【作者】 王康；

【导师】 吴小山；

【作者基本信息】 南京大学 ， 物理学， 2019， 博士

【摘要】 时间分辨实验技术的发展,使得人们能够在纳秒、皮秒甚至更短的时间尺度上探究新型功能材料中电子态、晶格结构、以及磁基态等在不同泵浦脉冲源激发下的瞬态响应。实验分辨技术与数值模拟相结合,不仅让我们有机会从微观的角度去了解材料的宏观物性,也为我们提供了一个研究平台,用以探索功能材料在未来信息功能器件中的潜在应用。在本文中,我们利用不同的时间分辨实验方法,包括激光脉冲泵浦-X射线脉冲探测(超快X射线衍射)、激光脉冲泵浦-激光脉冲探测、以及纯自旋流脉冲泵浦-静态探测的实验方法,在不同的材料体系,包括强关联的钌酸锶(SrRuO3)薄膜材料和重金属/铁磁多层膜体系中,研究电子、声子和自旋等自由度在不同泵浦脉冲激发后的瞬态响应,取得一些创新性成果:(1)理论与实验结合,获悉高品质SrRuO3薄膜在脉冲激光激发下电子、声子耦合的动力学行为。SrRuO3由于其巡游电子特性、高温下的非费米液体行为等特殊的电磁物性而受到广泛关注。在SrRuO3薄膜材料中,光激发可以产生大于1%的光致应力和光电阻变化,为应用于未来光机械器件和光电子器件中提供可能。但在SrRuO3薄膜中,光致应力的产生并不完全来自于亚皮秒时间尺度上光激发电子与声子之间能量转移所产生的热致应力,更多的电子与声子在光激发下的动力学行为仍有待研究。我们通过超快X射线衍射实验和超快光学反射率测量的实验方法,结合第一性原理计算和数值模拟研究了在光脉冲激发下SrRuO3薄膜中电子的激发与弛豫、应力的产生和弛豫过程。我们发现电子与声子除了在亚皮秒时间尺度上相互作用,在亚纳秒时间尺度上也存在相互耦合,与热致应力共同贡献了光致应力的产生。我们研究了温度对光致应力弛豫过程的影响。随着温度的升高,电子与声子之间的相互作用增强。我们讨论了电子态对光激发载流子弛豫过程的影响。在SrRuO3中,光激发载流子的弛豫不能通过费米液体理论进行描述,表明了在室温下SrRuO3薄膜的坏金属性。(2)理论与实验结合,以Ta/CoFeB/MgO为例,研究了重金属/铁磁多层膜体系在纯自旋流驱动下的动力学行为,获悉了拓扑缺陷运动与磁畴壁内部自旋极化的翻转对磁畴结构相变的影响。为了降低磁存储器件中“0”态和“1”态之间的翻转势垒,具有垂直磁各向异性的重金属/铁磁多层膜材料受到越来越多的关注与研究。在从垂直磁各向异性到面内磁各向异性的转变过程中,会有多畴结构的产生。其中,多畴结构中的磁斯格明子以及磁拓扑缺陷,由于其不为零的拓扑电荷密度,在纯自旋流脉冲作用下,存在垂直于自旋流驱动力方向的运动。我们在实验上通过纯自旋流脉冲泵浦-静态极化磁光克尔显微镜(polar magneto optical Kerr effect microscope,P-MOKE microscope)探测的实验方法对在纯自旋流驱动下磁畴结构的相变进行探究,探索磁拓扑缺陷在纯自旋流脉冲驱动下对磁畴结构相变的影响。实验结果表明,初始无序的磁畴结构可以向有序的磁畴结构、或斯格明子态发生转变。不同的末态磁畴结构取决于施加的自旋流密度以及垂直磁场大小。最后通过微磁学模拟研究了拓扑缺陷运动与磁畴壁内部自旋极化的翻转对磁畴结构相变的影响。(3)通过微磁学模拟提出了一个具有实验可行性的三端输运轨道模型,实现对斯格明子存储信息的修改与调控。传统的磁存储器件在向高密度存储的发展中受限于量子效应遇到了一定瓶颈。相应地,轨道存储器件的概念应运而生。在未来基于斯格明子的新型轨道存储器件中,“1”态和“0”态分别由存在和不存在一个斯格明子进行表示。因此,在磁存储器件中,写入、删除一个斯格明子、以及对一串斯格明子信息的有效调控受到了越来越多的关注。我们通过微磁学模拟提出了一个在实验上可实现的三端输运轨道模型。在三端输运轨道模型中,一串斯格明子信息在产生端(generator)写入,然后在纯自旋流脉冲作用下,传输到控制端(controller),通过控制端门电压实现“1”态与“0”态之间的相互转换,实现对斯格明子信息的修改与调控。修改后的斯格明子信息在收集端(controller)探测得到。我们讨论了三端输运轨道模型在实验上的可实施性。最后理论研究了斯格明子的呼吸模(breathing mode)和回旋振荡模(gyration motion),以及这两种振荡模之间的相互耦合。我们发现两种振荡模之间的耦合可以通过Thiele方程进行描述。更多还原

