CAREER:Toward ultra-low energy switching in spintronic devices

自旋电子学明确利用电子固有的量子力学特性自旋来实现新的功能。自旋电子器件的一个独特优势是它们的非易失性：它们在自旋中存储的信息即使在设备断电后也能保留和记忆。非易失性对于新一代的数字器件来说尤其重要，因为晶体管的尺寸将缩小到只有几纳米。在这种纳米尺度下，传统互补金属氧化物半导体（CMOS）晶体管的功耗将由漏电流主导；然而，对于自旋电子结构，这种能量浪费可以完全消除。自旋电子学的一个重要缺点是，在二进制计算机指令中，将电子自旋从上或下状态翻转到1和0的最低记录开关能量仍然比CMOS晶体管高出两个数量级，这严重限制了自旋电子学器件的当前应用。这个CAREER项目的重点是探索新的电压效应，以大大降低基于自旋的器件的开关能量。通过成功展示超低能量开关，该项目广泛影响了广泛的自旋电子设备以及可穿戴计算机和物联网等新兴技术，对于待机状态下的零功耗至关重要，并且非常需要。该研究的多学科性质影响了从高中到研究生的多个教育层次。教育活动建立在一个行之有效的计划上，鼓励和激励当地高中中代表性不足的少数民族学生在大学里攻读STEM专业。此外，PI将利用这项CAREER研究继续他的外展努力，通过“物理开放日”和“物理Phun Nite”等活动提高公众对自旋电子学的理解。该职业奖探索自旋电子结构中的节能开关机制。本文将研究三种不同但互补的方法：(1)基于电压控制的层间交换耦合的开关；(2)基于电压感应有效场的开关；(3)基于电压辅助自旋传递力矩的开关。在压控各向异性和压控磁性的背景下，研究了这三种开关场景。电压控制的各向异性是一种电子效应，其中磁各向异性可以大大改变，但饱和磁化强度基本保持不变。电压控磁是一种离子效应，其磁场各向异性和饱和磁化强度均可由电压控制。这些效应对Rashba自旋-轨道耦合和Dzyaloshinskii-Moriya相互作用的依赖性将被研究。结合铁磁体中电压诱导电荷再分配的实验，将对不同的开关机制有一个完整的了解。此外，这些大幅度降低开关能量的新方法将直接应用于具有强垂直磁各向异性的高质量磁隧道结上，它不仅可以作为独立的自旋电子存储器或逻辑单元，而且还可以作为许多其他自旋电子器件的重要组成部分，如横向自旋阀和自旋霍尔存储器。在这个项目中获得的结果将改变我们对超薄磁性薄膜中电子和离子如何介导磁电耦合的理解。这个项目的成功为超低开关能量的自旋电子器件铺平了道路，这种器件可以与传统的CMOS器件互补，甚至优于传统的CMOS器件。

Spintronics makes explicit use of spin, an inherent quantum-mechanical property of electrons, to achieve novel functionalities. A unique advantage of spintronic devices is their nonvolatility: the information they store in spin is retained and remembered even after power is removed from the devices. Nonvolatility is particularly important for the nascent generation of digital devices in which transistor dimensions will be reduced to only a few nanometers. At this nanoscale the power consumption of traditional complementary metal-oxide semiconductor (CMOS) transistors will be dominated by leakage current; whereas, for spintronic structures such energy waste can be completely eliminated. An important draw back for spintronics is the lowest recorded switching energy to flip electron spins between their up or down states to 1s and 0s in binary computer instructions, is still more than two orders of magnitude higher than a CMOS transistor, which severely limits the present applications of spintronic devices. This CAREER project is focused on exploring novel voltage effects to greatly reduce the switching energy of spin-based devices. By successfully demonstrating ultra-low energy switching, this project broadly impacts a wide range of spintronic devices as well as emergent technologies such as wearable computers and the Internet of Things, for which zero power consumption in the standby state is critical and highly desired. The multidisciplinary nature of the research impacts multiple levels of education, from high school to graduate students. The education activities build on a proven plan to encourage and inspire underrepresented minority students in local high schools to pursue STEM majors in college. Moreover, the PI will leverage this CAREER research to continue his outreach efforts to improve the general public's understanding of spintronics through activities such as 'Physics Open House' and 'Physics Phun Nite'.This CAREER award explores energy-efficient switching mechanisms in spintronic structures. Three different, but complementary approaches will be studied: (1) switching based on voltage-controlled interlayer exchange coupling; (2) switching based on voltage-induced effective field; and (3) switching based on voltage-assisted spin transfer torque. These three switching scenarios are studied under the context of voltage-controlled anisotropy and voltage-controlled magnetism. Voltage-controlled anisotropy is an electronic effect where the magnetic anisotropy field can be substantially modified, but the saturation magnetization remains largely the same. Voltage-controlled magnetism is an ionic effect where both the magnetic anisotropy field and the saturation magnetization can be controlled by voltage. The dependence of these effects on the Rashba spin-orbit coupling and Dzyaloshinskii-Moriya Interaction will investigated. Combined with the experiments that characterize the voltage-induced charge redistribution in ferromagnets, a complete understanding on different switching mechanisms will be achieved. Furthermore, these new approaches to dramatically decrease the switching energy will be directly implemented on high quality magnetic tunnel junctions with strong perpendicular magnetic anisotropy, which can not only perform as stand-alone spintronic memories or logic cells, but can serve also as the essential part of many other spintronic devices, such as lateral spin valves and spin-Hall memories. The results obtained in this project will transform our understanding of how magnetoelectric coupling is mediated by both electrons and ions in ultra-thin magnetic films. Success for this project paves a path to ultra-low switching energy spintronic devices that could be complementary, or even superior, to traditional CMOS devices.