Non-volatile magnetic memory devices based on field-free spin-orbit torque switching of perpendicularly polarized magnets.

开发固态系统中的自旋自由度是实现超高速、高能效和密集排列的磁性数据存储设备的关键。在这些磁存储器件中，信息被编码在双稳态铁磁薄膜层的磁化方向上。为了获得热稳定和紧密堆积的磁比特，优选垂直磁化的铁磁化层。通过对相邻的自旋源材料施加电荷电流而产生的自旋轨道扭矩，已经成为操纵或切换垂直磁化铁磁层的一种有效手段。然而，要实现垂直磁化铁磁层的确定性自旋轨道扭矩转换，需要外加磁场，因为传统的自旋源材料中的电荷电流感应的自旋电流由于晶体的对称性而在薄膜平面内极化。传统的自旋源材料的这一基本限制阻碍了利用自旋轨道力矩来实现垂直磁各向异性铁磁体的无场确定性开关。为了克服自旋电子学领域的这一长期障碍，本研究计划提出在一类特殊的材料--Weyl半金属中探索低对称性的晶体结构和拓扑电子结构，以演示各种垂直磁化铁磁体的无场确定磁化转换。这一研究计划的成功成果将导致下一代高能效磁存储器和自旋逻辑器件的诞生。这项研究计划将支持研究生和本科生的教育和培训。通过拟议的研究活动，研究生和本科生将接受关于非易失性磁记忆设备自旋轨道扭矩的实验技术和技能集的培训。将计划开展教育推广活动和活动，以激发科学兴趣，并为初中生和高中生提供互动和指导，特别是那些属于宾夕法尼亚州西南部代表性不足的少数族裔的学生。此外，还将开发实际操作的实验演示，以解释与自旋和磁相关的现象。利用新兴量子材料中的拓扑和晶体对称性，获得大电流诱导的自旋轨道扭矩，从而高效地和无场地操纵FM材料中的磁化，在自旋电子器件应用方面很有希望。在这种背景下，由于强的自旋-轨道耦合、对称性破缺和基于拓扑的现象，Weyl半金属提供了获得高效和非传统的电荷到自旋转换的独特机会。这项研究的目标是展示基于自旋轨道扭矩的原型磁记忆器件的无场操作，该器件利用Weyl半金属候选材料中低对称性晶体结构和拓扑电子结构的相互作用来产生具有面外自旋极化的自旋流。本计划的主要研究目标是利用Weyl半金属中电场感应的平面外取向自旋电流来演示三种不同类型的垂直磁各向异性铁磁体的无场磁化操作，即：(1)van der Waals基半金属铁磁体；(2)van der Waals基半导体铁磁体；(3)铁磁绝缘体。在建议的设备中，集成静电门将用于调节Weyl半金属薄膜中的电荷载流子密度，控制半导体铁磁材料中的磁性，以增强基于自旋轨道扭矩的设备功能，并探测电场相关的开关相图，以绘制用于磁记忆应用的低能量自旋轨道扭矩切换的最佳参数空间。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。

Exploitation of spin degree of freedom in solid state systems is key to realizing ultrafast, energy efficient, and densely packed magnetic data storage devices. In these magnetic memory devices, the information is encoded in the magnetization direction of a bistable ferromagnetic thin film layer. To achieve thermally stable and closely packed magnetic bits, a perpendicularly magnetized ferromagnetic layer is preferred. Spin orbit torques, generated by applying a charge current to an adjacent spin source material, has emerged as an efficient means to manipulate or switch the perpendicularly magnetized ferromagnetic layer. However, an external magnetic field is required to achieve deterministic spin orbit torque switching of a perpendicularly magnetized ferromagnetic layer because the charge-current induced spin current in conventional spin-source materials is polarized in the film plane due to the crystal symmetry. This fundamental limitation in conventional spin-source materials has prevented the use of spin orbit torque to achieve field-free deterministic switching of ferromagnets with perpendicular magnetic anisotropy. To overcome this longstanding hurdle in the field of spintronics, this research program proposes to explore low-symmetry crystal structure and topological electronic structure in a special class of materials, namely Weyl semimetals, to demonstrate field-free deterministic magnetization switching of variety of perpendicularly magnetized ferromagnets. A successful outcome of this research program will lead to next generation energy efficient magnetic memory and spin-logic devices. This research program will support the education and training of a graduate student and undergraduate students. Through proposed research activities, the graduate student and undergraduate students will be trained in experimental techniques and skill sets for spin orbit torques for nonvolatile magnetic memory devices. Educational outreach activities and events will be planned to kindle scientific interest and provide interactions and mentorship for middle school and high school students, especially those belonging to underrepresented minorities in southwestern Pennsylvania. Moreover, hands-on experimental demos to explain spin and magnetism-related phenomena will be developed for outreach activities.Utilization of topology and crystal-symmetry in emergent quantum materials, to obtain large current induced spin orbit torques for an energy efficient and field-free manipulation of the magnetization in FM materials, is promising for spintronic device applications. In this context, Weyl semimetals provide a distinct opportunity to obtain highly efficient and unconventional charge to spin conversion owing to strong spin-orbit coupling, symmetry breaking, and topology-based phenomena. The goal of this research program is to demonstrate field-free operation of spin orbit torque-based prototype magnetic memory devices that exploit the interplay of low-symmetry crystal structure and topological electronic structure in Weyl semimetal candidate materials to generate spin current with out-of-plane spin polarization. The main research objectives of this program are to use electric field induced out-of-plane oriented spin current in Weyl semimetals to demonstrate field-free magnetization manipulation in three different classes of ferromagnets with perpendicular magnetic anisotropy, namely: (1) van der Waals based semi-metallic ferromagnets; (2) van der Waals based semiconducting ferromagnets; and (3) a ferromagnetic insulator. In proposed devices, integrated electrostatic gates will be used to tune the charge carrier density in Weyl semimetal thin films, to control magnetism in semiconducting ferromagnets for enhanced spin orbit torque-based device functionalities, and to probe electric-field dependent switching phase diagram to map out the optimal parameter space for low-energy spin orbit torque switching for magnetic memory applications.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.