薄膜磁性与磁力显微镜针尖场的微磁学研究
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摘要
磁力显微镜（MFM）的发展与磁记录工业的发展相互依赖，相互促进。MFM是磁记录介质和磁头的常规测试工具。MFM探针的磁性覆盖层多为CoCr基磁性材料，目前高分辨的探针也以FePt材料为磁性覆盖层，这两种材料分别是当前硬盘中的磁薄膜介质和下一代硬盘介质的重要备选材料。随着磁记录密度的急剧增长，对磁畴微细磁结构进行高分辨的定量表征，是当前MFM观测技术面临的最大难题。MFM图像的定量解释及探测分辨率的提高，都依赖于对MFM探针磁性的理解和改善，而实验方法难于对探针尖端磁性材料进行结构和磁性测量，并且常用的磁偶极子针尖模型中没有给出有效偶极子的大小和位置，难于对MFM图像进行定量分析。据此，本文以MFM针尖为研究对象，推导针尖有效偶极子的位置，并结合微磁学方法确定有效偶极矩的大小；建立精确的针尖模型，对针尖形状、覆盖层的磁性和微结构与图像分辨的关系进行系统的研究，解释当前实验制备的针尖具有高分辨特性的物理原因，并对高分辨MFM针尖的设计提供指导。
本文首先对磁记录介质材料L10-FePt及实际的CoCrPt基硬盘的内禀磁性进行研究，为针尖模型和所观测样品模型的建立奠定基础。磁记录薄膜的模型中包括晶粒尺寸、晶粒间耦合及晶格错配等因素。L10-FePt薄膜的磁滞回线与实验数据拟合的很好，并对其翻转机制及商用可行性进行了探讨。在CoCrPt基薄膜中，引入了非晶软磁层（a-SUL），建立了实际的硬盘模型，研究发现a-SUL能够降低磁记录层的矫顽力（Hc），有利于信息的写入；利用转矩曲线法测量硬盘磁记录层磁晶各向异性常数（K1）时，a-SUL的影响很大，需要剥离a-SUL进行测量。
基于微磁学建立了准确的金字塔型针尖模型，其中，不规则的微磁学单元中涉及到的三角形退磁矩的解析解是本课题组之前的工作。根据推导的有效偶极子的位置公式，结合微磁学方法计算的针尖场和有效偶极矩的大小均与实验吻合。在此基础上，建立了含有覆盖层薄膜晶粒结构的金字塔型针尖模型，对针尖覆盖层材料的磁性及微结构进行了系统的讨论，研究发现垂直各向异性（PMA）的针尖能够在低扫描高度（2~8nm）下很好地表征线密度为1600kfci的颗粒薄膜介质的表面磁畴结构；而对于面密度为2.5TB/in~2的比特图形介质，面内各向异性（IMA）的针尖能够在低扫描高度（2nm）下更加准确地读出其记录信息。
Magnetic force microscope (MFM) has been widely used to characterize thedomain structures of the hard disk and read/write head in magnetic recording industrysince it was invented. Nowadays, the coating materials of high-resolution MFM tipinclude cobalt-based alloy and iron-platinum alloy, which are the recording layer usedas the current perpendicular recording media and as a candidate for next generationmagnetic recording media, respectively. As the rapid growth of the areal density, it isurgent to develop higher resolution MFM tips to image and quantitatively explain thenano-scaled domain structures of magnetic materials in the magnetic recording system.However, it is difficult to characterize the microstructure and magnetic properties oftip-coating by experiments. Moreover, the effective magnetic dipole and its exactposition are not well discussed in thepoint probe model of the tip. In this thesis, theposition of the effective dipole in a MFM tip is determined by the multipole expansiontheory in classical electromagnetism. The magnetic moment distribution in a MFM tipcan be calculated by micromagnetic simulation. An accurate3-D micromagnetic tipmodel is set up, and the relationshipsamong the tip shape, magnetic properties andmicrostructure of tip-coating and the MFM image resolution have been systematicallystudied. The result obtained is in good agreement with experiments and explains furtherfor the improved resolution in the recent experimental reports. The understanding fromthis study throws light on designing MFM tips for high resolution.
Intrinsic magnetic properties of theL10-FePt thin film media and the CoCrPt-basedperpendicular media with soft magnetic underlayer (SUL) are studied. This study is thefoundation for accurately setting up the MFM tip model and the sample model. Grainsize, inter-grain exchange coupling and lattice mismatch between different layers areincluded in the model. The simulated out-of-plane loop of L10-FePt thin film matchesthe experimental loop very well. The mechanism responsible for magnetizationreversalandpotential application of the thin film are studied and discussed. Amicromagnetic model of CoCrPt-based perpendicular disk media is set up, whichincludes a thick amorphous soft under layer (a-SUL). The a-SUL will decrease thecoercivity (Hc) of the recording layer (RL), which is beneficial for the writing process.However, due to the large saturation magnetization (Ms) and its large volume, a-SUL affects much on the measurement of crystalline anisotropy constant (K1) of RL bymeasuring torque curves of the whole disk media, which should be etched away beforemeasuring.
