Hu and Xiang Nanoscale Res Lett (2020) 15:226 https://doi.org/10.1186/s11671-020-03458-y NANO REVIEW Open Access Recent Advances in Two-Dimensional Spintronics Guojing Hu1,2 and Bin Xiang1,2* Abstract Spintronics is the most promising technology to develop alternative multi-functional, high-speed, low-energy elec- tronic devices. Due to their unusual physical characteristics, emerging two-dimensional (2D) materials provide a new platform for exploring novel spintronic devices. Recently, 2D spintronics has made great progress in both theoretical and experimental researches. Here, the progress of 2D spintronics has been reviewed. In the last, the current chal- lenges and future opportunities have been pointed out in this feld. Keywords: 2D spintronics, Graphene, Topological insulator, Van der Waals magnet, Spin-charge conversion, Spin transport, Spin manipulation Introduction In parallel with the boom of spintronics, two-dimen- With the discovery and application of the giant magne- sional (2D) van der Waals (vdW) materials have been toresistance efect (GMR), spintronics has quickly been at the frontier of material research since the isolation of developed into an attractive feld, aiming to use the spin graphene [7–9]. Distinct from their bulk materials, 2D degree freedom of electrons as an information carrier to vdW materials exhibit many novel physical phenomena. achieve data storage and logical operations [1–3]. Com- Some 2D materials have already shown great potential pared to conventional microelectronic devices based on for the engineering of next-generation 2D spintronic charge, spintronic devices require less energy to switch devices [10–12]. For example, graphene exhibits high a spin state, which can result in faster operation speed electron/hole mobility, long spin lifetimes, and long dif- and lower energy consumption. Terefore, spintronics fusion lengths, which make it a promising candidate for is the most promising technology to develop alterna- a spin channel [13–15]. However, due to its character- tive multi-functional, high-speed, low-energy electronic istics of zero gap and weak spin–orbit coupling (SOC), devices. Although spin-transfer-torque magnetoresistive graphene has limitations in building graphene-based random-access memory (STT-MRAM) has been com- current switches. In contrast, 2D transition metal dichal- mercially produced, various technical issues still need to cogenides (TMDCs) have varied band gaps, strong SOC be resolved. Major challenges include the efcient gener- efect, and, especially, unique spin-valley coupling, pro- ation and injection of spin-polarized carriers, long-range viding a platform to manipulate spin and valley degrees transmission of spin, and manipulation and detection of of freedom for nonvolatile information storage [16, 17]. spin direction [4–6]. Topological insulators (TIs) with topologically pro- tected surface states have strong spin–orbit interactions to achieve spin-momentum locking, which can suppress scattering and enhance spin and charge conversion ef- *Correspondence: [email protected] ciency [4, 12, 18]. Emerging 2D magnets with intrinsic 1 Department of Materials Science and Engineering, CAS Key magnetic ground states down to atomic-layer thicknesses Lab of Materials for Energy Conversion, Hefei National Research Center for Physical Sciences at the Microscale, University of Science open up new avenues for novel 2D spintronic applica- and Technology of China, Hefei 230026, Anhui, China tions [19–21]. Full list of author information is available at the end of the article © The Author(s) 2020. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creat iveco mmons .org/licen ses/by/4.0/. Hu and Xiang Nanoscale Res Lett (2020) 15:226 Page 2 of 17 With the development of 2D spintronics, it is nec- Magnetism in 2D Materials essary to review the latest experimental and theoreti- Magnetism has important meanings in data storage tech- cal work in the field. In this article, the progress of nologies. However, most 2D materials like graphene are 2D spintronics has been reviewed, and some current not intrinsically magnetic. Two methods have been pro- challenges and future opportunities have also been posed to make nonmagnetic materials magnetic. Te discussed in this emerging field. The first section frst method is to generate spin polarization by introduc- reviews magnetism in 2D materials, including induced ing vacancies or adding adatoms [22–24]. Te other one magnetic moments in graphene, TIs, and some other is to introduce magnetism via the magnetic proximity 2D materials via the methods of doping or proximity efect with the adjacent magnetic materials [18, 25, 26]. effect, and some intrinsic 2D magnets. The