Spintronics in Two-Dimensional Materials

ISSN 2311-6706 e-ISSN 2150-5551 CN 31-2103/TB REVIEW https://doi.org/10.1007/s40820-020-00424-2 Spintronics in Two‑Dimensional Materials Cite as Nano-Micro Lett. (2020) 12:93 Received: 29 January 2020 Accepted: 18 March 2020 © The Author(s) 2020 Yanping Liu1,2,3 *, Cheng Zeng1, Jiahong Zhong1, Junnan Ding1, Zhiming M. Wang4 *, Zongwen Liu5 * Yanping Liu and Cheng Zeng contributed equally to this work. * Yanping Liu, ; Zhiming M. Wang, ; Zongwen Liu, 1 School of Physics and Electronics, Hunan Key Laboratory for Super‑Microstructure and Ultrafast Process, Central South University, 932 South Lushan Road, Changsha 410083, Hunan, People’s Republic of China 2 Shenzhen Research Institute of Central South University, A510a, Virtual University Building, Southern District, High‑Tech Industrial Park, Yuehai Street, Nanshan District, Shenzhen, People’s Republic of China 3 State Key Laboratory of High‑Performance Complex Manufacturing, Central South University, 932 South Lushan Road, Changsha 410083, Hunan, People’s Republic of China 4 5 Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Chengdu 610054, People’s Republic of China School of Chemical and Biomolecular Engineering, The University of Sydney, Sydney, NSW 2006, Australia HIGHLIGHTS • The recent progress of spin injection, spin transport, spin manipulation, and application in 2D materials was summarized. • The current challenges and outlook of future studies in spintronics based on 2D materials and related heterostructures were discussed. ABSTRACT Spintronics, exploiting the spin degree of electrons as the information vector, is an attractive field for implementing the beyond Complemetary metal-oxide-semiconductor (CMOS) devices. Recently, two-dimensional (2D) materials have been drawing tremendous atten‑ tion in spintronics owing to their distinctive spin-dependent properties, such as the ultra-long spin relaxation time of graphene and the spin–val‑ ley locking of transition metal dichalcogenides. Moreover, the related heterostructures provide an unprecedented probability of combining the different characteristics via proximity effect, which could remedy the limitation of individual 2D materials. Hence, the proximity engineering has been growing extremely fast and has made significant achievements in the spin injection and manipulation. Nevertheless, there are still chal‑ lenges toward practical application; for example, the mechanism of spin relaxation in 2D materials is unclear, and the high-efficiency spin gating is not yet achieved. In this review, we focus on 2D materials and related heterostructures to systematically summarize the progress of the spin injection, transport, manipulation, and application for information storage and processing. We also highlight the current challenges and future perspectives on the studies of spintronic devices based on 2D materials. KEYWORDS Spintronics; 2D materials; TMDCs; Heterostructure; Proximity effect Vol.:(0123456789) 13 93 Page 2 of 26 1 Introduction With the imminent end of Moore’s law, exploiting new degrees of freedom has become an essential research direc‑ tion to promote further development of electronic devices. The aim of spintronics is to utilize the spin degree of free‑ dom of electrons to realize novel information storage and logic devices. A spintronic device has the superiority of faster speed, ultra-low heat dissipation, and non-volatility, making it an ideal candidate for future electronics. Addition‑ ally, 2D materials, such as graphene [1], black phosphorus (BP) [2], transition metal dichalcogenides (TMDCs) [3], and silicene [4], have created an excellent platform for spintronic research due to their unique spin-dependent properties, like ultra-long spin relaxation time and spin diffusion length, Rashba spin–orbit coupling (SOC), spin–valley locking, and quantum spin Hall effect. Furthermore, stacking individual 2D materials in a precisely designed order can combine the best of different components in one ultimate material [5, 6]. For example, the heterostructure of graphene and TMDCs enable graphene to have both excellent spin transport perfor‑ mance and larger SOC [7–9]. Along the way, 2D materials and related heterostructures can accomplish long-distance spin transport and effective spin manipulation, thereby real‑ izing magnetic logic gates, magnetic random access memory (MRAM) [10, 11], and other spintronic devices. However, several challenges remain to be solved in 2D material spintronics. Firstly, the 2D materials used in spin‑ tronics are mostly non-ferromagnetic [or the Curie temper‑ ature (Tc) is far below room temperature]. Consequently, this requires a spin injection into the 2D materials by vari‑ ous methods, which brings a new issue on improving the polarization efficiency of the spin injection. On the other hand, efficiently manipulating the spin and maintaining the spin state are not yet achieved. The inability to transmit spin information and switch spin states means that the applica‑ tion of spin is impossible. Over the past few years, much of the effort has gone into seeking solutions to these topics and much progress has been made, such as the continuous improvement of spin parameters [12], the discovery and research of the novel 2D materials [13], the magnetic engi‑ neering of non-magnetic 2D materials [14], and the in-depth study of various spin effects. In this review, while we reca‑ pitulate the pioneering work of spintronics in 2D materials, we focus on the recent research and development in this area. © The authors Nano-Micro Lett. (2020) 12:93 The first section of this review presents the spin injection in 2D materials, while the second section reviews the research of spin transport in 2D materials. The third section describes the ways to manipulate spin, and the final section discusses the application of 2D materials in spintronic devices. 2 Spin Injection in 2D Materials Spin injection is a key and essential topic in the research and application of spintronics in 2D materials. A simple solution is to produce magnetism in 2D materials, thereby obtaining a spin-polarized state. Besides, many other approaches to inject spin have been proposed, including electrical injec‑ tion, optical injection, and spin–orbit coupling effect. 2.1 Magnetic Engineering of Non‑magnetic 2D Materials At present, the widely used 2D materials in spintronic research, such as graphene and TMDCs, are non-magnetic. Therefore, magnetic engineering is a significant topic to obtain spin-polarized states in 2D materials, especially through gating, doping, functionalization. Magnetism origi‑ nates from the moving charges and spin of elementary par‑ ticles and is commonly deemed unstable in 2D systems on account of fundamental hindrances, such as thermal distur‑ bance. Through the efforts of recent years, it is now possible to realize long-range magnetism in 2D systems and many achievements (...truncated)