【作者】 任杨；

【导师】 金庆原；

【作者基本信息】 复旦大学 ， 光学， 2008， 博士

【摘要】 由于超快自旋动力学在基础研究和技术应用上的重要性,人们做了大量的实验和理论研究。然而,超快退磁过程的机制到目前为止还没有研究清楚。基于磁光克尔效应的时间分辨的泵浦—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。本论文选择了几个特殊的磁性系统作为研究对象,深入研究了瞬态退磁过程和磁化恢复过程的机制。本论文主要包含以下几个内容:第一,我们研究了TbFeCo和TbFe合金薄膜的超快自旋动力学过程。首先,对于特定的TbFeCo薄膜,在低泵浦能量下,退磁只有一个阶段,退磁时间约为600飞秒。退磁时间增加随着泵浦能量的增加而增加,动力学退磁过程逐渐分为两个阶段,即快退磁阶段和慢退磁阶段。其次,在特定的泵浦能量下(1.0 mJ/cm~2),对于含Tb量低的样品,其瞬态退磁过程中只有快退磁阶段,并且退磁过程在600飞秒内完成。当Tb含量为21%时,快退磁阶段(持续时间约为600飞秒)结束后,出现慢退磁阶段,且慢退磁阶段的退磁时间达到最大值3.0皮秒。我们在TbFe薄膜中也发现相同的现象。最后,我们还研究对比了TbFeCo薄膜制备态与退火后的超快退磁过程,发现退火后退磁时间减小,这是由于退火可以减小磁致伸缩系数。结果表明,TbFeCo的巨磁致伸缩效应至少是造成退磁过程产生第二个慢退磁阶段和长退磁时间的重要原因之一。第二,我们研究了不同散热层(包括Ag,Cu,Pt,Ta和Cr)的FeCo薄膜的超快自旋动力学过程。首先,对于不同散热层材料和厚度的样品,瞬态退磁过程都是在大约500飞秒以内完成,所以退磁时间与散热层和泵浦能量无关。其次,对于单层的FeCo薄膜,在延迟时间为4.0皮秒时的磁化强度与最大退磁量的比值(简称磁化强度的恢复率)比有散热层样品的降低了约15%。我们还发现磁化强度的恢复率与散热层材料密切相关,且随着泵浦能量的增加而降低。在我们研究的散热层材料中,Cr散热层的样品具有最大的磁化强度恢复率,这是由于FeCo层和Cr层晶格失配度很小,从而促成了FeCo层在散热层上柱状生长,因此减小了电子和声子在界面的散射,并且增强了热传导效率。经过分析,我们还发现在皮秒量级时间内的磁化强度恢复率与散热层的厚度和热导率无直接关系。最后,我们研究了超快激光的损伤阈值,发现散热层的熔点越高,其损伤阈值越高。在以上研究的所有样品中,Ta具有最高的激光损伤阈值。实验中还发现退火也能提高散热层的热传导效率及激光损伤阈值。第三,我们选择FePt/CoFe复合双层膜结构为研究体系,研究其超快自旋动力学过程。实验中,我们发现瞬态克尔迴线出现两个翻转阶段,这是由于激光瞬态热效应破坏了FePt层与CoFe层之间的界面交换耦合,随着延迟时间增加到10-20皮秒,磁化翻转又出现类似于单层膜的一致转动。通过分析FePt与CoFe时间相关的磁化强度的比值,我们推断解耦合以及恢复缓慢的磁化强度归因于光诱导FePt层的自旋有序度的下降,并且界面交换耦合的重新建立归因于自旋—晶格弛豫过程。在实验中,我们还得到了研究光调制退耦合现象较合适的软磁层厚度。更多还原

【Abstract】 Extensive experimental and theoretical studies have been carried out in photo-induced ultrafast spin dynamics because of its importance in both science and technology.However,the mechanism of the ultrafast demagnetization process has not been clarified yet.An optical pump probe with MOKE(magnetic optical Kerr effect) capability is used to investigate the ultra-fast demagnetization phenomenon.In the research work here,several magnetic systems are chosen to study ultrafast spin dynamics.The dissertation mainly includes the following contents.Firstly,ultrafast spin dynamics in TbFeCo and TbFe alloy films is studied as a function of the Tb content and pumping fluence.For specific TbFeCo film with low fluences,the ultrafast demagnetization process only consists of one fast stage with the demagnetization time of about 500 fs.As the pumping fluence is enhanced,a slow stage occurs,in addition to the fast stage,and the demagnetization time is increased. At a fixed pumping fluence of 1.0 mJ/cm~2,only the fast stage occurs and the demagnetization time is as short as about 500 fs for low Tb contents.At 21%Tb,the fast and slow stages occur and the demagnetization time reaches a maximum of 3.0 ps. Similar results are also observed for TbFe film.The long demagnetization time is suggested to arise from the giant magnetostriction effect in Tb-based fills.After post-annealing,the demagnetization time is reduced due to a reduction of the magnetostriction coefficient.Secondly,for FeCo alloy thin films with Ag,Cu,Pt,Ta,and Cr as heat sink layers,ultrafast demagnetization and recovery processes of transient magnetization have been studied by time-resolved magneto-optical Kerr effect.For all samples,the ultrafast demagnetization process is accomplished within almost the same time interval of 500 fs,which is independent of materials of heat sink layers and pump fluences.With heat sink layers,the recovery rate is enhanced,compared with that of FeCo grown on Si(100) substrate.It is also found that the recovery rate is independent of heat sink layer thickness and decreases with increasing pump fluence.Among all heat sink layers,the sample with Cr layer achieves the highest recovery rate because of the same BCC structure as FeCo layer and small lattice mismatch.For sample with Ta layer,it has the largest damage threshold of pump fluence because of the highest melting point.Thirdly,photo-induced magnetization dynamics in FePt/CoFe bilayer is investigated by optical pump-probe measurements.The transient Kerr loops exhibit an instantaneous separated switching behavior due to exchange decoupling between FePt and CoFe layers induced by photoexcitation,which subsequently recover to the original uniform switching at a certain delay time of 10-20 picoseconds.By analyzing the temporal dependence of magnetization ratio of FePt to CoFe and coercivity of FePt phase,we infer that the decoupling and slow magnetization recovery phenomena are ascribed to the laser-created spin disordering mainly in FePt layer,while the following onset of interfacial coupling results from spin/lattice cooling process.In the study,we also found the proper thickness of CoFe layer to investigate transient decoupling phenomena.更多还原