Two-dimensional magnetic materials
当前, 尽管集成电路制造工艺不断提高, 但由于器件的不断缩小, 受到量子效应的限制, 业界遇到了可靠性低、功耗大等瓶颈, 微电子行业延续了近 50年的“摩尔定律”将难以持续. 因此, 寻求从材料到系统的各个层面探究突破集成电路性能瓶颈的方案是亟待解决的关键科学问题. 自旋电子学有望突破上述瓶颈, 已成为后摩尔时代集成电路领域的关键技术之一. 1988年巨磁阻效应的发现标志着自旋电子学的诞生, 并带来了信息存储领域的快速发展. 由于在自旋电子学领域的杰出贡献,艾尔伯-费尔和皮特-葛伦伯格两位教授荣获 2007年诺贝尔物理学奖. 磁性材料是自旋电子器件的基础, 不同于传统磁性薄膜, 二维磁性材料的出现和其优势为传感、存储、电子及医学等诸多领域打开了新的局面, 受到国内外的广泛关注. 二维磁性材料其特点在于以层状的形式存在, 通过范德瓦耳斯力即分子间作用力堆叠在一起, 层内原子以化学键进行连接, 在原子级厚度下依然在磁学、电学、力学、光学等方面保持新奇的物理和化学特性. 进一步地, 通过较弱的范德瓦耳斯相互作用与相邻层结合, 使得匹配度不同的原子层结合成为可能, 进而创建多种范德瓦耳斯异质结构, 摆脱晶格匹配和兼容性的限制, 从而为实现具有电路微型化、力学柔韧性、三维堆叠高密度、响应速率快和高开关比性能的磁传感器和非易失随机存储器等新型自旋电子学器件提供了新的契机.
应《物理学报》编辑部的邀请, 我们邀请了部分活跃在二维磁性材料研究第一线的中青年科学家, 组织了本期的专题, 大致涵盖如下几方面内容: 在关于二维磁性材料的居里温度方面, 聂天晓老师综述了二维磁性材料的发展过程、制备方法及其优越性能, 并着重阐述了调控二维磁性材料居里温度的方法. 在磁性拓扑材料方面, 何庆林老师以具有层状结构的本征磁性拓扑绝缘体、磁性外尔半金属、磁性狄拉克半金属等为例简要综述磁序与拓扑序之间的相互作用和近期部分的重要实验结果; 沈冰老师的实验结果表明了 EuIn2As2的金属态性质, 通过掺杂 Ca来调节体系的费米能级和磁性. 在二维磁性材料性能调控方面, 邵启明老师介绍了近几年来二维材料中新型磁响应的实验研究进展; 龙根和张广宇老师综述了 CrI3二维磁性材料的生长、磁性结构测量和调控, 并对下一阶段的工作从基础凝聚态物理研究以及电子工程应用角度做出展望; 王伟和王琳老师总结了二维磁性材料的种类类型、合成方法、基本特性以及表征手段, 系统归纳了关于二维磁性材料物性调控方面的研究工作, 并对二维磁性材料的未来研究方向和挑战进行了简单的展望; 王以林老师综述了近年来发现的各类本征二维磁性材料的晶体结构、磁结构和磁性能, 并讨论了由磁场、电场、静电掺杂、离子插层、堆叠方式、应变、界面等外场调控二维磁性材料磁性能的研究进展, 最后总结并展望了二维磁性材料未来发展的研究方向. 在基于二维磁性材料的异质结方面, 林晓阳老师基于密度泛函理论与非平衡格林函数方法, 研究了 Fe3GeTe2/石墨烯二维异质结在有无氮化硼作隧穿层情况下的输运性质; 王守国和于国强老师综述了与二维材料及其异质结构中自旋轨道矩研究相关的最新进展, 主要包括基于非磁性二维材料和磁性二维材料的异质结中自旋轨道矩的产生、表征和对磁矩的操控等.
本专题从不同的角度描述了二维磁性材料在理论与实验方面的进展, 反映了此领域当前的研究现状, 希望对读者了解此前沿课题有所帮助.
