软磁层各向异性对ECC磁记录介质性质的影响
摘要
随着磁记录的不断发展,记录密度逐渐提高,即将接近垂直磁记录的物理极限。交换耦合复合磁记录介质(Exchange coupled composite recording media,简称ECC media)作为一种新型的垂直磁记录介质,在具有较低的矫顽力的同时能保持较高的热稳定性,因而受到了研究者的广泛关注。ECC介质由交换耦合的软磁层与硬磁层构成,它是通过软磁层与硬磁层的磁矩在磁化反转过程中的非一致转动来达到降低矫顽力的效果,而无外场时热反转仍为一致转动,因而能保持较高的热稳定性。ECC介质中软磁层具有较高的饱和磁化强度和较大的退磁能,然而在以往的研究中软磁层的各向异性对介质性质的影响还没有系统的研究过。
本论文中使用理论计算的方法研究了双磁矩模型中软磁层各向异性和饱和磁化强度的变化对介质反转场的影响。研究发现软磁层的磁各向异性能为负时,即易磁化方向平行于记录薄膜的膜面时,有助于降低复合颗粒的反转场以及反转场对软磁层和硬磁层之间的交换耦合强度的敏感性,并使最佳耦合强度增大,然而也会导致增益(Gain)的降低。假设软磁层的磁各向异性能等于其最大退磁能,我们发现具有较大饱和磁化强度的软磁层除了能够保有负的各向异性能带来的好处之外,还会使Gain增大。反转场对外加磁场角度的依赖关系在软磁层各向异性能为负时不会受到明显的影响,而为正时则受影响较大。研究结果说明当软磁层的退磁能无法避免时,使用较大饱和磁化强度的软磁层有利于降低矫顽力,保持热稳定性,提高介质的均匀性,减小中间层和记录层的厚度,并且对信噪比没有不良影响。
在实验研究方面,我们使用磁控溅射装置制备了记录层结构为软磁/中间层/硬磁的垂直磁记录薄膜。薄膜的具体结构为C/Co-TiO_2/Pt/CoPt-TiO_2/Ru/Pt/CoZrNb/ glass-disk。通过调节溅射气压,成功的改变了软磁层的饱和磁化强度。对于强耦合,即没有Pt中间层的介质,发现Co-TiO_2软磁层能够有效降低矫顽力,矫顽力最低降至单层介质的60%左右。热稳定性由于面内交换耦合的增强而略有上升,同时磁畴结构没有明显变化。对于有Pt中间层的ECC介质,发现Pt能够有效降低软磁层和硬磁层之间的交换耦合强度,从而使矫顽力降低,并且矫顽力对其厚度的敏感度不高。同时还发现软磁层的饱和磁化强度越大,达到最佳耦合状态所需要的Pt中间层厚度越小。连续的Pt中间层使面内交换耦合增强,从而能够提高热稳定性,然而也会导致磁畴尺寸增大。对比单层介质,强耦合介质以及ECC介质的剩磁矫顽力随外场角度的变化关系,发现单层介质与强耦合介质的矫顽力随外场角度的变化趋势基本一致,而ECC介质对角度的敏感性更低。当外场偏转45°时,剩磁矫顽力约为0°时的85%左右。通过观测加反转场后剩磁态MFM图像,发现ECC介质的面内晶粒间交换耦合比较弱,并且磁化反转过程为磁性颗粒的反转而并非畴壁移动。实验结果说明我们得到的ECC介质具有良好的性质,Pt很适合作为ECC介质的中间层使用,而使用饱和磁化强度较大的软磁层更有助于降低记录层的总厚度。
The astonishing advancement of magnetic recording has brought the recordingdensity close to its physical limit.Thus,a new perpendicular magnetic recordingmedia called exchange coupled composite media (ECC media) has attracted muchattention since it can decrease the coercivity while maintaining the thermal stability.The ECC media,consisting of exchange-coupled hard and soft magnetic layers,canreach smaller coercivity through incoherent reversal of the magnetic moments of thetwo layers in an applied field,whereas the thermal stability can be maintained due tothe coherent reversal without field.The soft layer of ECC media usually possesseshigh saturation magnetization (M_s) and also high demagnetizing energy density.However,the effect of the anisotropy of soft layer has not yet been investigatedsystemically.
In this thesis the effect of the anisotropy of soft layer on the switching field ofECC media was investigated by calculation using a two-moment model.It was foundthe minus anisotropy energy of soft layer is useful to the decrease of switching fieldand the sensitivity of switching field to the exchange strength between soft and hardlayers.It can also increase the optimal exchange strength,but decrease the highestGain.Assuming the anisotropy energy of soft layer is equal to its highestdemagnetizing energy,it was found the application of high Ms soft magnetic materialas the soft layer may improve the best Gain while maintaining the benefits caused byminus anisotropy.The dependence of H_(sw) on field angle is not affected very muchwhen the anisotropy of soft layer is minus,but worsened by plus soft layer anisotropy.These results suggest that when the demagnetizing energy of soft layer is unavoidablethe application of high Ms soft layer can help to decrease the switching field,maintainthe thermal stability,improve the uniformity,reduce the thickness of interlayer andtotal recording layer,and will not worsen the signal to noise ratio.
