FePt基磁性薄膜的超快自旋动力学研究

【作者】 赵佳琦；

【导师】 金庆原；

【作者基本信息】 复旦大学 ， 光学， 2009， 硕士

【摘要】 96年至今,由于超快自旋动力学在基础研究和技术应用上的重要意义,人们对其做了大量的实验和理论研究,试图获悉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。FePt-基磁性薄膜以其优越的磁学特性被认为是下一代超高密度垂直磁记录介质的理想材料。本论文以FePt-基磁性薄膜作为研究对象,用时间分辨的磁光克尔效应研究了其光诱导瞬态退磁和磁化恢复的过程及机制。首先,我们用时间分辨的极向磁光克尔效应研究了高垂直各向异性L10相FePt薄膜的飞秒激光脉冲诱导超快自旋动力学过程,得到了激光泵浦功率、膜厚和生长温度对薄膜的瞬态退磁率和磁性恢复速率的影响,以及这些过程中材料的矫顽力与克尔迥线的演化规律。我们的结论指出,对于中等功率的激光泵浦,由于样品内不均匀FePt颗粒不同的退磁程度,以及激光的热效应引起的颗粒间交换耦合的减弱,高垂直各向异性的L10相FePt薄膜在超快退磁时保留了一个大的矫顽力,这将不利于热辅助磁记录(HAMR)技术的应用。因此,为了降低材料在最大退磁点的剩余矫顽力,实现HAMR技术的真实超快写入,必须使得样品在激光脉冲诱导下达到完全退磁,这可以通过调节泵浦光的激发强度和样品厚度来完成。对于较薄的薄膜在强激发功率下磁化恢复速率较慢的问题,可以通过附加合适的非磁性散热层来解决。其次,我们用时间分辨的纵向和极向磁光克尔效应,研究了铁磁耦合FePt/CoFe双层膜结构在超短激光脉冲激励下的自旋动力学过程。从不同延迟时刻瞬态磁光克尔迥线的分段翻转现象上,发现水平的交换耦合双层膜在超快激光脉冲激发下0-10 ps内出现瞬态的交换解耦合现象,这个过程依赖于泵浦强度、软硬磁层的交换耦合强度,以及覆盖的软磁层厚度,然后在10—20 ps内交换耦合被重新建立。耦合消退和重建过程是由于FePt层界面处的自旋有序度在激光脉冲作用下的热激发和弛豫过程造成的。我们用此对观察到的FePt层溅射和退火温度为425℃、退火时间为150分钟的Corning Glass/Fe0.5-Pt0.5 (6nm)／Co0.8Fe0.2(5,7nm)／Ta(3nm)样品在磁化恢复过程的前0-100 ps中的磁化进动现象提出了解释。在高磁晶各向异性的垂直取向L10-FePt/CoFe耦合的双层膜中,由于其很高的耦合强度,我们并没有发现激光诱导下的解耦合现象。更多还原

【Abstract】 Extensive experimental and theoretical studies have been carried out from 1996 in photo-induced ultrafast spin dynamics because of its importance in both science and technology. People are trying to investigate transient spin switch process in magnetic material and detect its speed limit. FePt-based magnetic material is one of the main candidates for the next generational ultra-high density perpendicular magnetic recording because of its outstanding magnetic features. Its laser-induced transient demagnetization and magnetic recovery process is studied in the dissertation by time-resolved magnetic optical Kerr effect (TRMOKE) of different geometry.Firstly, Laser-induced ultrafast magnetization dynamics in perpendicular magnetized L10-FePt films are investigated using time-resolved polar magneto-optical Kerr technique. We observed the coercivity and the transient loop evolution at different delay time. We demonstrate that there is a pronounced laser power and film thickness effect on magnetic softening and magnetization recovery rate. Our results indicate that due to the nonuniform demagnetization of the weakly coupled FePt grains upon photoexcitation, the hard FePt films hold a residual large coercivity that probably affects the desirable HAMR writing. We propose that it is necessary in the real ultrafast writing to achieve a full demagnetization in order to eliminate the residual large coercivity, by means of controlling the film thickness and laser energy. Additionally, the problem of slow magnetization recovery happened in thinner film pumped by high power laser could be solved by adding suitable nonmagnetic heat sink layers.Secondly, photo-induced magnetization dynamics in FePt/CoFe bilayer is investigated by Longitudinal-and Polar-TRMOKE. The transient Kerr loops of the in-plane coupled bilayers in 0-10 ps exhibit an instantaneous separated switching behavior indicating the decoupling between FePt and CoFe layers induced by photo-excitation. This process is related to the pump fluence, the coupling strength between the hard/soft interface, and the thickness of the soft layer. In the subsequent 10-20 ps, the loops recover to the original uniform switching, the coupling between the bilayer is rebuilt. The decoupling phenomena is due to the laser-created spin disordering mainly in FePt layer, while the following onset of interfacial coupling results from spin/lattice cooling process. We use this model to explain the observed magnetization precesional motion during the first 0-100 ps of magnetization recovery process in the Corning Glass/Feo.s-Pto.s (6 nm)/Co0.8Fe0.2(5,7nm)/Ta(3nm) sample whose FePt layer is sputtered at 425℃and annealed at the same temperature for 150 min. In the high magnetocrystalline anisotropy perpendicular magnetized L10-FePt/CoFe bilayers, because of its high coupling strength, we did not observed the later-induced decoupling phenomenon.更多还原