【作者】 李凡；

【导师】 潘峰；

【作者基本信息】 清华大学 ， 材料科学与工程， 2018， 博士

【摘要】 在信息技术向着高速、高密度、低功耗方向发展的过程中,对多功能材料与器件的需求急剧增长。锰氧化物,因具有多种微观自由度,为探寻新颖的磁电性能提供了宽广的舞台,推动了自旋电子学材料与器件的发展。与体系中多种多样的物理现象相比,锰氧化物磁电性能的调控方式仍然有限,探寻磁电性能调控的新机制、丰富磁电性能调控思路与技术,成为推进氧化物自旋电子学的多功能化进程所面临的关键科学问题。本工作以磁性锰氧化物薄膜及其异质结为对象,系统研究了由LED提供的可见光对自旋输运特性的影响,提出基于半金属电子结构的光控磁新机制,构建出具有显著光学响应的新型自旋电子学器件,并基于氧八面体旋转这一晶格自由度发展出磁性交换耦合与磁各向异性调控的新手段。研究发现光照下La_1/2Sr_1/2MnO_3-δ（LSMO）的磁电阻会发生显著变化,表明光照使LSMO饱和磁化场增大而磁各向异性减弱,基于电子在光致跃迁过程发生的自旋态转变实现了对半金属磁性的调控;通过磁电阻器件部分区域曝光的设计制备了一类光栅极自旋晶体管,观测到光照下器件电阻的瞬时增大,基于光致跃迁产生的态密度不连续性实现了光照对自旋输运的调控,展现了光学手段的瞬时调控能力。利用氧化还原调节了LSMO的化学价态,改变了LSMO光激发下的自旋极化,使光致跃迁下的自旋态转变增强,将光栅极自旋晶体管的光致电阻变化由0.3%提高到60%,光学响应的大幅提升为推动其应用提供了途径。通过分析理想G型反铁磁/铁磁界面氧八面旋转与交换耦合的关联,提出氧八面体旋转是该体系出现交换偏置的关键,通过基片选择和薄膜厚度设计调节了SrMnO₃/La_2/3Sr_1/3MnO₃界面附近的应力状态,改变了氧八面体旋转的类型与程度,基于对氧八面体旋转的控制定性与半定量地调控了交换偏置。研究表明通过基于氧八面体旋转的磁性交换耦合,面内易磁化的较厚的La_2/3Sr_1/3MnO₃能诱导较薄的SrCoO_2.5产生转向面外的反铁磁磁矩,反过来,面内易磁化的较厚的SrCoO_2.5能诱导较薄的La_2/3Sr_1/3MnO₃出现垂直易磁化趋势,利用氧八面体旋转诱导的磁矩垂直排列状态实现了对磁矩排布的操控,为探索氧八面体旋转的多功能化应用提供了新的视角。更多还原

【Abstract】 With the development of information technology towards high speed,high density and low consumption,the demands for multifuctional materials and devices are rapidly increasing.Manganites demonstrate large amounts of novel and interesting physics behavior due to various degrees of freedom,which provides a large stage for design of magnetic and electric properties and promotes the advances in spintronic materials and devices.In comparison to the rich physics phenomena,the methods for manipulation of magnetic and electric behavior are still limited.Thus,how to explore new mechanisms and techiniques for the manipulation of magnetic and electric behavior is the key problem during the development of spintronics towards multifunctional applications.This dissertation focuses on manganite-based thin films and heterostructures and systematically studies the influence of light on spin transport properties via the light illumination of LED.A new mechanism of optical control of mangetism is proposed on the basis of the half-metallic electronic structure.Followingly,a novel type of spintronic devices with remarkable optical response is proposed and achieved.Meanwhile,on the basis of oxygen octahedral tilting,new methods are developed for the control of exchange coupling and magnetic anisotropy.A remarkable variation in magnetoresistance of La_1/2Sr_1/2MnO_3-δ（LSMO）is observed under light illumination,indicating an increased saturation field and a weakened magnetic anisotropy in LSMO induced by light.Then it’s proposed that based on the photon-induced spin transition,the magnetism in half-metallic systems can be modulated.Followingly,a type of photon-gated spin transistor is proposed and designed with partial area of the device under light exposure,where a sudden increase of resistance is observed under light illumination.In this way,opitcal control of spin transport is achieved taking advantage of the discontinuity of DOS caused by the photon-induced spin transition,demonstrating the transient advantage of optical control.The chemical valence of LSMO is altered via redox,accompanied by the variation of the spin polarization at the excited state,which leads to an enhancement of the photon-induced spin transition.Thus,the photon-induced resistance change of the photon-gated spin transistor is increased from 0.3%to60%,offering a possible way for the application of this type of spintronic devices.According to the analysis about the relationship between octahedral tilting and exchange coupling at the ideal G-type antiferromagnet/ferromagnet interface,it’s proposed that oxygen octahedral tilting is the key to the formation of exchange bias at the interface.Then the pattern and scale of octahdral tilting at the interface of SrMnO₃/La_2/3Sr_1/3MnO₃ are modulated via substrate selection and thickness design,consequently achieving a qualitative and semi-quantitative control of exchange bias via the control of octahedral tilting.It is observed that taking advantage of the exchange coupling due to octahedral tilting,the antiferromagnetic moments in thin SrCoO_2.5 turn to the out-of-plane direction when the ferromagnetic moments in thick La_2/3Sr_1/3MnO₃align in plane.In turn,the in-plane alignment of antiferromagnetic moments in thick SrCoO_2.5 results in a tendency of perpendicular magnetocrystalline anisotropy in thin La_2/3Sr_1/3MnO₃.In this way,utilizing the perpendicular alignment of magnetic moments induced by octahedral titling,the alignment of magnetic moments is manipulated,offering a novel way to explore the multifunctional applications of oxygen octahedral tilting.更多还原