应《物理学报》编辑部的邀请, 我们邀请了部分活跃在二维磁性材料研究第一线的中青年科学家, 组织了本期的专题, 大致涵盖如下几方面内容: 在关于二维磁性材料的居里温度方面, 聂天晓老师综述了二维磁性材料的发展过程、制备方法及其优越性能, 并着重阐述了调控二维磁性材料居里温度的方法. 在磁性拓扑材料方面, 何庆林老师以具有层状结构的本征磁性拓扑绝缘体、磁性外尔半金属、磁性狄拉克半金属等为例简要综述磁序与拓扑序之间的相互作用和近期部分的重要实验结果; 沈冰老师的实验结果表明了 EuIn2As2的金属态性质, 通过掺杂 Ca来调节体系的费米能级和磁性. 在二维磁性材料性能调控方面, 邵启明老师介绍了近几年来二维材料中新型磁响应的实验研究进展; 龙根和张广宇老师综述了 CrI3二维磁性材料的生长、磁性结构测量和调控, 并对下一阶段的工作从基础凝聚态物理研究以及电子工程应用角度做出展望; 王伟和王琳老师总结了二维磁性材料的种类类型、合成方法、基本特性以及表征手段, 系统归纳了关于二维磁性材料物性调控方面的研究工作, 并对二维磁性材料的未来研究方向和挑战进行了简单的展望; 王以林老师综述了近年来发现的各类本征二维磁性材料的晶体结构、磁结构和磁性能, 并讨论了由磁场、电场、静电掺杂、离子插层、堆叠方式、应变、界面等外场调控二维磁性材料磁性能的研究进展, 最后总结并展望了二维磁性材料未来发展的研究方向. 在基于二维磁性材料的异质结方面, 林晓阳老师基于密度泛函理论与非平衡格林函数方法, 研究了 Fe3GeTe2/石墨烯二维异质结在有无氮化硼作隧穿层情况下的输运性质; 王守国和于国强老师综述了与二维材料及其异质结构中自旋轨道矩研究相关的最新进展, 主要包括基于非磁性二维材料和磁性二维材料的异质结中自旋轨道矩的产生、表征和对磁矩的操控等.
本专题从不同的角度描述了二维磁性材料在理论与实验方面的进展, 反映了此领域当前的研究现状, 希望对读者了解此前沿课题有所帮助.
2021, 70 (12): 127303.
doi: 10.7498/aps.70.20202132
Abstract +
The magnetic response in a two-dimensional material has received increasing attention in recent years. The magnetic effects and related quantum transport originate from Berry curvature, which is associated with crystal symmetry and many quantum effects including electrons’ orbital magnetism, spin-orbit coupling, and magnetoelectricity. The importance of studying the magnetic response in the two-dimensional material lies in two aspects. First, the magnetic response of two-dimensional material provides a platform to investigate the coupling between the above-mentioned intrinsic quantum effects and their couplings. Second, it possesses the potential applications in energy-efficient quantum and spintronic devices. Here, we review the experimental research progress made in recent years. In particular, we focus on the research progress of the valley Hall and magnetoelectric effect, quantum non-linear Hall effect, anomalous Hall, and quantum anomalous Hall effect in two-dimensional materials such as graphene, transition-metal chalcogenides, and twisted bilayer graphene. For each session, we first introduce these phenomena and their underlying physics by using crystal symmetries and band structures. Then, we summarize the experimental results and identify unsolved problems. At last, we provide an outlook in this emerging research direction.
2021, 70 (12): 127502.
doi: 10.7498/aps.70.20210042
Abstract +
The study of two-dimensional (2D) magnetic materials has driven the development of modern nano-electronic devices. Exploration of novel intrinsic layered materials with 2D magnetic order will provide a material candidate pool for fabricating 2D devices and searching for new quantum phases. Recently the layered antiferromagnetic (AF) topological insulators have aroused the great interest of researchers. As one of the proposed axion insulators, EuIn2As2 exhibits a layered structure and 2D AF order. It is found that the parent compound EuIn2As2 exhibits metallic behavior instead of the predicted insulating feature. To pursuit the predicted non-trivial topological state and novel feature, in this paper, we use various elements to dope the system to adjust the Fermi level. It is found that only Ca is successfully doped into the EuIn2As2 system. The systematic transport and magnetization studies are performed on the single crystal of Eu1–xCaxIn2As2. The long-range AF order is revealed to be similar to the parent compound. Above the AF transition, the magnetization violated Curie-Weiss behavior and magnetoresistance keeps negative, indicating the ferromagnetic order. With doping nearly 20% non-magnetic Ca, the magnetic properties of the system barely change, which is favorable to keeping the former predicted nontrivial topological properties in EuIn2As2. Although Ca shares the same valence with Eu, the carrier density of Eu1–xCaxIn2As2 is one order lower than that of EuIn2As2. The Ca doping brings electrons in and lifts the Fermi level. The results enrich the 2D magnetic material candidate pool and provide useful information for realizing the nontrivial topological state in the 2D AF system.
