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    SPECIAL TOPIC — Two-dimensional magnetic materials and devices

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    Magnetic and electronic properties of two-dimensional metal-organic frameworks TM3(C2NH)12
    Zhen Feng(冯振), Yi Li(李依), Yaqiang Ma(马亚强), Yipeng An(安义鹏), and Xianqi Dai(戴宪起)
    Chin. Phys. B, 2021, 30 (9): 097102.   DOI: 10.1088/1674-1056/ac0cdb
    Abstract220)   HTML4)    PDF (4889KB)(186)      
    The ferromagnetism of two-dimensional (2D) materials has aroused great interest in recent years, which may play an important role in the next-generation magnetic devices. Herein, a series of 2D transition metal-organic framework materials (TM-NH MOF, TM=Sc-Zn) are designed, and their electronic and magnetic characters are systematically studied by means of first-principles calculations. Their structural stabilities are examined through binding energies and ab-initio molecular dynamics simulations. Their optimized lattice constants are correlated to the central TM atoms. These 2D TM-NH MOF nanosheets exhibit various electronic and magnetic performances owing to the effective charge transfer and interaction between TM atoms and graphene linkers. Interestingly, Ni- and Zn-NH MOFs are nonmagnetic semiconductors (SM) with band gaps of 0.41 eV and 0.61 eV, respectively. Co- and Cu-NH MOFs are bipolar magnetic semiconductors (BMS), while Fe-NH MOF monolayer is a half-semiconductor (HSM). Furthermore, the elastic strain could tune their magnetic behaviors and transformation, which ascribes to the charge redistribution of TM-3d states. This work predicts several new 2D magnetic MOF materials, which are promising for applications in spintronics and nanoelectronics.
    Vertical WS2 spin valve with Ohmic property based on Fe3GeTe2 electrodes
    Ce Hu(胡策), Faguang Yan(闫法光), Yucai Li(李予才), and Kaiyou Wang(王开友)
    Chin. Phys. B, 2021, 30 (9): 097505.   DOI: 10.1088/1674-1056/ac078b
    Abstract213)   HTML1)    PDF (628KB)(176)      
    The two-dimensional (2D) transition-metal dichalcogenides (TMDCs) have been recently proposed as a promising class of materials for spintronic applications. Here, we report on the all-2D van der Waals (vdW) heterostructure spin valve device comprising of an exfoliated ultra-thin WS2 semiconductor acting as the spacer layer and two exfoliated ferromagnetic Fe3GeTe2 (FGT) metals acting as ferromagnetic electrodes. The metallic interface rather than Schottky barrier is formed despite the semiconducting nature of WS2, which could be originated from the strong interface hybridization. The spin valve effect persists up to the Curie temperature of FGT. Moreover, our metallic spin valve devices exhibit robust spin valve effect where the magnetoresistance magnitude does not vary with the applied bias in the measured range up to 50 μA due to the Ohmic property, which is a highly desirable feature for practical application that requires stable device performance. Our work reveals that WS2-based all-2D magnetic vdW heterostructure, facilitated by combining 2D magnets, is expected to be an attractive candidate for the TMDCs-based spintronic applications.
    Spin orbit torques in Pt-based heterostructures with van der Waals interface
    Qian Chen(陈倩), Weiming Lv(吕伟明), Shangkun Li(李尚坤), Wenxing Lv(吕文星), Jialin Cai(蔡佳林), Yonghui Zhu(朱永慧), Jiachen Wang(王佳晨), Rongxin Li(李荣鑫), Baoshun Zhang(张宝顺), and Zhongming Zeng(曾中明)
    Chin. Phys. B, 2021, 30 (9): 097506.   DOI: 10.1088/1674-1056/ac0908
    Abstract188)   HTML3)    PDF (1182KB)(127)      
    Spin orbit torques (SOTs) in ferromagnet/heavy-metal heterostructures have provided great opportunities for efficient manipulation of spintronic devices. However, deterministically field-free switching of perpendicular magnetization with SOTs is forbidden because of the global two-fold rotational symmetry in conventional heavy-metal such as Pt. Here, we engineer the interface of Pt/Ni heterostructures by inserting a monolayer MoTe2 with low crystal symmetry. It is demonstrated that the spin orbit efficiency, as well as the out-of-plane magnetic anisotropy and the Gilbert damping of Ni are enhanced, due to the effect of orbital hybridization and the increased spin scatting at the interface induced by MoTe2. Particularly, an out-of-plane damping-like torque is observed when the current is applied perpendicular to the mirror plane of the MoTe2 crystal, which is attributed to the interfacial inversion symmetry breaking of the system. Our work provides an effective route for engineering the SOT in Pt-based heterostructures, and offers potential opportunities for van der Waals interfaces in spintronic devices.
    Strain drived band aligment transition of the ferromagnetic VS2/C3N van der Waals heterostructure
    Jimin Shang(商继敏), Shuai Qiao(乔帅), Jingzhi Fang(房景治), Hongyu Wen(文宏玉), and Zhongming Wei(魏钟鸣)
    Chin. Phys. B, 2021, 30 (9): 097507.   DOI: 10.1088/1674-1056/ac0cd1
    Abstract161)   HTML0)    PDF (3542KB)(207)      
    Exploring two-dimensional (2D) magnetic heterostructures is essential for future spintronic and optoelectronic devices. Herein, using first-principle calculations, stable ferromagnetic ordering and colorful electronic properties are established by constructing the VS2/C3N van der Waals (vdW) heterostructure. Unlike the semiconductive properties with indirect band gaps in both the VS2 and C3N monolayers, our results indicate that a direct band gap with type-Ⅱ band alignment and p-doping characters are realized in the spin-up channel of the VS2/C3N heterostructure, and a typical type-Ⅲ band alignment with a broken-gap in the spin-down channel. Furthermore, the band alignments in the two spin channels can be effectively tuned by applying tensile strain. An interchangement between the type-Ⅱ and type-Ⅲ band alignments occurs in the two spin channels, as the tensile strain increases to 4%. The attractive magnetic properties and the unique band alignments could be useful for prospective applications in the next-generation tunneling devices and spintronic devices.
