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。低损耗(最小损耗能量)的磁化转换是开发高能效数字存储器件的关键。近年来，一种名为“自旋-轨道扭矩”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式。然而，突出的问题是，更强的自旋轨道扭矩需要极薄的、有损耗的磁性薄膜，在这种薄膜中，开关涉及大量浪费的能量。这项拟议的研究将通过开发一种新的磁性薄膜家族来解决这个长期存在的问题，这种薄膜具有定制的化学成分分布，同时能够实现强大的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-扭矩磁存储器的能效提高100倍以上。此外，研究将促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，不仅在数字存储器中，而且在脑启发和量子计算技术中也有更广泛的应用。此外，还建议为小学生开展一项动手活动，用廉价的材料制作有声扬声器。本活动将帮助学生对电、磁和声的物理概念如何应用于日常技术产生持久的鉴赏力。用于存储和计算应用的自旋轨道扭矩(SOT)设备通常是双层结构，由与自旋轨道材料连接的磁性薄膜组成。这种器件结构的问题是，较强的SOT需要厚度低至~1 nm的较薄磁体，但较薄的磁体表现出较高的阻尼，从而导致高功耗和较差的性能。这项拟议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大的SOT和低阻尼来解决这个长期存在的问题。这项研究将对对称性破缺采取一种根本不同的方法，对称性破缺是SOTS出现的关键因素。具体地说，与在薄膜界面处破坏对称性的传统双层膜不同，所提出的方法故意破坏磁性薄膜本身的对称性--通过沿厚度轴的连续成分梯度。这种块状对称性破缺被假设为直接在厚厚的、低阻尼的磁性薄膜中产生强烈的SoT。这项研究的目标是：(1)生长和表征垂直梯度磁性薄膜，并确定其组成和结构如何影响SOTS和阻尼值；以及(2)量化垂直梯度磁体的SOTS和阻尼值如何影响自旋电子存储器、振荡器和自旋波通道的性能。这项研究的成功成果将使SOT驱动器件取得革命性进展，包括功耗降低两个数量级，以及更高的稳定性、更高的信号输出和与商业制造工艺的出色兼容性。更广泛地说，这项研究将促进利用分级材料中的自旋轨道现象的设备开发，这有可能取代依赖原子尖锐界面的异质结构。这一奖项反映了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。

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.