介质微结构与未来磁记录方式中记录过程的微磁学研究
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摘要
在2004-2006年间，计算机硬盘中的数据记录方式逐渐由水平磁记录方式转向垂直磁记录方式。可是随着记录密度的进一步提高，垂直磁记录方式也在接近超顺磁性的限制，因此，对下一代磁记录方式的研究势在必行。能量辅助磁记录、比特图形磁记录和叠层瓦片式磁记录是记录密度有望超过1Tbit/in2的新型磁记录方式。这三种磁记录方式对介质的磁性参数和微结构的要求是各不相同的，相应的磁记录过程也有较大的差异。因此，在综合考虑介质的热稳定性、信噪比和可写入性的前提下，本文研究了比特图形磁记录和叠层瓦片式磁记录所采用的介质、记录过程及密度极限，同时还分析了能量辅助磁记录所使用的L10FePt介质中的孪晶结构对磁记录性能的影响。
本文首先从实验中观察到的纳米颗粒中的五重孪晶结构出发，建立了一个解析的模型来理解孪晶结构的存在对磁晶各向异性场的影响。计算结果表明，孪晶结构的存在会大幅降低颗粒的磁晶各向异性场，同时造成磁晶各向异性场的分布，这会降低介质的热稳定性，增大反转场的分布宽度，降低信噪比。这个问题的研究对理解磁晶各向异性场的分布有普遍的意义。
使用双颗粒模型对交换耦合（ECC）介质的反转特性进行了研究，分析了介质磁性参数与反转场的关系，为设计热稳定性和可写入性都良好的ECC介质提供了指导。随后，分别对垂直薄膜介质和ECC介质进行了叠层瓦片式磁记录过程的模拟，统计了不同记录密度下的信噪比，发现ECC介质具有更高的信噪比和密度极限，同时还对ECC介质的磁性参数作了进一步的优化。
在对光刻法制备的比特图形介质（BPM）的磁记录过程的模拟中，研究了实现写入同步的时间窗口和对磁道偏移量的容忍度，发现降低由介质非均匀性造成的磁晶各向异性场的涨落有利于比特的正确写入。在对离子辐照法制备的BPM介质的磁记录模拟过程中发现，大幅降低比特之间被辐照区域的磁性质、对各个比特进行磁性隔离是实现正确写入的关键。通过优化系统参数和磁性参数，对不同密度的BPM介质的磁记录结果进行分析，可以预测离子辐照法制备的BPM介质的记录密度极限大约是3.0Tbit/in~2。
During2004to2006，the industry of hard disk drive switched from longitudinalmagnetic recording (LMR) to perpendicular magnetic recording (PMR). However, withthe continual increase of areal density, PMR will approach its superparamagnetic limit.The study of new magnetic recording modes is imperative. Energy assisted magneticrecording (EAMR), bit patterned magnetic recording (BPMR) and Shingled magneticrecording (SMR) are the three recording modes to achieve the density above1Tbit/in2.The three magnetic recording modes need different media designs with differentmagnetic parameters and microstructures, and their own recording processes are alsodifferent. In this thesis, under comprehensive consideration of thermal stability, signalto noise ratio (SNR) and writeability, the media, recording process and density limit ofBPMR and SMR are studied. The effect of twin structure in L10FePt medium used byEAMR on magnetic recording properties is also analyzed.
Based on the measured five-fold twin structure in L10FePt nanoparticle, ananalytical model is established to study the effect of twin structure on the anisotropyfield of nanoparticle. The calculation indicates that the existence of twin structure willconsiderably reduce the anisotropy field and cause a wide distribution of the anisotropyfield, which will decrease the thermal stability of FePt medium, broaden the switchingfield distribution, and decrease SNR. This study has general significance on the Hkdistribution in HDD media.
A Two-particle model is used to study the switching properties of exchangecoupled composites (ECC) media, and analyze the relationship between the switchingfield and the media magnetic parameters. The results will help the design of ECC mediawhich has both good thermal stability and writeabilty. Then, shingled magneticrecording is carried out on the conventional thin film media and ECC media,respectively. SNR of both media at different recording densities is studied, and it isfound that ECC media has a higher SNR and a higher density limit. The magneticparameters of ECC media are further optimized to attain a higher SNR.