2021, 70 (12): 127503.
doi: 10.7498/aps.70.20202204
Abstract +
The recently discovered two-dimensional magnetic materials have attracted tremendous attention and become a cutting-edge research topic due to their long-range magnetic ordering at a single-unit-cell thickness, which not only provide an ideal platform for studying the magnetism in the two-dimensional limit and other novel physical effects, but also open up a new way to develop low-power spintronics/magnetic storage devices. In this review, first, we introduce the crystal structures, magnetic structures and magnetic properties of various recently discovered intrinsic two-dimensional magnetic materials. Second, we discuss the research progress of controlling the magnetic properties of two-dimensional magnetic materials by magnetic field, electric field, electrostatic doping, ion intercalation, stacking, strain, interface, etc. Finally, we give a perspective of possible research directions of the two-dimensional magnetic materials. We believe that an in-depth understanding of the origin and mechanism of magnetism of the two-dimensional magnetic materials as well as the study of the relationship between magnetic properties and microstructures are of great significance in exploring a magnetic material with a substantially high Curie temperature (Néel temperature), and designing multifunctional new concept devices.
2021, 70 (12): 127302.
doi: 10.7498/aps.70.20210133
Abstract +
The intersection between layered magnetic materials and topological materials combines the advantages of the two, forming a material system with both the magnetic orders and topological properties within the minimum two-dimensional unit, i.e. layered magnetic topological materials. This type of material may host Dirac points, Weyl points, nodal lines, etc. which are associated with helical or chiral electronic states ranging from insulator, semimetal to metal. This results in lots of novel physical problems and effects, which attract much attention of scientists. In this paper, we focus our attention on intrinsic magnetic topological insulator, magnetic Weyl semimetal, magnetic Dirac semimetal, and take them for example to briefly review the interplay between magnetic orders and topological orders and recent experimental results. This emergent area requires further studies to explore more new material candidates, which is a challenging frontier of condensed matter physics.
2021, 70 (12): 127504.
doi: 10.7498/aps.70.20202197
Abstract +
For a long time, it has been generally acknowledged that low-dimensional (lower than three-dimensions) long-range orders cannot stay stable at any finite temperature, because temperature-induced fluctuations can destroy any long-range orders in low-dimensional systems supported by isotropic short-range interactions. However, this theorem requires that the interaction must be short-range and isotropic. In fact, many low-dimensional systems do not meet these two requirements. For example, due to the strong anisotropy in two-dimensional CrI3 crystals, there is a band gap in the magnon spectrum. When the excitation energy from temperature is much lower than the band gap, the magneton cannot be excited by temperature on a large scale, and the long-range magnetic order in the two-dimensional system will not be destroyed. Various methods have been used to characterize the magnetic order in atomically thin CrI3 crystals, and a lot of attempts have been made to manipulate the magnetic structure in the system. Focusing on CrI3, in this article we review the recent studies on growth, magnetic structure measurement and manipulation of two-dimensional magnetic materials, and also discuss the prospects for the next phase of research from the perspectives of basic condensed matter physics research and electronic engineering applications.