【Abstract】 Transient changes in electronic states,lattice and magnetic structures of functional materials,can be revealed by well-developed pump-probe techniques.The time resolution of pump-probe techniques can reach picosecond,and even faster.This leads it to be possible to gain microscopic understandings on macroscopic properties that cannot be fully understood through static studies,and to explore potential applications of functional materials in devices.In this thesis,we study dynamics of electron and phonon in SrRuO3,and evolutions of chiral magnetic structures in heavy metal/ferromagnetic multilayer stacks,through different pump-probe techniques.The main contents of the thesis are summarized as follows:(1)SrRuO3 attracts a lot of attention because of its typical properties including itinerant ferromagnetism and non-Fermi liquid behavior at high temperature,etc.Photo-induced strain above 1%has been observed in SrRuO3 films with the optical excitation intensity above 60 W cm-2,suggesting the great potential of SrRuO3 in applications of optomechanical and optoelectronic devices.Photoexcitation and energy transfer via electron-phonon scattering have been studied,but recent studies inferred that more processes that remain to be identified contribute to the light-induced deformation.We image excitation and relaxation characteristics of SrRuO3,through two complementary methods that are ultrafast X-ray diffraction and ultrafast optical reflectivity measurements.Upon photoexcitation,photo-carriers redistribute both in space and energy,and then relaxation with fast direct recombination,phonon-assisted relaxation and phonon-phonon scattering dominating on a longer time scale.The phonon-assisted relaxation process,together with the thermal effect resulting from the electron-phonon scattering,contributes to the photo-induced strain in SrRuO3.The bottleneck effects of phonons on relaxations are proposed and demonstrated by temperature dependent measurements.The fast carrier relaxatio supports a scaled behavior depending on the electron correlation.The scaled relationship for SrRuO3 shows a divergence from that derived from the Fermi-liquid model,giving a hint that phonon effect occurs in the bad-metal region of SrRuO3.(2)The heavy metal/ferromagnetic multilayer system with perpendicular magnetic anisotropy(PMA)has been attracting much attention in past few years.In the vicinity of the spin reorientation transition from the perpendicular magnetization to in-plane magnetization,labyrinthine phases appear,wherein multiple topological defects and magnetic skyrmions are present.Because of the non-zero topological charge densities of topological defects,we expect their typical responses to the spin current,therefore causing transitions of magnetic domain phases.Using the polar magneto optical Kerr effect microscope,we observe in a prototypical multilayer system of Ta/CoFeB/MgO a spin current-driven transformation of an initial labyrinthine domain pattern.The labyrinthine domain phase can either transform into a dense array of skyrmions,or simply has a change in the orientational order,depending on the strengths of the applied spin current pulse and the external magnetic field.We conclude both from experimental studies and micromagnetic simulations that the applied spin current leads to the generation/deletion of topological defects through modifying the motion of domain walls and the magnetic configurations within the domain walls,leading to the observed transformations.(3)A train of skyrmions information with codes "1"/"0" defined by existence/absence of a single skyrmion has been proposed to be digital information carriers in spintronic devices.However,correcting or revising skyrmions information in a controlled manner has not been studied so far as we know,which is of great significance and high-demand for future skyrmion-based data storage and memory systems.In our work,we propose and demonstrate through micromagnetic simulations studies an effective and experimentally feasible method to modify a train of skyrmions information by a low spin polarized current.We use a "generator" to write skyrmions information,a "collector"to detect it,and a "controller" to correct a single skyrmion.In studying the skyrmion manipulation processes,we have observed and further described the skyrmion’s gyrating motion(mode)and its breathing mode.The dynamics is analysed via a Thiele equation derived from the micromagnetic model.From the equation,the gyration mode of a skyrmion is suggested to be coupled to the breathing mode dynamics.更多还原