合成反铁磁结构中磁性斯格明子的研究

【作者】 李勇；

【导师】 马付胜；

【作者基本信息】 南京师范大学 ， 凝聚态物理， 2020， 硕士

【摘要】 近几年来,磁性斯格明子已经成为凝聚态物理领域的热门课题,引起了磁学界和拓扑物理学界研究人员的广泛关注。磁性斯格明子是一种具有拓扑保护的非平庸自旋结构,拓扑保护的稳定性、纳米尺度的几何尺寸以及低电流的可驱动性使得它展现出了作为信息载体的巨大潜力,对下一代信息存储和逻辑器件等自旋电子器件的发展有着重大的意义。要实现基于磁性斯格明子的信息处理,必须能够有效的控制它们的产生和湮灭,既要控制它们出现的位置,又要控制它们的空间排列。但是,目前实验报道的薄膜材料中产生的磁性斯格明子都是随机且杂乱无序排布的,这阻碍了基于磁性斯格明子的赛道存储器和磁子晶体的发展。尽管人工斯格明子可以解决空间排布问题,但是其尺寸受到顶部纳米盘的限制而无法进行调控。本论文提出并实验验证了一种新的人工斯格明子的形成机制,既可以实现磁性斯格明子在空间上的有序排布又可以实现其尺寸的有效调控。本论文利用磁成像、磁光效应、霍尔电阻实验测量和微磁模拟的方法对合成反铁磁多层膜中人工斯格明子的产生机制、可控性和热稳定性等几个方面进行了研究。首先,对合成多层膜的磁化特性进行了研究。实验探究了具有垂直磁各向异性的[Pt/Co]₂/Ru/[Co/Pt]₄多层膜中的层间交换耦合场H_ex和耦合强度J_iec的热稳定性。通过改变非磁性间隔层钌（Ru）的厚度,我们发现多层膜的层间交换耦合强度J_iec对于Ru厚度的变化非常敏感,层间交换耦合强度可以实现从铁磁(J_iec>0)到反铁磁(J_iec<0)的转变。另外,实验测量了温度从100 K到600 K的范围内层间交换耦合场的变化情况并理论计算了相应温度下的层间交换耦合强度。研究发现,[Pt/Co]₂/Ru/[Co/Pt]₄多层膜的层间交换耦合不仅对温度非常敏感,而且具有非常好的热稳定性。其次,利用优化的具有较强的反铁磁层间交换耦合的多层膜结构,我们提出了一种人工斯格明子的成核机制。通过直接的磁力显微镜成像测量,间接的磁光克尔光谱仪磁滞回线测量、间接的电输运霍尔电阻测量以及微磁模拟等方法对这种机制进行了有效的验证。与基于Dzyaloshinskii-Moriya（DM）相互作用的斯格明子不同的是,我们提出的机制依赖于多层膜结构中的反铁磁层间交换耦合场,顶部铁磁层纳米盘的退磁场,以及外磁场之间的相互竞争。这种新机制产生的人工斯格明子不仅是有序可控排布的,而且其尺寸可以通过外加磁场进行有效调节。最后,对基于反铁磁层间交换耦合产生的人工斯格明子机制的鲁棒性进行了研究。通过改变温度（4.5～300 K）、纳米盘直径（400～1200 nm）和纳米盘的空间排布方式,发现这种新机制产生的人工斯格明子对温度、纳米盘的尺寸和空间排布方式都具有较强的鲁棒性。更多还原

【Abstract】 In recent years,magnetic skyrmion has become a hot topic in the field of condensed matter physics,and is attracting extensive attention from researchers in the field of magnetism and topology physics.Magnetic skyrmion is a topologically nontrivial spin structure,it shows great potential as information carrier owing to their topological stability,nanoscale geometry size and smaller driven current density.Magnetic skyrmion is of great significance to the development of the next generation spintronic memory and logic devices.In order to realize the processing of information based on magnetic skyrmion,we must be able to effectively control the creation and annihilation of them,as well as their position and spatial arrangement.However,their existence in ferromagnetically coupled multilayers is spatially disordered arrangement as reported,which hinders the development of skyrmion based racetrack memory and magnonic crystal.Although the artificial skyrmion can solve the problem of spatially ordered arrangement,its size is limited by the physical size of the top nanodisk and cannot be tuned.In this thesis,a nucleation scenario of ordered skyrmions with size tunability in nanostructured synthetic antiferromagnetic（SAF）multilayers is proposed and experimentally verified.Magnetic imaging,magneto-optical Kerr,hall resistance measurement and micromagnetic simulation were used to make a research of the generation mechanism,size tunability and thermal stability of artificial skyrmion in nanostructured SAF multilayers.Firstly,we studied the magnetic properties of synthetic multilayers.We experimentally investigated the thermal stability of the interlayer exchange coupling field H_ex and strength J_iec in SAF[Pt/Co]₂/Ru/[Co/Pt]₄ multilayers with perpendicular magnetic anisotropy.By changing the thickness of the non-magnetic spacing ruthenium（Ru）layer,we find that the J_iec of the multilayer is very sensitive to the thickness of Ru and the property of J_ieccan be transformed from ferromagnetic(J_iec>0)to antiferromagnetic(J_iec<0).In addition,temperature-dependent H_ex was studied by measuring the magnetization hysteresis loops in the temperature range from 100 K to600 K.It is found that the interlayer exchange coupling of the[Pt/Co]₂/Ru/[Co/Pt]₄multilayers is not only very sensitive to temperature but also has very high thermal stability.Secondly,we proposed and experimentally demonstrated a nucleation scenario of ordered artificial skyrmion in nanostructured SAF multilayers.Direct magnetic microscope imaging,indirect magnetization curve measurement by magneto-optic Kerr,indirect measurement of hall resistance of electrical transport and micromagnetic simulation were used to verify this mechanism.Instead of relying on Dzyaloshinskii–Moriya interaction,the proposed mechanism depends on the competition between the antiferromagnetic interlayer exchange coupling field in SAF multilayers,the demagnetizing field of the top nanodisk,and the external magnetic field.The artificial skyrmion produced by this new mechanism is not only ordered but also tunable by external magnetic field.Finally,we investigated the robustness of the proposed artificial skyrmion nucleation scenario.By changing the temperature（4.5～300 K）of experiment environment and the diameter（400～1200 nm）and spatial arrangement of the top nanodisks,we find that the proposed artificial skyrmion nucleation scenario is robust against the temperature and the dimension and the spatial arrangement of the top nanodisks.更多还原