In experimental investigation,we prepared films with the recoding layerstructure of soft/interlayer/hard by magnetron sputtering.The structure of films isC/Co-TiO_2/Pt/CoPt-TiO_2/Ru/Pt/CoZrNb/ glass-disk.By changing the sputtering gas pressure,the M_s of soft layer is adjusted successfully.For the films without a Ptinterlayer,it was found Co-TiO_2 soft layers reduce the coercivity efficiently and thelowest coercivity is about 60% of that of single recording layer.The thermal stabilityis enhanced by the strengthened intergranular exchange coupling,but the domainstructure is not affected obviously.For the films with a Pt interlayer,it was found thePt interlayer can reduce the exchange strength between soft and hard layers efficiently,so that the coercivity is decreased.And the coercivity has a small sensitivity to thethickness of Pt interlayer.It was also found that the soft layer with a high M_s is usefulto reduce the interlayer thickness of the optimal state,which can decrease thethickness of ECC media.The continuous Pt interlayer strengthened intergranularexchange coupling,that lead to the improvement of thermal stability and also theaverage domain size.Comparing to the the single recording layer film and strongexchange coupled film,the ECC film has much smaller sensitivity of coercivity tofield angle.When the field angle is 45°,coercivity decreases to about 85% of thatobtained at the angle of 0°.By measuring the remanent state MFM images of optimalECC film,it was realized that the intergranular exchange coupling is not strong,andthe reversal process of media is not the domain wall movement but the reverse ofmagnetic clusters.The experimental results suggest that the prepared ECC mediapossess favorable properties,Pt is appropriate for the interlayer of ECC media,andthe application of high M_s soft layer is useful to the reduction of recording layer.
本论文中使用理论计算的方法研究了双磁矩模型中软磁层各向异性和饱和磁化强度的变化对介质反转场的影响。研究发现软磁层的磁各向异性能为负时,即易磁化方向平行于记录薄膜的膜面时,有助于降低复合颗粒的反转场以及反转场对软磁层和硬磁层之间的交换耦合强度的敏感性,并使最佳耦合强度增大,然而也会导致增益(Gain)的降低。假设软磁层的磁各向异性能等于其最大退磁能,我们发现具有较大饱和磁化强度的软磁层除了能够保有负的各向异性能带来的好处之外,还会使Gain增大。反转场对外加磁场角度的依赖关系在软磁层各向异性能为负时不会受到明显的影响,而为正时则受影响较大。研究结果说明当软磁层的退磁能无法避免时,使用较大饱和磁化强度的软磁层有利于降低矫顽力,保持热稳定性,提高介质的均匀性,减小中间层和记录层的厚度,并且对信噪比没有不良影响。
在实验研究方面,我们使用磁控溅射装置制备了记录层结构为软磁/中间层/硬磁的垂直磁记录薄膜。薄膜的具体结构为C/Co-TiO_2/Pt/CoPt-TiO_2/Ru/Pt/CoZrNb/ glass-disk。通过调节溅射气压,成功的改变了软磁层的饱和磁化强度。对于强耦合,即没有Pt中间层的介质,发现Co-TiO_2软磁层能够有效降低矫顽力,矫顽力最低降至单层介质的60%左右。热稳定性由于面内交换耦合的增强而略有上升,同时磁畴结构没有明显变化。对于有Pt中间层的ECC介质,发现Pt能够有效降低软磁层和硬磁层之间的交换耦合强度,从而使矫顽力降低,并且矫顽力对其厚度的敏感度不高。同时还发现软磁层的饱和磁化强度越大,达到最佳耦合状态所需要的Pt中间层厚度越小。连续的Pt中间层使面内交换耦合增强,从而能够提高热稳定性,然而也会导致磁畴尺寸增大。对比单层介质,强耦合介质以及ECC介质的剩磁矫顽力随外场角度的变化关系,发现单层介质与强耦合介质的矫顽力随外场角度的变化趋势基本一致,而ECC介质对角度的敏感性更低。当外场偏转45°时,剩磁矫顽力约为0°时的85%左右。通过观测加反转场后剩磁态MFM图像,发现ECC介质的面内晶粒间交换耦合比较弱,并且磁化反转过程为磁性颗粒的反转而并非畴壁移动。实验结果说明我们得到的ECC介质具有良好的性质,Pt很适合作为ECC介质的中间层使用,而使用饱和磁化强度较大的软磁层更有助于降低记录层的总厚度。
The astonishing advancement of magnetic recording has brought the recordingdensity close to its physical limit.Thus,a new perpendicular magnetic recordingmedia called exchange coupled composite media (ECC media) has attracted muchattention since it can decrease the coercivity while maintaining the thermal stability.The ECC media,consisting of exchange-coupled hard and soft magnetic layers,canreach smaller coercivity through incoherent reversal of the magnetic moments of thetwo layers in an applied field,whereas the thermal stability can be maintained due tothe coherent reversal without field.The soft layer of ECC media usually possesseshigh saturation magnetization (M_s) and also high demagnetizing energy density.However,the effect of the anisotropy of soft layer has not yet been investigatedsystemically.