2021, 70 (12): 127501.
doi: 10.7498/aps.70.20210004
Abstract +
The spin-orbit torque generated by charge current in a strong spin-orbit coupling material provides a fast and efficient way to manipulate the magnetic moment in adjacent magnetic layers, which is expected to be used for developing low-power, high-performance spintronic devices. Two-dimensional materials have attracted great attention, for example, they have abundant species, a variety of crystal structures and symmetries, good adjustability of spin-orbit coupling strength and conductivity, and good ability to overcome the lattice mismatch to form high-quality heterojunctions, thereby providing a unique platform for studying the spin-orbit torques. This paper covers the latest research progress of spin-orbital torques in two-dimensional materials and their heterostructures, including their generations, characteristics, and magnetization manipulations in the heterostructures based on non-magnetic two-dimensional materials (such as MoS2, WSe2, WS2, WTe2, TaTe2, MoTe2, NbSe2, PtTe2, TaS2, etc.) and magnetic two-dimensional materials (such as Fe3GeTe2, Cr2Ge2Te6, etc.). Finally, some problems remaining to be solved and challenges are pointed out, and the possible research directions and potential applications of two-dimensional material spin-orbit torque are also proposed.
2021, 70 (12): 129101.
doi: 10.7498/aps.70.20202136
Abstract +
Recently, the discovery of intrinsic two-dimensional (2D) ferromagnetism has accelerated the application of spintronics in ultra-low power electronic device. Particularly, the Curie temperature of Fe3GeTe2 can be improved to room-temperature in several ways, such as interfacial exchange coupling and ionic liquid gating, which makes Fe3GeTe2 desirable for the practical application. In this work, we investigate the transport properties of Fe3GeTe2/graphene heterostructures with or without h-BN layers by utilizing the density functional theory combined with nonequilibrium Green’s function method. The results show that due to electronic orbital hybridization, the spin can be effectively injected into graphene with ± 0.1 V bias at the transparent contact interface of Fe3GeTe2/graphene. What is more, the efficient spin tunneling injection can be achieved in a wider bias range [–0.3 V, 0.3 V] by adding h-BN as a tunneling layer, where the spin filter effect that is induced by mismatched distribution of spin-dependent electronic states in the Brillouin zone, leads a spin polarizability to approach 100%. These results are helpful in the applications of 2D all-spin logic and the development of ultra-low power spintronic devices.
2021, 70 (12): 127301.
doi: 10.7498/aps.70.20210223
Abstract +
To date, despite the continuous improvement of integrated circuit manufacturing technology, it has been limited by quantum effects and the shrinking of device size has caused the industry to encounter bottlenecks such as low reliability and high power consumption. The “Moore’s Law” that has lasted for nearly 50 years in the microelectronics industry will not be sustainable. In 2004, the advent of graphene, a two-dimensional (2D) material, brought new opportunities to break through the power consumption bottleneck of integrated circuits. Due to the low dimensionality, 2D materials exhibit a variety of fasinatingly electrical, ferromagnetic, mechanical, and optical properties at an atomic level. Among them, ferromagnetism has a wide range of applications in information processing, magnetic memory and other technologies. However, only a few 2D ferromagnetic materials are successfully synthesized. Meanwhile, the magnetic long-range order will be strongly suppressed within a limited temperature range due to thermal fluctuations, and thus bringing non-ignorable limitations and challenges to subsequent work. Therefore, the realization and control of room-temperature ferromagnetism in 2D magnetic materials is the major concern at this stage. In light of the above, this review first introduces the development process, preparation methods and superior properties of 2D magnetic materials in detail, and then focuses on the methods of manipulating the Curie temperature of 2D magnetic material. Finally, we briefly give an outlook of the application prospects in the future.
2021, 70 (12): 127801.
doi: 10.7498/aps.70.20202146
Abstract +
Two-dimensional (2D) materials represented by graphene and molybdenum disulfide (MoS2) have attracted much attention in recent years due to their advantages in electrical, thermal, optical and mechanical properties. As a branch of 2D materials, 2D magnetic materials have special properties such as magnetic anisotropy and single-layer magnetic order. Especially, their magnetism can also be controlled by a variety of physical fields, and it possesses various physical properties and potential applications. Therefore, they have received widespread attention of researchers gradually. In this article, we summarize the types, synthesis methods, basic characteristics and characterization methods of 2D magnetic materials in detail, and the magnetism controlling of 2D magnetic materials as well. Finally, a simple outlook on the research directions and future challenges of 2D magnetic materials is given.
