Thermodynamics of nanomagnetic devices driven by spin currents

在未来的电子设备中，电子自旋是一种很有前途的传输、处理和存储信息的介质。在一些所追求的实现中，信息通过自旋波在磁性材料中传递，自旋波可以通过向纳米磁铁中注入自旋电流产生。自旋电流注入可以同时产生多种不同的自旋波模式，但其谱分布及其控制机制尚不清楚，这阻碍了自旋器件相干操作的实现。本计划将发展新的实验方法，以表征自旋电流产生的自旋波的光谱分布，并建立控制自旋电流的方法。由有效热力学参数-温度和化学势描述的自旋波形成准平衡分布的可能性将被测试。这将使所提出的研究能够确定由自旋电流驱动的纳米磁系统中动态状态形成的基本机制。由此产生的实现高相干磁化动力学的能力将有助于实现高效自旋基器件的进展。该项目将通过为新的动手实验材料科学课程开发培训模块，为埃默里大学迅速发展的工程科学学位做出贡献，并通过为公众开发教育演示，为非常成功的亚特兰大科学节做出贡献。该项目的主要目标是建立由自旋电流驱动的纳米磁系统的动力学和热力学特性之间的关系，这将使纳米器件应用的这些特性的有效工程和优化成为可能。磁光微焦点布里渊光谱学技术将用于确定自旋波量子的光谱分布，即磁振子。为了实现该技术的广谱灵敏度，将利用动量空间压缩和等离子体效应将探测光集中到深亚波长区域。所得结果将用于定量验证自旋电流驱动的纳米磁系统可以形成以化学势和温度等有效热力学参数为特征的准平衡态的假设。这将使该项目能够建立先前实现的自旋电流诱导的相干动力学和玻色-爱因斯坦凝聚之间的关系，当化学势等于最低的磁振子能量时，一种自发形成的相干动力学状态。通过建立这种关系，将解决许多基本和实际重要的问题，例如不同的动力谱模式在磁系统形成相干态中的作用，非线性磁振子-磁振子相互作用如何阻止或促进这些态的形成，它们是否可以通过工程动力谱来控制？是否有可能实现由自旋电流驱动的玻色-爱因斯坦凝聚，以及这种凝聚是否可以在大体积的磁系统中形成，或者它是否总是局限于纳米级区域。通过解决这些问题，将实现对动态磁化状态的前所未有的理解，以及控制它们用于基于自旋的器件应用的能力。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Electron spin is a promising medium for the transmission, processing, and storage of information in future electronic devices. In some of the pursued implementations, information is carried by coherent spin dynamics in magnetic materials through the spin waves, which can be generated by injecting spin current into nanomagnets. It is now well known that many different spin wave modes are simultaneously produced by spin current injection, but their spectral distribution, or the mechanisms controlling it, are not well known, hindering the progress in achieving coherent spin-based device operation. The proposed Project will develop new experimental approaches enabling the characterization of spectral distribution of spin waves generated by spin current, and establish the methods to control it. The possibility that the spin waves form a quasi-equilibrium distribution, described by the effective thermodynamic parameters - temperature and chemical potential will be tested. This will allow the proposed research to identify the fundamental mechanisms underlying the formation of dynamical states in nanomagnetic systems driven by spin currents. The resulting ability to achieve highly coherent magnetization dynamics will contribute to the progress in the implementation of efficient spin-based devices. The project will contribute to the burgeoning Engineering Sciences degree at Emory University, by developing training modules for the new hands-on experimental Materials Science course, and to the highly successful Atlanta Science Festival, by developing educational demos for the general public.The main goal of the Project is to establish the relation between the dynamical and the thermodynamic characteristics of nanomagnetic systems driven by spin current, which will enable efficient engineering and optimization of these characteristics for nanodevice applications. Magneto-optical micro-focus Brillouin Light spectroscopy technique will be utilized to determine the spectral distribution of spin wave quanta known as the magnons. To achieve a broad spectral sensitivity of the technique, momentum-space squeezing and plasmonic effects will be utilized to concentrate the probing light into deep sub-wavelength regions. The obtained results will be used to quantitatively test the hypothesis that nanomagnetic systems driven by spin current can form a quasi-equilibrium state characterized by the effective thermodynamic parameters such as chemical potential and temperature. This will allow the project to establish the relationship between the previously achieved coherent spin current-induced dynamics and Bose-Einstein condensation, a coherent dynamical state spontaneously formed when the chemical potential becomes equal to the lowest magnon energy. By establishing this relation, number of fundamentally and practically important questions will be addressed, such as the role of different dynamical spectral modes play in a magnetic systems in the formation of coherent states, what nonlinear magnon-magnon interactions prevent or facilitate the formation of these states, can they be controlled by engineering the dynamical spectrum, is there a possibility to achieve Bose-Einstein condensation driven by spin current and can the condensate be formed in a large volume of the magnetic system, or is it always localized in nanoscale regions. By addressing these questions, an unprecedented level of understanding of dynamical magnetization states, and the ability to control them for spin-based device applications, will be achieved.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.