 磁性纳米膜微波物性是薄膜磁学及其应用的研究热点之一。由于高密度磁记录工 业和无线通讯系统等相关高科技领域的飞速发展，促使应用在 GHz 频段下的磁性纳 米薄膜材料备受关注。针对磁性薄膜高频电感和磁性微波吸收剂等不同的应用目标， 必须研究不同性能要求的磁性薄膜材料。在薄膜高频电感应用中，为了提高高频电感 性能，需要软磁薄膜材料具有高的自然共振频率，窄的铁磁共振线宽和高的低频磁导 率实部。而在微波吸收剂中，则需要软磁薄
2、膜材料具有较大的铁磁共振线宽，以提高 电磁波吸收带宽。因而，系统地开展对磁性薄膜的阻尼因子和磁谱形状研究是高性能 磁性薄膜应用中至关重要的问题。为此，本研究利用微磁学方法计算了偶极作用颗粒 薄膜和交换作用磁性纳米颗粒膜的高频磁导率，并着重研究了纳米软磁薄膜中的随机 各向异性、磁化色散和退磁场等非均匀性对薄膜阻尼因子的影响规律和作用机制，且 建立了用于阐明相关实验现象的理论模型。为指导磁性薄膜的设计与制备工艺制定， 进一步对薄膜生长进行了模拟。 用微磁学方法研究了孤立磁性纳米颗粒膜体系微波磁性能。结果表明，磁偶极作 用对体系的静态磁结构影响显著，而交换相互作用影响不明显。进一步利用有效媒质 理论
3、的计算分析表明， 纳米颗粒膜中磁性颗粒体积分数含量上升时， 微波磁导率实部、 虚部均增大，共振频率增高。而搭接磁性颗粒体系中，交换作用对体系磁结构影响显 著，搭接颗粒的磁矩趋向一致排列。 利用微磁学方法计算了单相薄膜的静态磁结构，得到了典型的微磁 ripple 结构， 其磁色散角变化规律与线性 ripple 理论分析基本符合。计算得到的微波磁导率能用单 畴磁性颗粒的磁导率公式较好地描述。薄膜的等效阻尼因子随磁色散角的增大而线性 增大，这与理论结果一致。对两相薄膜的研究发现，两相薄膜的交换耦合区具有独特 的微细磁结构，这种微细磁结构增大了磁性薄膜的阻尼因子。 利用微磁学方法计算了具有表面各向异性
4、的磁性颗粒的磁导率。研究表明，表面 各向异性为 Aharoni表面各向异性时，磁性颗粒自旋结构为共线性单畴结构。为 Nel 表面各向异性时，颗粒自旋形成所谓“throttled”结构，自然共振频率与颗粒直径倒 数1/ D成正比。其次，以具有不同磁晶各向异性易轴的 4个磁性纳米颗粒嵌入在磁性 基体为模型，用蒙特卡洛方法模拟了磁性纳米薄膜的微磁结构。计算结果表明，不同 II 强度的交换和表面作用时，体系存在 3种典型的自旋结构：共线性磁结构； “zigzag” 磁结构； “throttled”磁结构。 利用微磁学方法计算了单面粗糙薄膜的静态磁结构和磁谱。结果表明，粗糙表面 导致了薄膜面内的微磁 r
5、ipple 结构，从而增大了薄膜的阻尼因子。对特定均方根粗糙 度，相关长度的薄膜，随着纵向外加磁场的增大，薄膜阻尼因子先减小后增大。在此 基础上，还利用分析的手段得到了具有表面粗糙度的薄膜磁导率公式，进一步讨论了 杂散场和面内退磁场对薄膜磁谱的影响。当薄膜面内退磁因子和磁色散角变化时，薄 膜磁谱有可能表现为单峰或双峰结构，由此可用来解释实际薄膜测量呈现的多种多样 的磁谱结构。 对 Fe 多晶薄膜生长进行了模拟。在利用分子动力学，能量最小方法来计算吸附 原子的表面激活能的基础上， 应用动力学蒙特卡洛方法进行了一维单颗粒生长。 由此， 进一步应用 FACET 程序进行多晶薄膜生长。由此，可以确定薄
6、膜 rms 粗糙度和颗粒 尺寸随相关制备工艺参数的关系，如薄膜厚度，薄膜沉积温度，斜入射角度，初始成 核态等，从而指导磁性薄膜的设计与制备工艺制定。 关键词：微磁学 纳米颗粒膜 阻尼因子 微波磁谱 随机各向异性 表面各向异性 表面粗糙度 分子动力学 动力学蒙特卡洛 薄膜生长 III Abstract In recent decades, attention was mainly paid to the microwave properties of magnetic nanostructural films in the field of thin films magnetism invest
7、igation. The development of high technologies related to the high density magnetic recording industry and the widespread of wireless communication systems is an important incentive to focus on magnetic materials, which can be used in gigahertz-frequency applications. To make the requirements of the
8、different applications, these materials can vary considerably depending on the properties of various magnetic nanostructural films. For instance, in order to design of a high-frequency inductor we can use a soft magnetic material with high ferromagnetic resonance (FMR) frequency. A narrow FMR line a
9、nd a high real component of the magnetic permeability are in this case desirable. Somewhat different properties must be satisfied for microwave absorber. The next-generation of thin film microwave absorber could mainly depend on the absorption of microwave power at GHz frequency; the FMR linewidth m
10、ust be very broaden to increase the absorbing band width. Therefore, understanding damping and magnetic spectra in nanostructural magnetic media is critical to wide applications. In this dissertation, the microwave permeability of the dipolar and exchange granular film is calculated systematically u
11、sing micromagnetic method. And the influence and mechanism of some anisotropy, magnetization and demagnetizing, inhomogenous in films on damping factor, are extensively examined. The theory model to explain the relative experiment is described. Furthermore, the process of film growth is simulated to
12、 guide the design of magnetic film for particular applications. The isolated nano-granular magnetic films are extensively investigated using the micromagnetic method. The computational results showed that effect of the dipolar interaction of system on the static magnetic structure is remarkable. The
13、 calculation according to the effective medium theory shows the real and imaginary part of permeability and resonance frequency increase with the increase of fill factor of magnetic particle. Nevertheless, in the connected granular system, the effect of the exchange interaction on static magnetic st
14、ructure is remarkable. The magnetization of the connected granular tends to collinear. The micromagnetic structures of the single-phase soft magnetic films are simulated using micromagnetic method. The typical micromagnetic ripple structure of magnetic films is obtained. The relationship between the
15、 magnetic dispersion angle and the corresponding IV magnetic parameters of the film is in agreement with the linear ripple theory. The obtained magnetic spectra could well fit by the permeability equation which was deduced from the LLG equation when the film is considered as a single domain. And the
16、 effective damping factor increases with the increase of dispersion angle, which follows the theory law. In two-phase system, the micromagnetic structure in exchange coupling region show fine structure, which increases the effective damping factor. The resonance frequency of single ferromagnetic particle with surface anisotropy is investigated by micromangetic simulation. In Aharoni surface anisotropy, the spin structure of the single
