Thermodynamics of nanomagnetic devices driven by spin currents

电子自旋是未来电子设备中传输、处理和存储信息的一种很有前途的介质。在一些被追求的实现中，信息是由磁性材料中的相干自旋动力学通过自旋波来携带的，自旋波可以通过将自旋流注入纳米磁体来产生。众所周知，许多不同的自旋波模式是由自旋电流注入同时产生的，但它们的谱分布或控制它的机制还不是很清楚，这阻碍了实现基于自旋的相干器件操作的进展。拟议的项目将开发新的实验方法，能够表征由自旋流产生的自旋波的光谱分布，并建立控制它的方法。自旋波形成准平衡分布的可能性将被测试，由有效热力学参数-温度和化学势来描述。这将使拟议的研究能够确定在自旋流驱动的纳米磁系统中形成动力学状态的基本机制。由此产生的实现高度相干磁化动力学的能力将有助于实现基于自旋的高效器件的进展。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，通过为普通公众开发教育演示，为埃默里大学蓬勃发展的工程科学学位做出贡献，并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，这将使有效的工程设计和这些特性的优化应用于纳米设备应用。磁光微聚焦布里渊光谱学技术将被用来确定被称为磁子的自旋波量子的光谱分布。为了实现该技术的宽光谱灵敏度，将利用动量空间压缩和等离子体激元效应将探测光集中到深亚波长区。所获得的结果将被用来定量检验自旋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状态的假设。这将使该项目能够建立之前获得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，玻色-爱因斯坦凝聚是一种当化学势等于最低磁振子能量时自发形成的相干动力学状态。通过建立这一关系，将解决许多基本和实际的重要问题，如不同的动力学谱模式在磁系统相干态形成中的作用，非线性磁振子相互作用如何阻止或促进这些态的形成，它们是否可以通过动力学谱工程来控制，是否有可能实现由自旋流驱动的玻色-爱因斯坦凝聚，凝聚体是否可以在磁系统的大体积中形成，或者它总是在纳米尺度范围内定域。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水平，以及为基于自旋的设备应用控制动态磁化状态的能力。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.