【作者】 李正华；

【导师】 魏福林；

【作者基本信息】 兰州大学 ， 凝聚态物理， 2009， 博士

【摘要】 硬盘核心系统包括存储信息的磁介质、读写信息的磁头和实现快速读写的机械转动系统，其中涉及的学科非常广泛，包括材料、机械、电子、自动化、计算机等工科的研究，也与物理、化学这些理科的基础研究密切相关。本文的研究主要集中在对硬盘工业最关键的核心磁性器件的研究领域。微磁学（micromagnetics）可以计算磁滞回线、磁导率、磁畴和信息的读写过程，通过计算和设计，可以大幅度地减少器件制备、材料制备过程中的盲目性，并可以设计硬盘系统的重要参数。本论文的研究内容从超高密度磁记录介质到CPP-GMR器件，再到热辅助磁记录系统（包括磁性记录层，中间层，软磁衬底层，SPT写磁头以及激光光源）。根据具体的研究步骤，论文主要分为以下三个部分：第一部分超高密度磁记录用记录介质的微磁学模拟分析这一部分主要以磁晶各项异性能和磁弹性能为核心对实验室制备出来的磁性能良好的垂直取向L1₀相FePt合金单层膜和多层膜以及水平取向的CoCrPt合金单层膜进行微磁学分析，主要得到以下一些结论：1)在FePt/Pt/CrW单层膜中，随着衬底层中W含量从0增加到15 at．%，CrW合金的晶格常数变大，Cr（200）面和FePt（001）面之间的错配度也相应增大，从5．4％增加到7．2%，这也就表明，在FePt合金薄膜的面内，存在着伸张内应力σ_i。通过应力定性分析，我们得到了FePt单层膜面内总的应力大小为1．079 GPa（包括内应力和外应力），其中沿a轴方向的总应力大小约为763MPa。2)在FePt/Pt/CrW单层膜中，我们通过微磁学分析的方法，研究了四角磁晶各项异性，晶粒内部的交换相互作用以及面内应力对FePt合金单层薄膜M-H回线的影响。3)在Fe/Pt多层膜中，我们对L1₀相FePt合金多层磁性薄膜的磁致回线进行了模拟。和单层膜相比，多层膜中c轴的取向度更好一些，而且晶粒内部和晶粒之间具有较大的交换相互作用。我们模拟得到的垂直方向的矫顽力约为7．8 kOe，略小于面内的矫顽力，这主要是因为四角磁晶各项异性在面内导致了较大的立方各向异性的缘故。4)在Fe/Pt多层膜中，我们通过计算矫顽力与外场和取向间夹角的关系，对Fe/Pt多层膜的反磁化过程进行了简单的研究。研究表明，多层膜表现出与畴壁移动模型相近的变化趋势，这说明它的反磁化过程主要是以畴壁位移的方式进行的。5)此外，我们以六角网格为基本单元，以单轴磁晶各项异性能和磁弹性能为核心，对实验室制备出来的水平取向的CoCrPt合金单层磁性薄膜进行了微磁学分析。微磁学研究表明，当Pt含量超过15 at%时，由应力导致的矫顽力将起主导作用。第二部分超高密度磁记录用CPP-GMR读磁头的微磁学分析这一部分主要致力于对纳米尺度的CPP-GMR器件进行微磁学模拟，主要得到了以下结论：1)为了提供最佳的永磁偏置场，抑制多层膜自由层中磁畴结构的产生，使得自由层的磁矩只随外场转动，水平永磁体顶角θ的最优化值为70°。2)当CPP-GMR磁头的磁迹宽度（track-width）减小到40nm时，在不同的h/w（自由层高度与宽度比）比下，自由层的磁矩在信号场的作用下发生了一致转动，然而，当h/w比大于1时，形状各向异性在磁矩的翻转过程中起到了非常重要的作用，导致了磁阻曲线中磁滞现象和不可逆跳跃的发生，并且恶化了磁头的输出信号。3)研究了CPP-GMR磁头的h/w比值（形状各项异性）和永磁体的Mrd值对MR响应曲线的影响。4)研究了CPP-GMR磁头的磁屏蔽层镜像效应。当软磁屏蔽层间隙G减小到20nm时，要获得良好的读出特性，需要H_pinned≥700Oe和H_syAF≥1000Oe。第三部分热辅助磁记录系统的微磁学模拟这一部分主要通过建立三维的热传导模型和微磁学模型，对记录密度超过1Tb/in²的热辅助磁记录系统进行分析研究，主要得到了以下结论：1)研究发现，记录介质的加热和冷却过程是由磁性层和软磁衬底层之间的热传导（-▽²T）过程决定的，而且和软磁衬底层的热传导系数κ有着非常密切的关系。κ值的增加，加速了HAMR介质内部热量传导的过程，导致了冷却时间的迅速减小，同时，也使得HAMR介质局域温度的最大值(T_max)有所降低。2)研究表明，在记录层上数据位的分布图样不仅与SPT写磁头的形状和写入场的大小有关，而且还与记录介质层对温度的响应有关。对于FePt合金来说，最优化的写入温度为722K，这个温度稍低于FePt合金的居里温度（770K）。值得注意的是，在局域温度高于722K时，HARM介质不能进行数据的写入，因为此时介质中心的饱和磁化强度大小几乎为0．3)在对HARM系统单磁道写入过程的模拟中，SPT磁头和介质之间的相对速度为35m/s，记录位的道宽比（the bit-aspect radio）是2．7，记录密度超过了1Tb/in²．为了进行最佳的数据写入操作，激光加热斑点的中心与SPT磁头主极前后边缘的距离应该小于1．4/41．4nm，这对于热辅助磁记录系统的设计有很重要的指导意义。本论文在对L1₀相FePt合金单层膜和多层膜微磁学分析中取得了一定的进展，首先在传统微磁学模型的基础上，根据晶体的对称性，引入了四角磁晶各项异性能密度的唯象表达形式，紧接着，依据薄膜生长过程中晶格对称性的破坏，在薄膜面内考虑了应力，并且引入了磁弹性能。我们以四角磁晶各项异性能和磁弹性能为核心，对L1₀相FePt合金单层膜和多层膜的磁滞回线作出了很好的解释，并且确定了薄膜面内应力的大小，从而为实验工作者的研究提供了理论依据。此外，我们对CPP-GMR读磁头器件和热辅助磁记录系统的微磁学分析，对它们的优化设计具有很重要的理论指导意义。更多还原

