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

自旋动量转移是纳米磁性器件中研究最活跃的现象之一。 这种现象被广泛认为是关键的，以提高下一代的磁性随机存取存储器器件的密度，以及为实现未来的芯片上的微波振荡器的信号处理。 在多层异质结构中，图案化成亚微米柱，短脉冲（皮秒至纳秒）的自旋极化电流的响应，确定了器件的性能。智力优势：在拟议的研究中，将通过基于同步加速器的频闪X射线透射显微镜和磁力计获得对磁化动力学和自旋扭矩器件操作的新见解。 X射线进入金属的穿透深度，结合在Fe，Co和Ni边缘吸收的磁性圆二色性，使埋层磁化选择性地访问;泵浦-探测（频闪）测量，与射频驱动场同步，照亮它们的动态。 通过短波长的软X射线和使用聚焦光学器件，可以获得40纳米的空间分辨率，低至15纳米; PI和同事在未图案化薄膜上的先前工作中已经证明了提高时间分辨率（至2皮秒）和磁对比度的创新技术，并将应用于拟议的纳米结构研究。 然后，自旋扭矩的影响将在皮秒时间尺度和~40纳米长度尺度下可视化，允许设备设计者在真实的设备中在设备切换和旋进磁化动力学中“拍摄操作电影”。技术目标包括澄清自由和固定层动力学，自旋波传播远离点接触的第一个成像，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：拟议的研究将提供一个独特的和广泛的研究生培训经验，两个博士。学生，他们将有机会在创业公司和国家实验室现场工作。研究活动将与当地K-12学生的外展活动紧密结合;哥伦比亚将为一名研究生研究助理的支持提供50%的成本匹配，该研究生研究助理将积极参与海登K-12外展计划的维护，并为通常来自哈莱姆的学生提供“科学星期六”的充实。拟议的活动将加强国家实验室的研究基础设施，开发用于磁化动力学研究的新型同步加速器仪器。通过提供对新型磁电子产品的理解和协助开发，可以为社会带来好处。

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.