Probing spin torque and domain wall scattering with XPEEM and transport measurements

2007年诺贝尔物理学奖庆祝1988年在磁性多层膜中发现了巨磁电阻(GMR)，这导致了自旋电子学一个令人兴奋的新领域。GMR磁头将磁数据存储密度提高了20倍以上。自旋电子学预计还将对微电子、汽车传感器、通信和量子计算产生重大影响，其方式可与50年前晶体管的发展相媲美。DTI表示，到2015年，自旋电子技术将是使电子设备变得更小、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，将耗尽它们的小型化能力。[《DTI全球观察》杂志，2005年7月/8月。然而，使用外部磁场切换磁体的传统方法有几个缺点，最重要的是串扰和高功耗。这引起了人们对使用自旋极化电流而不是外部磁场来切换自旋电子器件的日益增长的兴趣--这是继GMR效应之后自旋电子学的又一重大发现。电流感应磁化切换，即众所周知的自旋-扭矩效应，可以局部切换磁性元件，避免串扰，降低功耗。由于自旋和动量的转移，自旋极化电流可以移开捕获在磁性纳米接触中的磁畴壁，或者在GMR类型的纳米柱结构中切换磁性亚层。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及其巨大的潜力的发现，去年11月在佛罗里达州坦帕市举行的第52届磁性和磁性材料年会期间，大约有150名受邀和投稿的专家就自旋-扭矩效应及其应用发表了演讲。然而，有许多基本和具有挑战性的问题，包括临界电流的机制，动量转移和自旋转移的贡献，轨道矩的影响，以及自旋-轨道耦合对自旋相关散射的作用，这些问题不是单一的实验技术能完全解决的。在这个项目中，我们打算通过探索同步辐射和基于实验室的测量技术来研究磁性纳米接触。约克自旋电子学团队在磁性纳米接触和同步辐射技术方面拥有国际领先的专家，能够很好地开展这项工作。约克的自旋电子学小组一直是SRS达累斯伯里实验室的定期用户，并与联合博士生一起使用。该大学最近投资约500万美元建立了约克纳米制造和分析中心，拥有最先进的纳米制造设施。在这个项目中，我们将利用新建立的钻石纳米科学光束线I06，利用光电子显微镜(XPEEM)中的X射线磁性圆二色谱来探测自旋扭矩和磁化壁散射。XPEEM技术是非侵入性的，并提供磁化方向的直接成像，与传统的磁力显微镜相比具有几个优势。更重要的是，XPEEM技术能够探测磁性纳米接触中的自旋构型以及自旋和轨道矩，并将提供独特的信息来理解自旋-扭矩效应和磁畴壁散射的机制，这两者都依赖于自旋-轨道交换耦合。通过与约克的输运测量相结合，该项目将首次在实验上探索轨道矩、临界电流和磁畴壁磁阻之间的关系，这将对理解自旋-扭矩效应和磁性纳米接触的迷人物理产生重大影响。

The 2007 Nobel Prize in Physics celebrated the discovery of giant magneto-resistance (GMR) in magnetic multilayers in 1988, which lead to an exciting new area of spintronics. The GMR heads have increased the magnetic data storage density by more than 20 times. Spintronics is also expected to have a major impact on microelectronics, automotive sensors, communication and quantum computing in a way comparable to the development of the transistor 50 years ago. 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]. There are, however, several drawbacks associated with the conventional approach of switching a magnet using external magnetic fields, and the most important being cross-talk and high power consumption. This has generated a growing interest in the use of spin-polarised current rather than the external magnetic fields to switch the spintronic devices - another major discovery in spintronics after the GMR effect. 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. There are, however, many fundamental and challenging issues including the mechanism of the critical current and the contribution of momentum transfer and spin-transfer, the effect of the orbital moment, and role of the spin-orbital coupling on the spin dependent scattering and these issues can not fully addressed with a single experimental technique. In this project, we propose to study the magnetic nanocontact by exploring both Synchrotron Radiation and laboratory based measurement techniques. York spintronics team is well positioned to carry out this work with their internationally leading expertises in both magnetic nanocontact and Synchrotron Radiation techniques. The York's Spintronics group had been a regular user of the SRS Daresbury Laboratory with joint PhD students. The university have recently invested about 5M in establishing the York Center of Nanofabrication and Analysis with the state-of-the-art facilities for nanofabrication. In this project, by taking advantages of the newly established Diamond NanoScience beamline I06, we will probe the spin torque and domain wall scattering using x-ray magnetic circular dichroism in photoemission electron microscopy (XPEEM). Being non-intrusive and providing direct imaging of the magnetization orientations, the XPEEM technique has several advantages over the conventional magnetic force microscope. More importantly, the XPEEM technique is capable of probing both the spin configuration and spin and orbital moments in magnetic nanocontacts and will provide unique information to understand the mechanisms of spin-torque effect and domain wall scattering, both depending on the spin-orbital exchange coupling. By combining with the transport measurements in York, this project will thus explore experimentally for the first time the correlation between the orbital moments, the critical current and the domain wall magneto-resistance, which will have a major impact on the understanding of the fascinating physics of the spin-torque effect and magnetic nanocontacts.