second Te recently discovered 2D magnetic vdW crystals have section presents the three elementary functionalities intrinsic magnetic ground states at the atomic scale, pro- to achieve 2D spintronic device operations, includ- viding unprecedented opportunities in the feld of spin- ing spin-charge conversion, spin transport, and spin tronics [20, 27]. manipulation in 2D materials and at their interfaces. The third section overviews applications of 2D spin- Induced Magnetic Moments in Graphene tronics. The fourth section introduces several poten- Pristine graphene is strongly diamagnetic, so a large tial 2D spintronic devices for memory storage and number of theoretical and experimental studies explore logic applications. The final section discusses some the magnetism of graphene. Introducing vacancies and current challenges and future opportunities in 2D adding hydrogen or fuorine have been used to induce spintronics to achieve practical application. magnetic moments in graphene [23, 25, 28]. For exam- ple, Kawakami’s group utilized hydrogen adatoms to dope the graphene (Fig. 1a) and detected pure spin Fig. 1 Induced magnetic moment in graphene. a Theoretical prediction of magnetic moments in graphene due to hydrogen. b Magnetic moments due to hydrogen doping detected by spin transport measurements at 15 K. The device was measured after 8 s hydrogen doping. c Schematic of graphene exchange coupled to an atomically fat yttrium iron garnet (YIG) ferromagnetic thin flm. d Anomalous Hall resistance measurements on magnetic graphene at diferent temperatures. a, b Reproduced with permission from McCreary et al., Phys. Rev. Lett. 109, 186,604 (2012). Copyright 2012 American Chemical Society [23]. (c) and (d) reproduced with permission from Wang et al., Phys. Rev. Lett. 114, 016,603 (2015). Copyright 2015 American Chemical Society [25] Hu and Xiang Nanoscale Res Lett (2020) 15:226 Page 3 of 17 current by nonlocal spin transport measurement to Induced Magnetic Moments in TIs demonstrate magnetic moment formation in graphene 2D materials are susceptible to environmental condi- [23]. As shown in Fig. 1b, the characteristic dip appear- tions, such as moisture and oxygen. Te conductive sur- ing at zero magnetic feld in the nonlocal spin transport face state in TI surface regions is considered to be a more measurement shows that the pure spin current is scat- stable 2D material [30]. In addition, the surface state of tered by exchange coupling between conduction elec- TIs exhibits the spin-momentum locking property, which trons and local hydrogen-induced magnetic moments. provides a way to manipulate the spin signal via the In addition, graphene with fuorine adatoms and charge current direction. More interestingly, breaking the vacancy defects has paramagnetic moments, which can time-reversal symmetry by the doping of magnetic atoms be measured by a SQUID (superconducting quantum or the magnetic proximity efect can give rise to some interference device) [28]. Nevertheless, the realization exotic phenomena such as the quantum anomalous Hall of long-range ferromagnetic order in doped graphene efect (QAHE) [18, 31]. Chang et al. [24] frst observed is still an overwhelming challenge. Some researchers QAHE in Cr doped magnetic TI, Cr 0.15(Bi0.1Sb0.9)1.85Te 3. have proposed using the magnetic proximity efect to As demonstrated in Fig. 2a, by tuning the Fermi level of make graphene gain magnetism [29]. When graphene is magnetically induced TI bands, we can observe a plateau adjacent to a magnetic insulator, the π orbitals of gra- of Hall conductance of e2/h. Te measured results show phene and the neighboring spin-polarized d orbitals in the gate-tunable anomalous Hall resistance reaches the the magnetic insulator have an exchange interaction to quantized value of h/e2 at zero magnetic feld (Fig. 2b). generate long-range ferromagnetic coupling. As shown However, the spin scattering efect of doped magnetic in Fig. 1c, in the graphene/yttrium iron garnet (YIG) atoms is limited to achieve a robust long-range mag- heterostructure, the measured anomalous Hall efect netic order at the surface of the TI. Te magnetic prox- signal can persist to 250 K (Fig. 1d) [25]. imity between TIs and magnetic materials can avoid the introduction of doping atoms or defects, gaining a long- range magnetic order by interfacial exchange coupling. Fig. 2 Induced magnetic moment in TIs. a Schematic of the QAHE in a magnetic TI thin flm. The magnetization direction (M) is indicated by red arrows. The chemical potential of the flm can be controlled by a gate voltage applied on the back side of the dielectric substrate. b Magnetic feld dependence of QAHE at diferent gate voltages in Cr0.15(Bi0.1Sb0.9)1.85Te3 flm.