CAREER: Low-Loss Spintronic Devices with Vertically Engineered Magnets

在个人计算机和数据中心中，信息通常存储在磁性薄膜中，其中两个数字状态（“0”和“1”）由相反的磁化方向表示。以低损耗（最小的能量浪费）转换磁化是开发高能效数字存储器件的关键。近年来，一种被称为“自旋轨道转矩”的效应被认为是切换下一代磁存储器的一种很有前途的方法。然而，突出的问题是，更强的自旋轨道扭矩需要极薄的、有损耗的磁薄膜，在这种薄膜中，开关涉及大量浪费的能量。这项提议的研究将通过开发一种具有定制化学成分谱的新型磁性薄膜来解决这个长期存在的问题，同时实现强自旋轨道扭矩和低损耗。这项研究的成功成果将使自旋轨道转矩磁存储器的能量效率提高一百倍以上。此外，研究将推进对磁性材料中自旋轨道扭矩和损耗如何产生的基本理解，不仅在数字记忆中，而且在大脑启发和量子计算技术中具有更广泛的应用。此外，本研究建议发展小学生课堂动手活动，利用廉价的材料制作音响。这项活动将帮助学生培养对电、磁和声的物理概念如何应用于日常技术的持久欣赏。用于存储和计算应用的自旋轨道扭矩（SOT）器件通常是双层的，由磁膜和自旋轨道材料组成。这种器件结构的问题是，更强的sot需要更薄的磁体，厚度可降至~ 1nm，但更薄的磁体具有更高的阻尼，导致高功耗和较差的性能。提出的研究将通过在几纳米厚的单层磁性金属薄膜中同时设计强sot和低阻尼来解决这个长期存在的问题。这项研究将采用一种完全不同的方法来研究对称破缺，这是sot出现的一个重要因素。具体来说，与传统双层膜在膜界面处的对称性被打破不同，该方法通过沿厚度轴的连续成分梯度，故意打破磁性膜本身的对称性。假设这种体对称破缺可以直接在厚的低阻尼磁膜中产生强SOTs。本研究的目的是：(1)生长和表征垂直梯度磁膜，并确定其成分和结构如何影响SOTs和阻尼；(2)量化垂直梯度磁体的sot和阻尼如何影响自旋电子存储器、振荡器和自旋波通道的性能。这项研究的成功结果将使sot驱动器件的变革性进步-包括降低两个数量级的功耗，以及更高的稳定性，更高的信号输出，以及与商业制造工艺的出色兼容性。更广泛地说，这项研究将催化利用梯度材料中的自旋轨道现象的器件开发，这有可能取代依赖于原子尖锐界面的异质结构。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。

In personal computers and data centers, information is often stored in magnetic films where the two digital states (“0” and “1”) are represented by opposite magnetization directions. Switching the magnetization with low loss (minimal wasted energy) is key to developing energy-efficient digital memory devices. In recent years, an effect called “spin-orbit torque” has been envisioned as a promising way to switch next-generation magnetic memories. However, the outstanding problem is that stronger spin-orbit torques require extremely thin, lossy magnetic films, in which switching involves a large amount of wasted energy. The proposed research will resolve this longstanding problem by developing a new family of magnetic films with tailored chemical composition profiles, which simultaneously enable strong spin-orbit torques and low loss. A successful outcome of this research will improve the energy efficiency of spin-orbit-torque magnetic memories by more than a hundredfold. In addition, research will advance the basic understanding of how spin-orbit torques and losses arise in magnetic materials, with broader applications in not only digital memories but also brain-inspired and quantum computing technologies. Moreover, it is proposed to develop a hands-on in-class activity for elementary school students to build audio speakers with inexpensive materials. This activity will help students develop a long-lasting appreciation for how the physical concepts of electricity, magnetism, and sound apply to everyday technologies. Spin-orbit torque (SOT) devices for memory and computing applications are typically bilayers, consisting of a magnetic film interfaced with a spin-orbit material. The problem with this device structure is that stronger SOTs require thinner magnets with thicknesses down to ~1 nm, but thinner magnets exhibit higher damping that results in high power consumption and poor performance. The proposed research will address this longstanding problem by simultaneously engineering strong SOTs and low damping in several-nm-thick, single-layer magnetic metal films. The research will take a fundamentally different approach to symmetry breaking, which is an essential ingredient for the emergence of SOTs. Specifically, in contrast to the conventional bilayers where symmetry is broken at film interfaces, the proposed approach deliberately breaks symmetry within the magnetic film itself – via a continuous compositional gradient along the thickness axis. Such bulk symmetry breaking is hypothesized to yield strong SOTs directly within a thick, low-damping magnetic film. The objectives of this research are to: (1) grow and characterize vertically graded magnetic films and determine how their compositions and structures impact SOTs and damping; and (2) quantify how the SOTs and damping of vertically graded magnets impact the performance of spintronic memories, oscillators, and spin-wave channels. A successful outcome of this research will enable transformative advances in SOT-driven devices – including two-orders-of-magnitude lower power dissipation, along with higher stability, higher signal output, and excellent compatibility with commercial fabrication processes. More broadly, this research will catalyze device development that leverages spin-orbit phenomena in graded materials, which have the potential to supersede heterostructures relying on atomically sharp interfaces.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.