Long-distance spin transport in disordered insulators and low-damping metals

摘要：集体磁化激发自旋输运的研究是凝聚态物理学的一个重要新前沿。事实证明，磁性绝缘体、金属或碳纳米材料中的自旋输运通过自由度而不是简单地由传播的电子携带，是富含潜在技术创新的肥沃基础。这些领域的新发现为未来的纳米电子模型提供了信息，这些模型有朝一日可能会取代当前的技术。例如，由无序的磁性绝缘体薄膜形成的自旋信息管道可以使材料集成变得容易，并在未来的全自旋计算机处理器中实现必要的互连。类似地，低阻尼铁磁体中的大自旋波效应可以提供新的方法来产生驱动这样一个处理器所需的自旋电流。优化的碳纳米管网络薄膜提供了自旋传输应用的潜力，但如果更好地了解其基本的热学和热电物理，则可以将废热转化为可用能量。这个项目探索了这些潜在的变革性进展的基础科学。技术摘要：本项目的重点是测量三类材料之间的自旋和热输运(及其相互作用)：无序磁性绝缘体、低阻尼铁磁性金属和单壁碳纳米管网络。其中每一项都探索了新的、潜在的变革性方向。对无序磁绝缘体中自旋输运的研究是在初步结果的基础上进行的，结果表明，由自旋霍尔效应驱动并通过逆自旋霍尔效应探测到的自旋输运，在存在反铁磁关联的情况下，但在没有磁性和结构长程有序的情况下，是令人惊讶的有效的。这一领域的具体任务包括：1)非晶态Cr2O_3和NiO的初始测试，2)距离和几何关系的详细表征，3)非晶态YIG的退火效应，4)热梯度相关性的理解，5)等待时间和老化实验，以及6)直接测试无序磁性薄膜中的非平衡自旋热导率。对低阻尼金属中磁振子驱动的自旋和热输运以及热电效应的探索主要集中在Co-Fe合金系统上，并受到潜在的新的自旋热电子效应的推动。这里的具体任务包括：1)使用“标准”的Zink Lab热绝缘平台进行研究；2)表征AMR、MTEP、PNE和ANE；以及3)使用电子束光刻定义的热绝缘结构来探索磁非驱动的自旋流。最后，研究碳纳米材料中的自旋和热输运利用了高度可调的单壁纳米管网络薄膜的制备和研究方面的专业知识。这里的具体任务包括：1)研究Wiedemann-Franz“破坏”和热导率与掺杂的依赖关系，2)设计更敏感的热隔离平台，以及3)使用非局域自旋阀几何结构测试网络中的自旋输运。

Non-Technical Abstract:The study of spin transport by excitation of collective magnetization is an important new frontier in condensed matter physics. Spin transport in magnetic insulators, metals, or carbon nanomaterials via degrees of freedom not simply carried by propagating electrons is proving to be fertile fundamental ground rich with potential technological innovation. New discoveries in these areas inform future nanoelectronic models that could one day replace current technologies. For example, a spin information conduit formed from a disordered magnetic insulator thin film could allow easy materials integration and an essential inter-connect in a future all-spin computer processor. Similarly, large spin-wave effects in low-damping metallic ferromagnets could provide new ways to generate the spin currents needed to drive such a processor. Optimized carbon nanotube network films offer potential for spin transport applications, but could convert waste heat to usable energy if their fundamental thermal and thermoelectric physics is better understood. This project explores the fundamental science of these potentially transformative advances. Technical Abstract:This project focuses on measuring spin and heat transport (and their interplay) across three classes of materials: disordered magnetic insulators, low-damping ferromagnetic metals, and single-wall carbon nanotube networks. Each of these explores new and potentially transformative directions. The study of spin transport in disordered magnetic insulators follows on preliminary results showing that spin transport, driven by the spin Hall effect and detected via the inverse spin Hall effect, is surprisingly efficient where antiferromagnetic correlations exist but in the absence of magnetic and structural long range order. Specific tasks in this area include: 1) initial tests of amorphous Cr2O3 and NiO, 2) detailed characterization of distance and geometry dependence, 3) effects of annealing of amorphous YIG, 4) understanding of thermal gradient dependence, 5) waiting time and aging experiments, and 6) direct testing for non-equilibrium spin thermal conductivity in the disordered magnetic films. Exploration of magnon-driven spin and heat transport and thermoelectric effects in low-damping metals focuses on the Co-Fe alloy system and is motivated by potentially dramatic new spincaloritronic effects. Specific tasks here include: 1) studies with the "standard" Zink Lab thermal isolation platform, 2) characterization of AMR, MTEP, PNE, and ANE, and 3) exploration of magnon-driven spin currents using e-beam lithographically defined thermal isolation structures. Finally, study of spin and heat transport in carbon nanomaterials leverages expertise in preparation and study of highly-tuned single-wall nanotube network films. Specific tasks here include: 1) study of Wiedemann-Franz "violation" and doping dependence of thermal conductivity, 2) designing more sensitive thermal isolation platforms, and 3) testing spin transport in the networks using a non-local spin valve geometry.