【Abstract】 Nowadays, micromagnetic modeling has become a useful tool in studyingand designing the magnetic devices in hard disk drives for ultra-high densitymagnetic recording, such as the magnetic recording media, the magnetic read andwrite heads and the magnetic recording systerms, etc. In this dissertation, weusing the micromagnetic simulation method for analysing the core magneticdevices in magnetic hard disk drives, including the L1₀ ordered FePt perpendicularrecording media, the current-perpendicular-to-plane giant magnetoresistance readheads, and the heat assisted magnetic recording system. Thus, the dissertation isdivided into three parts:Part 1: Micromagnetic Analysis of The Magnetic Recording Media Used forUltra-High Density RecordingIn this work, a 3D micromagnetic model is set up for the FePt monolayer andmultilayers deposited by magnetic sputtering experimental methods. The basicmagnetic characteristics of the FePt medium is analyzed according to therelationship between the magnetic property and the microstructure, with carefuldiscussions of the tetragonal structure and the magnetostriction. The simulationresults agree well with experiments. The main results are as follows:1) In the FePt/Pt/CrW monolayer, when the W content in the Cr underlayer is increased from 0% to 15%, the misfit between the Cr（200） and FePt （001） planes increases from 5.4% to 7.2%, which suggests that the intrinsic stressσ_i exits in the FePt phase’s film plane. Through the qualitative analysis of the film stress, it is concluded that the total stress in the FePt phase’s film plane is about 1.079 GPa, which contains both the thermal and intrinsic components of stress, and total film stress along the FePt phase’s a-axis is about 763 MPa. 2) In the FePt/Pt/CrW monolayer, we studied the role of tetragonal anisotropy, stress and inter-grainular exchange on the M-H loop properties by using the micromagnetic method.3) In the Fe/Pt Multilayers, the simulated M-H loops agree well with the experiments. It is found that the c-axis deviation from the film normal direction is smaller in Fe/Pt Multilayers, compared with the FePt/Pt/CrW monolayer,however,the monolayer has narrower H_k^a distribution and larger exchange interactions among the magnetic grains and across the grain boundaries, both of which agree with the experiments. The simulated perpendicular coercivity of Fe/Pt Multilayers is about 7.8 kOe, a little smaller than the in-plane coercivity, which is mainly caused by the large in-plane crystalline anisotropy field.4) In the Fe/Pt Multilayers, The coercivity mechanism is analyzed by using the micromagnetic model, it is found that the domall wall motion plays an important role on the coercivity mechanisms.5) Furthermore, a 3D micromagnetic model is built up to analyze the relationship between the magnetic property and the microstructure of CoCrPt thin films, with careful discussions of the uniaxial symmetry and the magnetostriction. It is found that magnetostriction effects play an important role on the coercivity mechanisms when the Pt content is more than 15 at%Part 2: Micromagnetic Modeling For Current-Perpendicular-to-Plane GiantMagnetoresistance Read HeadIn this part, a micromagnetic model is performed to find