Overseas Travel Grant: XMCD-PEEM study of the spin structure and spin torque in magnetic nanocontacts at the Advanced Light Source (ALS)

本次海外旅行资助申请是为了向EPSRC寻求支持，以支付我们在先进光源（ALS）的光束时间的旅行和住宿费用，以研究磁性纳米接触中的自旋扭矩效应，这是自旋电子学中最激动人心的课题之一。DTI表示，“到2015年，自旋电子学技术将是使电子设备更小、更高效的唯一途径——到那时，现有的半导体和半导体材料，如硅，将耗尽其小型化能力。”[DTI Global Watch杂志，2005年7 / 8月]。利用自旋极化电流而不是外部磁场来开关自旋电子器件是一个日益增长的兴趣。电流感应磁化开关，即众所周知的自旋转矩效应，可以局部开关磁性元件，以避免串扰并降低功耗。由于自旋和动量转移，自旋极化电流可以移动被磁性纳米触点困住的畴壁，或者在gmr型纳米柱结构中切换磁性子层。在过去的几年里，有几篇《自然》和《科学》杂志的论文报道了这种效应的发现及其巨大的潜力，去年11月在佛罗里达州坦帕市举行的第52届磁学和磁性材料年会上，约有150人应邀发表了关于自旋扭矩效应及其应用的演讲。我们获得了美国先进光源（Advanced Light Source， USA）在光束线11.0.1.1处的12次光束时间偏移，利用x射线磁圆二色性光电子显微镜（XMCD-PEEM）研究磁性纳米接触中的自旋结构和自旋扭矩（提案号：ALS-02778，见附件）。我们还获得了一个新的EPSRC项目（EP/G0100641/1），有一名博士生研究这个主题（注意：这个新的EPSRC不包括ALS波束时间的旅行和住宿费用）。11.0.1 PEEM3波束线在ALS上的空间分辨率优于20 nm，提供了一种优越的技术来探测磁性纳米接触中的自旋构型和轨道矩，这有望为理解自旋-扭矩效应和畴壁散射的机制提供独特的信息，这两种效应都依赖于自旋-轨道交换耦合。具体来说，在ALS的这个项目中，我们将测量FeNi磁性纳米触点的自旋构型和自旋和轨道矩，并研究FeNi纳米触点中临界电流密度和畴壁磁阻纳米触点尺寸的依赖关系。对于未来的工作，我们正在申请钻石纳米科学光束线（I06）的光束时间，正如我们新的EPSRC拨款提案所描述的那样。本项目所描述的从ALS的波束时间中获得的经验将有助于钻石的波束线I06在这个新领域开展工作。在这个项目中，除了在ALS中测量的FeNi外，我们还计划研究自旋结构以及自旋和轨道矩作为磁性纳米接触尺寸和金刚石光束线I06中多晶Ni和单晶Fe和Co的薄膜厚度的函数。我们的最终目标是确定合适的材料，晶体结构，几何形状和磁性纳米触点的尺寸，以实现自旋电子学应用的大电流诱导电阻变化和低临界电流密度。

This overseas travel grant application is to seek the support from the EPSRC to cover the travel and accommodation costs of our beam time at the Advanced Light Source (ALS) to study the spin torque effect in magnetic nanocontact, one of the most exciting topics of spintronics. The DTI has stated that, 'by 2015 spintronics technology will be the only way to make electronic devices smaller and more efficient -by then, existing semiconductors and semiconductor materials, like silicon, will have exhausted their capability for miniaturization.' [DTI Global Watch magazine, July /August 2005]. This is a growing interest in the use of spin-polarised current rather than the external magnetic fields to switch the spintronic devices. The current-induced magnetization switching, well known as spin-torque effects, can locally switch a magnetic element to avoid cross-talk and reduce the power consumption. Due to the spin and momentum transfer, the spin-polarised current can move away a domain wall trapped in a magnetic nanocontact, or switch the magnetic sub-layer in a GMR-type nanopillar structure. Over the last few years, there are several Nature and Science papers reporting the discovery of this effect and its great potentials, and during the 52nd Annual Conference on Magnetism and Magnetic Materials last November in Tampa, Florida, there were around 150 invited and contributed presentations on spin-torque effect and its applications. We have been awarded 12 shifts of beamtime at the Beamline 11.0.1.1 at the Advanced Light Source, USA, to study the spin structure and spin torque in magnetic nanocontact using x-ray magnetic circular dichroism in photoemission electron microscopy (XMCD-PEEM) (Proposal Number: ALS-02778, see the attached letter). We have also been awarded a new EPSRC project (EP/G0100641/1) with a PhD student to work on this topic (Note: This new EPSRC doesn't cover the travel and accommodations costs for the ALS beamtime). The 11.0.1 PEEM3 beamline at ALS with a spatial resolution of better than 20 nm offers a superior technique to probe both the spin configuration and orbital moment in magnetic nanocontacts, which is expected to provide unique information to understand the mechanisms of spin-torque effect and domain wall scattering, both depending on the spin-orbital exchange coupling. In specifically during this project in the ALS, we will measuring the spin configurations and spin and orbital moments in FeNi magnetic nanocontacts, and to investigating the dependence of the critical current density and domain wall magneto-resistance nanocontacts sizes in the FeNi nanocontacts.For the future work, we are applying for the beamtime to the Diamond NanoScience beamline (I06), as described in our new EPSRC grant proposal. The experiences to be obtained from our beamtime in ALS as described in this project will help the Diamond's beamline I06 to carry out the work in this new area. Apart from the FeNi to be measured in ALS in this project, we are planning to study both the spin structures and the spin and orbital moments as a function of the magnetic nanocontact size and the film thickness in polycrystalline Ni and single crystal Fe and Co in the Diamond's beamline I06. Our final objective is to determine suitable materials, crystal structures, geometries and sizes of magnetic nanocontacts with the aim to achieve large current-induced resistance changes and low critical current density for spintronics applications.