Visualizing spin torque in nanoscale magnetic devices using ultrafast x-ray microscopy

自旋动量转移是纳米级磁性器件中研究最为活跃的现象之一。人们普遍认为，这种现象对于提高下一代磁性随机存取存储器器件的密度以及实现用于信号处理的片上微波振荡器至关重要。在亚微米柱的多层异质结构中，超薄膜磁化对短脉冲（皮秒到纳秒）自旋极化电流的响应决定了器件性能。智力优势：在提出的研究中，将通过基于同步加速器的频闪x射线透射显微镜和磁强计来获得对自旋扭矩器件的磁化动力学和操作的新见解。x射线对金属的穿透深度，结合Fe， Co和Ni边缘的磁性圆二色性吸收，使得埋层磁化选择性可达；泵-探针（频闪）测量，与射频驱动场同步，阐明其动态。否则，通过软x射线的短波长和聚焦光学的使用，可以获得40纳米至15纳米的空间分辨率；提高时间分辨率（至2皮秒）和磁对比度的创新技术已经在PI及其同事之前的无图案薄膜上得到了证明，并将应用于纳米结构的研究中。然后，自旋扭矩的影响将在皮秒时间尺度和~40纳米长度尺度上可视化，允许器件设计人员在实际器件中“拍摄”器件开关和进动磁化动力学中的操作。技术目标包括澄清自由层和固定层动力学，首次成像自旋波在远离点接触的地方传播，以及纳米柱阵列中的锁相。观测结果可以直接与详细的微磁模拟进行比较，首次使用观测到的不均匀性作为输入。更广泛的影响：拟议的研究将为两名博士生提供独特而广泛的研究生培训经验，他们将有机会在一家初创公司和国家实验室现场工作。研究活动将与本地K-12学生的外展活动紧密结合；哥伦比亚大学将提供50%的费用来支持一名研究生研究助理，该助理将积极参与海登K-12外展项目的维护，为来自哈莱姆区的学生提供“科学周六”的丰富内容。拟议的活动将加强国家实验室的研究基础设施，为磁化动力学研究开发新的基于同步加速器的仪器。通过提供对新型磁电产品的理解和协助开发，可以为社会带来利益。

Spin momentum transfer is one of the most actively studied phenomena in nanoscale magnetic devices. This phenomenon is widely believed to be critical for enhancing the density of a next generation of magnetic random access memory devices, as well as for the realization of future on-chip microwave oscillators for signal processing. The response of ultrathin film magnetization in multilayer heterostructures, patterned into submicron pillars, to short-pulse (picosecond to nanosecond) spin polarized currents, determines the device performance. Intellectual merits:In the proposed research, new insight into the magnetization dynamics and operation of spin torque devices will be gained through synchrotron-based, stroboscopic x-ray transmission microscopy and magnetometry. The penetration depth of x-rays into metals, combined with the magnetic circular dichroism of absorption at Fe, Co, and Ni edges, makes buried layer magnetization selectively accessible; pump-probe (stroboscopic) measurement, synchronized with radio frequency driving fields, illuminates their dynamics. Otherwise-unobtainable spatial resolution of 40 nanometers, down to 15 nanometers, can be attained through the short wavelengths of soft x-rays and the use of focusing optics; innovative techniques to enhance both temporal resolution (to 2 picoseconds) and magnetic contrast have been demonstrated in prior work by the PI and coworkers on unpatterned films, and will be applied in the proposed research on nanostructures. The effects of spin torque will then be visualized at picosecond time scales and ~40 nanometer length scales, allowing device designers to "take movies" of operation, both in device switching and precessional magnetization dynamics, in real devices. Technical goals include the clarification of free and fixed layer dynamics, the first imaging of spin wave propagation away from a point contact, and of phase locking in arrays of nanopillars. Observed results can be compared directly with detailed micromagnetic simulations, using observed inhomogeneity as an input for the first time. Broader impacts: The proposed research will provide a unique and broad graduate training experience for two Ph.D. students, who will have the opportunity to work on site at both a startup company and national laboratories. Research activities will be closely integrated with outreach activities to local K-12 students; Columbia will provide a 50% cost match for the support of one graduate research assistant who will be actively involved in the maintenance of the Hayden K-12 outreach program, with "Science Saturdays" enrichment for students typically drawn from Harlem. The proposed activities will enhance research infrastructure at national laboratories, developing novel synchrotron-based instrumentation for the study of magnetization dynamics. Benefits to society are possible by providing understanding of, and assisting in the development of, novel magnetoelectronic products.