In this thesis the effect of the anisotropy of soft layer on the switching field ofECC media was investigated by calculation using a two-moment model.It was foundthe minus anisotropy energy of soft layer is useful to the decrease of switching fieldand the sensitivity of switching field to the exchange strength between soft and hardlayers.It can also increase the optimal exchange strength,but decrease the highestGain.Assuming the anisotropy energy of soft layer is equal to its highestdemagnetizing energy,it was found the application of high Ms soft magnetic materialas the soft layer may improve the best Gain while maintaining the benefits caused byminus anisotropy.The dependence of H_(sw) on field angle is not affected very muchwhen the anisotropy of soft layer is minus,but worsened by plus soft layer anisotropy.These results suggest that when the demagnetizing energy of soft layer is unavoidablethe application of high Ms soft layer can help to decrease the switching field,maintainthe thermal stability,improve the uniformity,reduce the thickness of interlayer andtotal recording layer,and will not worsen the signal to noise ratio.
In experimental investigation,we prepared films with the recoding layerstructure of soft/interlayer/hard by magnetron sputtering.The structure of films isC/Co-TiO_2/Pt/CoPt-TiO_2/Ru/Pt/CoZrNb/ glass-disk.By changing the sputtering gas pressure,the M_s of soft layer is adjusted successfully.For the films without a Ptinterlayer,it was found Co-TiO_2 soft layers reduce the coercivity efficiently and thelowest coercivity is about 60% of that of single recording layer.The thermal stabilityis enhanced by the strengthened intergranular exchange coupling,but the domainstructure is not affected obviously.For the films with a Pt interlayer,it was found thePt interlayer can reduce the exchange strength between soft and hard layers efficiently,so that the coercivity is decreased.And the coercivity has a small sensitivity to thethickness of Pt interlayer.It was also found that the soft layer with a high M_s is usefulto reduce the interlayer thickness of the optimal state,which can decrease thethickness of ECC media.The continuous Pt interlayer strengthened intergranularexchange coupling,that lead to the improvement of thermal stability and also theaverage domain size.Comparing to the the single recording layer film and strongexchange coupled film,the ECC film has much smaller sensitivity of coercivity tofield angle.When the field angle is 45°,coercivity decreases to about 85% of thatobtained at the angle of 0°.By measuring the remanent state MFM images of optimalECC film,it was realized that the intergranular exchange coupling is not strong,andthe reversal process of media is not the domain wall movement but the reverse ofmagnetic clusters.The experimental results suggest that the prepared ECC mediapossess favorable properties,Pt is appropriate for the interlayer of ECC media,andthe application of high M_s soft layer is useful to the reduction of recording layer.
引文
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[1]周瑞发，韩雅芳，陈祥宝，《纳米材料技术》，国防工业出版社，2003。
[2]杨邦朝，王文生，《薄膜物理与技术》，电子科技大学出版社，1994。
[3]陈光华，邓金祥，《纳米薄膜技术与应用》，化学工业出版社，2004。
[4]黄兰友，刘绪平，《电子显微镜与电子光学》，1991。
[5]赵家政，徐涛，分析电子显微镜使用手册，宁夏人民出版社，1996。
[6]李树棠，晶体X射线衍射学基础，冶金工业出版社，1990。
[7]张建中，杨传铮，晶体的射线衍射基础，南京大学出版社，1992。
[8]李培森，《物性测量原理与测试分析方法》，兰州大学出版社，1994。
[9]周文生，《磁性测量原理》，电子工业出版社，1988。
[10]Model 2900 MicroMagTM Instruction Manual．
[11]韩宝善，磁力显微镜的发展历史、原理和应用，理化检验-物理分册，34，24(1998)。
[12]刘公强，乐志强，沈德芳，《磁光学》，上海科学技术出版社，2001。
[13]马云贵博士学位论文，兰州大学，2005。
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[2]N．Honda， K．Ouchi， Digest of the ISAMMA2007 in Jeju， Korea， 115 (2007)．
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[4]H．J．Richter， and A．Yu．Dobin， J．Appl．Phys．99 08Q905 (2006)．
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[2]J．P．Wang， W．K．Shen， J．M．Bai， IEEE Trans．Magn．41 3181 (2005)．
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[7]M．P．Sharrock， J．Appl．Phys．76， 6413 (1994)．