the propergeometrical and magnetic parameters for current-perpendicular-to-plane giantmagnetoresistance （CPP-GMR） spin-valve heads with suitable longitudinalbiasing schemes at higher recording densities. The main results are as follows: 1) For optimum biasing, at the side edges of the free layer, the biasing field in the film norm direction must be reduced to prevent the multi-domain formation in the free layer. So the optimal value of the pitch angleθfor the PM design calculated here is 70°.2) When the GMR sensor width is reduced to 40nm, the magnetization of the free layer rotates coherently in response to the excitation fields at various height to width ratios. When h/w（height to width ratio）is more than 1, the shape anisotropy plays an important role in the magnetization reversal process, which will cause hysterical and irreversible jumps in the MR transfer curves, and will deteriorate the sensitivity of the head signal output.3) The respective calculated MR transfer curves is studied at various h/w ratios and different Mrδvalue of the abutted permanent magnets with track width W=40 nm.4) We also studied the imaging effect related to the two magnetic shields on the calculated M-H loops of the spin-valve multilayers. When the shield-to-shield gap length G is reduced to 20nm, the H_pinned≥700Oe and H_syAF≥1000Oe are needed for optimal designing of the spin value GMR sensors.Part 3: Micromagnetic Modeling for Heat Assisted Magnetic RecordingHere, a micromagnetic model and a heat transfer model are introduced tostudy the heating and cooling processes in the HAMR media; then, by integrationof the SPT head and the laser heating source, the recording performance issimulated and investigated on a single track at area density of 1Tb/in². The mainresults are as follows:1) The cooling process is dominated by heat conduction that transfers heat from the magnetic layer to the soft underlayer, and increasing the thermal conductivityκof the SUL accelerates the heat transfer process, and then leads to a reduction of the local maximum temperature.2) It is found that the written bit pattern in the recording layer is determined not only by the head geometry and the head field, but by the way that the media respond to the laser. The optimum writing temperature is 722K, which is a little lower than the curie temperature of FePt alloy （770K）. It should also be noted that, at higher temperature above 722K, bit patterns can not be written properly in the magnetic layer, since the saturation magnetization is nearly zero at the center of the medium.3) In the recording process,the head-medium speed is 35m/s, the bit-aspect radio is 2.7, and the area density is beyond 1Tb/in². The distance from laser waveguide to rear/front edge of SPT head should be less than 41.4/1.4nm respectively, for a proper write-in.更多还原