An accurate pyramid tip model is set up, which includes polyhedron cells in the tipsurfaces and edges. The analytical solution of the demagnetizing matrix of a trianglewas obtained earlier by our group. Calculated tip stray field and the effective dipole arein good agreement with experiments. Furthermore, a micromagnetic model of pyramidtip coated by hard magnetic thin film is set up with the grain structure included in thethin film;and magnetic properties and microstructure of tip-coating arestudied. The tipwith perpendicular magnetic anisotropy (PMA tip) can measure the micromagneticstructures of granular thin film medium with a linear density of1600kfci accurately atlow scan heights of2nm to8nm, and tip with in-plane magnetic anisotropy (IMA tip) isa good choice for measuring the domain structure of bit patterned media with an areadensity of2.5TB/in~2at a low scan height of2nm.
本文首先对磁记录介质材料L10-FePt及实际的CoCrPt基硬盘的内禀磁性进行研究，为针尖模型和所观测样品模型的建立奠定基础。磁记录薄膜的模型中包括晶粒尺寸、晶粒间耦合及晶格错配等因素。L10-FePt薄膜的磁滞回线与实验数据拟合的很好，并对其翻转机制及商用可行性进行了探讨。在CoCrPt基薄膜中，引入了非晶软磁层（a-SUL），建立了实际的硬盘模型，研究发现a-SUL能够降低磁记录层的矫顽力（Hc），有利于信息的写入；利用转矩曲线法测量硬盘磁记录层磁晶各向异性常数（K1）时，a-SUL的影响很大，需要剥离a-SUL进行测量。
基于微磁学建立了准确的金字塔型针尖模型，其中，不规则的微磁学单元中涉及到的三角形退磁矩的解析解是本课题组之前的工作。根据推导的有效偶极子的位置公式，结合微磁学方法计算的针尖场和有效偶极矩的大小均与实验吻合。在此基础上，建立了含有覆盖层薄膜晶粒结构的金字塔型针尖模型，对针尖覆盖层材料的磁性及微结构进行了系统的讨论，研究发现垂直各向异性（PMA）的针尖能够在低扫描高度（2~8nm）下很好地表征线密度为1600kfci的颗粒薄膜介质的表面磁畴结构；而对于面密度为2.5TB/in~2的比特图形介质，面内各向异性（IMA）的针尖能够在低扫描高度（2nm）下更加准确地读出其记录信息。
Magnetic force microscope (MFM) has been widely used to characterize thedomain structures of the hard disk and read/write head in magnetic recording industrysince it was invented. Nowadays, the coating materials of high-resolution MFM tipinclude cobalt-based alloy and iron-platinum alloy, which are the recording layer usedas the current perpendicular recording media and as a candidate for next generationmagnetic recording media, respectively. As the rapid growth of the areal density, it isurgent to develop higher resolution MFM tips to image and quantitatively explain thenano-scaled domain structures of magnetic materials in the magnetic recording system.However, it is difficult to characterize the microstructure and magnetic properties oftip-coating by experiments. Moreover, the effective magnetic dipole and its exactposition are not well discussed in thepoint probe model of the tip. In this thesis, theposition of the effective dipole in a MFM tip is determined by the multipole expansiontheory in classical electromagnetism. The magnetic moment distribution in a MFM tipcan be calculated by micromagnetic simulation. An accurate3-D micromagnetic tipmodel is set up, and the relationshipsamong the tip shape, magnetic properties andmicrostructure of tip-coating and the MFM image resolution have been systematicallystudied. The result obtained is in good agreement with experiments and explains furtherfor the improved resolution in the recent experimental reports. The understanding fromthis study throws light on designing MFM tips for high resolution.
Intrinsic magnetic properties of theL10-FePt thin film media and the CoCrPt-basedperpendicular media with soft magnetic underlayer (SUL) are studied. This study is thefoundation for accurately setting up the MFM tip model and the sample model. Grainsize, inter-grain exchange coupling and lattice mismatch between different layers areincluded in the model. The simulated out-of-plane loop of L10-FePt thin film matchesthe experimental loop very well. The mechanism responsible for magnetizationreversalandpotential application of the thin film are studied and discussed. Amicromagnetic model of CoCrPt-based perpendicular disk media is set up, whichincludes a thick amorphous soft under layer (a-SUL). The a-SUL will decrease thecoercivity (Hc) of the recording layer (RL), which is beneficial for the writing process.However, due to the large saturation magnetization (Ms) and its large volume, a-SUL affects much on the measurement of crystalline anisotropy constant (K1) of RL bymeasuring torque curves of the whole disk media, which should be etched away beforemeasuring.
An accurate pyramid tip model is set up, which includes polyhedron cells in the tipsurfaces and edges. The analytical solution of the demagnetizing matrix of a trianglewas obtained earlier by our group. Calculated tip stray field and the effective dipole arein good agreement with experiments. Furthermore, a micromagnetic model of pyramidtip coated by hard magnetic thin film is set up with the grain structure included in thethin film;and magnetic properties and microstructure of tip-coating arestudied. The tipwith perpendicular magnetic anisotropy (PMA tip) can measure the micromagneticstructures of granular thin film medium with a linear density of1600kfci accurately atlow scan heights of2nm to8nm, and tip with in-plane magnetic anisotropy (IMA tip) isa good choice for measuring the domain structure of bit patterned media with an areadensity of2.5TB/in~2at a low scan height of2nm.
引文
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