The recording processes in BPM fabricated by lithography or by ion irradiation aresimulated. In the lithography prepared BPM, the time window for the correct written-inand the tolerance of the offset margin can be attained. We find that the distribution of anisotropy field caused by the inhomogenity of thin film can deteriorate the correctwritten-in and should be reduced for optimum performance. For the recording processof BPM fabricated by ion irradiation, it is recognized that the significant reduction ofmagnetic properties in the irradiated area and the magnetic isolation among bits are thekeys to realize the correct written-in. Through optimizing the system and magneticparameters of BPMR, the recording density limit of BPM fabricated by ion irradiation ispredicted to be3.0Tbit/in~2.
本文首先从实验中观察到的纳米颗粒中的五重孪晶结构出发，建立了一个解析的模型来理解孪晶结构的存在对磁晶各向异性场的影响。计算结果表明，孪晶结构的存在会大幅降低颗粒的磁晶各向异性场，同时造成磁晶各向异性场的分布，这会降低介质的热稳定性，增大反转场的分布宽度，降低信噪比。这个问题的研究对理解磁晶各向异性场的分布有普遍的意义。
使用双颗粒模型对交换耦合（ECC）介质的反转特性进行了研究，分析了介质磁性参数与反转场的关系，为设计热稳定性和可写入性都良好的ECC介质提供了指导。随后，分别对垂直薄膜介质和ECC介质进行了叠层瓦片式磁记录过程的模拟，统计了不同记录密度下的信噪比，发现ECC介质具有更高的信噪比和密度极限，同时还对ECC介质的磁性参数作了进一步的优化。
在对光刻法制备的比特图形介质（BPM）的磁记录过程的模拟中，研究了实现写入同步的时间窗口和对磁道偏移量的容忍度，发现降低由介质非均匀性造成的磁晶各向异性场的涨落有利于比特的正确写入。在对离子辐照法制备的BPM介质的磁记录模拟过程中发现，大幅降低比特之间被辐照区域的磁性质、对各个比特进行磁性隔离是实现正确写入的关键。通过优化系统参数和磁性参数，对不同密度的BPM介质的磁记录结果进行分析，可以预测离子辐照法制备的BPM介质的记录密度极限大约是3.0Tbit/in~2。
During2004to2006，the industry of hard disk drive switched from longitudinalmagnetic recording (LMR) to perpendicular magnetic recording (PMR). However, withthe continual increase of areal density, PMR will approach its superparamagnetic limit.The study of new magnetic recording modes is imperative. Energy assisted magneticrecording (EAMR), bit patterned magnetic recording (BPMR) and Shingled magneticrecording (SMR) are the three recording modes to achieve the density above1Tbit/in2.The three magnetic recording modes need different media designs with differentmagnetic parameters and microstructures, and their own recording processes are alsodifferent. In this thesis, under comprehensive consideration of thermal stability, signalto noise ratio (SNR) and writeability, the media, recording process and density limit ofBPMR and SMR are studied. The effect of twin structure in L10FePt medium used byEAMR on magnetic recording properties is also analyzed.
Based on the measured five-fold twin structure in L10FePt nanoparticle, ananalytical model is established to study the effect of twin structure on the anisotropyfield of nanoparticle. The calculation indicates that the existence of twin structure willconsiderably reduce the anisotropy field and cause a wide distribution of the anisotropyfield, which will decrease the thermal stability of FePt medium, broaden the switchingfield distribution, and decrease SNR. This study has general significance on the Hkdistribution in HDD media.
A Two-particle model is used to study the switching properties of exchangecoupled composites (ECC) media, and analyze the relationship between the switchingfield and the media magnetic parameters. The results will help the design of ECC mediawhich has both good thermal stability and writeabilty. Then, shingled magneticrecording is carried out on the conventional thin film media and ECC media,respectively. SNR of both media at different recording densities is studied, and it isfound that ECC media has a higher SNR and a higher density limit. The magneticparameters of ECC media are further optimized to attain a higher SNR.
The recording processes in BPM fabricated by lithography or by ion irradiation aresimulated. In the lithography prepared BPM, the time window for the correct written-inand the tolerance of the offset margin can be attained. We find that the distribution of anisotropy field caused by the inhomogenity of thin film can deteriorate the correctwritten-in and should be reduced for optimum performance. For the recording processof BPM fabricated by ion irradiation, it is recognized that the significant reduction ofmagnetic properties in the irradiated area and the magnetic isolation among bits are thekeys to realize the correct written-in. Through optimizing the system and magneticparameters of BPMR, the recording density limit of BPM fabricated by ion irradiation ispredicted to be3.0Tbit/in~2.
引文
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