垂直磁记录介质的制备和超快自旋动力学研究

【作者】 徐振；

【导师】 周仕明；

【作者基本信息】 复旦大学 ， 凝聚态物理， 2009， 博士

【摘要】 人类社会步入信息化时代后,信息总量不断增大,传输速度越来越快。硬盘是信息存储的最佳介质,人们对其存储密度和信息处理速度不断提出更高的要求。垂直磁记录作为这两年工业界的新星受到了广泛的关注。为了真正实现每平方英寸1Tbit的存储密度,人们需要不断进行材料创新和开发新的记录方式来满足大众的需求。基于磁光克尔效应的时间分辨泵浦—探测技术可以用来研究磁性材料自旋瞬态翻转的特性和磁性体系中磁化翻转的速度极限,对磁记录工业应用如热辅助磁记录有重要的意义。为了提高信噪比,磁记录介质必须是颗粒膜,而对其超快自旋动力学过程的研究还没有报道。本论文就是在上述背景下展开研究的。本论文主要包含以下几个内容:第一,我们研究了两种不同织构的种子层和热处理对CoCrPt垂直磁记录介质的影响,得到了性能较优的CoCrPt垂直磁记录介质。Cu和Cr两种种子层材料的运用较直接生长在玻璃上CoCrPt的矫顽力有很大的提高。而两类材料得到的CoCrPt介质的成核场和磁化翻转方式不同,以Cr为种子层的材料成核场为负,磁化翻转方式倾向于畴壁位移;以Cu为种子层的材料成核场为正,磁化翻转方式倾向于一致转动。我们分别从退火时间、生长温度和种子层厚度三个方面进行优化分析。第二,我们研究了L1₀ FePt-CoCrPt垂直交换耦合体系。CoCrPt颗粒膜能有效地降低FePt垂直磁记录介质的写入场。当软磁层厚度小于一个临界值时,双层薄膜表现为刚性磁体行为。当软磁层厚度大于临界厚度时,双层膜表现出弹性磁体行为。弹性磁体的磁化翻转过程分为三个阶段,首先是软磁的可逆磁化翻转,形成N（?）el壁,然后N（?）el壁被挤压推向硬磁,最后硬磁形成多畴以Bloch畴壁位移的方式进行反磁化以降低系统畴壁能。第三,我们研究了L1₀ FePt颗粒膜的自旋动力学和瞬态磁软化过程。当泵浦光激发薄膜的时候,样品发生了磁软化过程。反射率、克尔角、矫顽力,矩形度,磁化翻转方式都经历了快速变化随后缓慢恢复的过程。三个样品的热磁响应按强弱顺序依次为:FePt-MgO颗粒膜＞FePt单层＞FePt-Ag颗粒膜。FePt-Ag颗粒膜比FePt单层的热退磁效果弱主要是因为Ag提供了大量的自由电子分走了入射的pump光能量,所以FePt吸收到的能量就小。而FePt-MgO颗粒膜中由于存在表面等离子共振,电子的温度更高,热磁响应就更大,磁软化的效果就更佳。更多还原

【Abstract】 With the development of information age,the volume of information is consistently increasing very fast,and the transmission speed is also accelerating very fast.Hard disk recording is the best storage medium,so people are keeping asking for higher areal density and faster information processing speed.Perpendicular magnetic recording has been a super star in the hard disk industry for the past two years,and it receives widespread concern.In order to truly realize 1Tbit per square inch storage density,it requires constant innovation and development of new materials to meet the needs of the public.Magneto-optical Kerr effect with time-resolved pump-probe technique can be used to study the transient reversal of the magnetic spin and to explore the limit of magnetization reversal speed in magnetic systems.It has great impact on the magnetic recording industrial such as heat-assisted magnetic recording（HAMR）.In order to improve the signal to noise ratio,magnetic recording media must be of granular configuration.However,ultra-fast spin dynamics research on magnetic granular film has not yet been reported.This research work was carried out under the above background.The dissertation mainly includes the following contents.Firstly,we carried research on effect of seed layer texture and heat treatment on CoCrPt perpendicular magnetic recording media.We got optimized parameter of making better CoCrPt media.Both Cu and Cr seed layer materials are observed to greatly improve the coercivity of CoCrPt media,compared with samples grown on bare glass.These two types of materials brings to different nucleation field of the CoCrPt media.Nucleation field of sample with Cr seed layer is negative,which is due to the domain wall displacement dominated magnetization reversal.Nucleation field of sample with Cu seed layer is positive,which is due to the collective rotation dominated magnetization reversal.We finally optimized the sample through three aspects:annealing time,growth temperature and seed layer thickness.Secondly,we studied the L1₀ FePt-CoCrPt perpendicular exchange coupled system.CoCrPt granular soft layer is very effective in reducing the writing field of FePt hard layer.When the thickness of soft-magnetic layer is smaller than a critical value,the bilayer will act as a rigid magnet.When the thickness of soft layer becomes larger,the bilayer will act as an exchange spring media.For exchange spring media, the magnetization reversal process is divided into three stages.It begins with a reversible magnetization reversal in the soft layer and forms a N（?）el domain wall. Then,the domain wall is squeezed to the hard layer.Finally,multi-domain is formed in the hard layer to reduce the total energy of the system.Thirdly,we studied the spin dynamics and transient magnetic softening in L1₀ FePt granular films.After the pump excitation,the reflectivity,Kerr angle,coercivity, rectangular degree and the magnetization reversal mode all undergo rapid change followed by the subsequent slow recovery.The thermal response strength of three samples goes like:FePt-MgO granular films＞FePt single layer＞FePt-Ag granular films.The thermal demagnetization in FePt-Ag granular films is smaller than FePt single layer.It is mainly due to allocation of incident pump energy by the free electrons in Ag,and the energy absorbed by FePt becomes smaller.What is more important is that FePt-MgO granular films have greater magnetic thermal response.It is due to the existence of surface plasmon resonance which brings to higher electron temperature,and consequently better magnet softening effect.更多还原