Understanding antiferromagnetic spin-orbit heterostructures with a single-spin microscope

本研究旨在了解和控制一类磁性材料，即反铁磁体，它在自然界中含量丰富，但不产生宏观磁场。在这些材料中，原子尺度的磁源交替方向，因此从远处观察时完全抵消。直到最近，这些材料在磁性科学和技术中发挥的作用很小，主要应用于其他磁性材料的改性。然而，反铁磁材料作为存储和处理信息的活性元素的潜力已经出现。它们的优点，包括超快的动力学和对外加磁场的不敏感，使它们成为高性能信息存储的吸引力。研究小组的方法承认了材料接口在先进磁存储技术中的关键作用。此外，该项目解决了该领域的一个关键挑战——反铁磁性材料很难研究，因为它们除了与样品的原子尺度距离外不会产生磁场。该团队正在解决这一挑战，使用原子尺度的量子传感器在纳米尺度上扫描反铁磁性材料及其界面，从而能够在最小的长度尺度上测量否则无法检测到的磁场。对研究生和本科生的培训有助于培养具有材料物理和量子信息技术知识的劳动力。此外，该项目还为小学生提供了动手的科学教育机会，鼓励他们与榜样建立积极的关系，为科学素养的社会做出贡献。本项目研究了反铁磁材料和强自旋轨道耦合材料之间的界面，主要问题是：自旋轨道耦合是否可以改变自旋顺序？例如，研究小组正在研究界面Dzyaloshinskii-Moriya相互作用（或反对称交换）的潜在形成，这种相互作用可以诱导手性自旋顺序和界面上反铁磁各向异性的修改。该项目还试图了解对反铁磁体/铁磁体异质结构中界面磁性至关重要的未补偿磁矩在反铁磁体/自旋轨道耦合界面中的作用。研究小组研究的磁性材料包括反铁磁体，如氧化镍、铬、铱锰、铁铑等，以及接近补偿的铁磁体，如稀土合金和石榴石，它们可以调节净磁化的温度和成分。具有强自旋轨道耦合的材料包括重金属（Pt, Ta， W, Ir）和拓扑绝缘体（硒化铋及相关材料）。为了理解这些异质结构中的自旋顺序，研究小组必须克服与反铁磁性材料研究相关的实验困难——缺乏可以用传统技术检测到的平均磁化强度。该项目利用了扫描探针氮空位中心磁显微镜的灵敏度和空间分辨率。该团队正在使用这种量子增强传感器来探测表面和界面处的微小磁场，他们正在开发用于反铁磁材料成像的新方法，包括相干成像、弛豫测量和图像重建。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。

Nontechnical SummaryThis research seeks to understand and control a class of magnetic materials, antiferromagnets, that are abundant in nature but produce no macroscopic magnetic fields. In these materials, the atomic-scale sources of magnetism alternate directions and thus fully cancel when observed from a distance. Until recently, these materials have played little role in magnetic science and technology, with the principle application being the modification of other magnetic materials. However, the potential for antiferromagnetic materials as active elements that store and process information has emerged. Their advantages, including ultra-fast dynamics and insensitivity to applied magnetic fields, make them attractive for high-performance information storage. The research team’s approach acknowledges the critical role of material interfaces for advanced magnetic memory technologies. Additionally, this project addresses a key challenge in this field – that antiferromagnetic materials are difficult to study because they produce no magnetic fields except at atomic-scale distances from the sample. The team is tackling this challenge using an atom-scale quantum sensor scanned in nanoscale proximity to antiferromagnetic materials and their interfaces, thus enabling the measurement of otherwise undetectable magnetic fields at the smallest length scales. Training for graduate and undergraduate students contributes to a workforce with convergent knowledge in materials physics and quantum information technology. Additionally, this project provides hands-on science educational opportunities aimed at elementary school students that encourage positive relationships with role models and contributes to a scientifically literate society.Technical SummaryThis project examines interfaces between antiferromagnetic materials and materials with strong spin-orbit coupling with the main question: can spin-orbit coupling modify AF spin order? For example, the research team is examining the potential formation of an interfacial Dzyaloshinskii-Moriya interaction (or antisymmetric exchange) that induces chiral spin order and modifications to antiferromagnetic anisotropy at an interface. This project also seeks to understand how uncompensated magnetic moments, which are critical to the interfacial magnetism in antiferromagnet/ferromagnet heterostructures, contribute in the case of antiferromagnet/spin-orbit coupling interfaces. The magnetic materials that the research team work with include antiferromagnets such as nickel oxide, chromia, iridium manganese, iron rhodium and others, as well as ferrimagnets near compensation such as rare earth alloys and garnets, which enable temperature and compositional tuning of the net magnetization. The materials with strong spin-orbit coupling include heavy metals (Pt, Ta, W, Ir) and topological insulators (bismuth selenide and related materials). To understand spin order in these heterostructures, the research team must overcome the experimental difficulties associated with study of antiferromagnetic materials – the lack of an average magnetization that can be detected using conventional techniques. This project takes advantage of the exquisite sensitivity and spatial resolution that is possible using a scanning-probe nitrogen-vacancy center magnetic microscope. The team is using this quantum-enhanced sensor to detect the tiny magnetic fields at surfaces and interfaces, and they are developing new approaches designed for antiferromagnetic material imaging including coherence imaging, relaxometry, and image reconstruction.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.