    Controlled vapor growth of 2D magnetic Cr2Se3 and its magnetic proximity effect in heterostructures
    Danliang Zhang(张丹亮), Chen Yi(易琛), Cuihuan Ge(葛翠环), Weining Shu(舒维宁), Bo Li(黎博), Xidong Duan(段曦东), Anlian Pan(潘安练), and Xiao Wang(王笑)
    Chin. Phys. B, 2021, 30 (9): 097601.   DOI: 10.1088/1674-1056/ac0cd9
    Abstract279)   HTML3)    PDF (3458KB)(286)      
    Two-dimensional (2D) magnetic materials have aroused tremendous interest due to the 2D confinement of magnetism and potential applications in spintronic and valleytronic devices. However, most of the currently 2D magnetic materials are achieved by the exfoliation from their bulks, of which the thickness and domain size are difficult to control, limiting the practical device applications. Here, we demonstrate the realization of thickness-tunable rhombohedral Cr2Se3 nanosheets on different substrates via the chemical vapor deposition route. The magnetic transition temperature at about 75 K is observed. Furthermore, van der Waals heterostructures consisting of Cr2Se3 nanosheets and monolayer WS2 are constructed. We observe the magnetic proximity effect in the heterostructures, which manifests the manipulation of the valley polarization in monolayer WS2. Our work contributes to the vapor growth and applications of 2D magnetic materials.
    Review of Raman spectroscopy of two-dimensional magnetic van der Waals materials
    Yu-Jia Sun(孙宇伽), Si-Min Pang(庞思敏), and Jun Zhang(张俊)
    Chin. Phys. B, 2021, 30 (11): 117104.   DOI: 10.1088/1674-1056/ac1e0f
    Abstract287)   HTML4)    PDF (3052KB)(348)      
    Ultrathin van der Waals (vdW) magnets provide a possibility to access magnetic ordering in the two-dimensional (2D) limit, which are expected to be applied in the spintronic devices. Raman spectroscopy is a powerful characterization method to investigate the spin-related properties in 2D vdW magnets, including magnon and spin-lattice interaction, which are hardly accessible by other optical methods. In this paper, the recent progress of various magnetic properties in 2D vdW magnets studied by Raman spectroscopy is reviewed, including the magnetic transition, spin-wave, spin-lattice interaction, symmetry tuning induced by spin ordering, and nonreciprocal magneto-phonon Raman scattering.
    Magnetic two-dimensional van der Waals materials forspintronic devices
    Yu Zhang(张雨), Hongjun Xu(许洪军), Jiafeng Feng(丰家峰), Hao Wu(吴昊), Guoqiang Yu(于国强), and Xiufeng Han(韩秀峰)
    Chin. Phys. B, 2021, 30 (11): 118504.   DOI: 10.1088/1674-1056/ac2808
    Abstract374)   HTML2)    PDF (3012KB)(316)      
    Magnetic two-dimensional (2D) van der Waals (vdWs) materials and their heterostructures attract increasing attention in the spintronics community due to their various degrees of freedom such as spin, charge, and energy valley, which may stimulate potential applications in the field of low-power and high-speed spintronic devices in the future. This review begins with introducing the long-range magnetic order in 2D vdWs materials and the recent progress of tunning their properties by electrostatic doping and stress. Next, the proximity-effect, current-induced magnetization switching, and the related spintronic devices (such as magnetic tunnel junctions and spin valves) based on magnetic 2D vdWs materials are presented. Finally, the development trend of magnetic 2D vdWs materials is discussed. This review provides comprehensive understandings for the development of novel spintronic applications based on magnetic 2D vdWs materials.
    Carrier and magnetism engineering for monolayer SnS2 by high throughput first-principles calculations
    Qing Zhan(詹庆), Xiaoguang Luo(罗小光), Hao Zhang(张皓), Zhenxiao Zhang(张振霄), Dongdong Liu(刘冬冬), and Yingchun Cheng(程迎春)
    Chin. Phys. B, 2021, 30 (11): 117105.   DOI: 10.1088/1674-1056/ac2805
    Abstract157)   HTML0)    PDF (1152KB)(98)      
    Two-dimensional (2D) semiconducting tin disulfide (SnS2) has been widely used for optoelectronic applications. To functionalize SnS2 for extending its application, we investigate the stability, electronic and magnetic properties of substitutional doping by high throughput first-principles calculations. There are a lot of elements that can be doped in monolayer SnS2. Nonmetal in group A can introduce p-type and n-type carriers, while most metals in group A can only lead to p-type doping. Not only 3d, but also 4d and 5d transition metals in groups VB to VⅢB9 can introduce magnetism in SnS2, which is potentially applicable for spintronics. This study provides a comprehensive view of functionalization of SnS2 by substitutional doping, which will guide further experimental realization.
    Observation of magnetoresistance in CrI3/graphene van der Waals heterostructures
    Yu-Ting Niu(牛宇婷), Xiao Lu(鲁晓), Zhong-Tai Shi(石钟太), and Bo Peng(彭波)
    Chin. Phys. B, 2021, 30 (11): 117506.   DOI: 10.1088/1674-1056/ac1e1d
    Abstract228)   HTML4)    PDF (1106KB)(156)      
    Two-dimensional ferromagnetic van der Waals (2D vdW) heterostructures have opened new avenues for creating artificial materials with unprecedented electrical and optical functions beyond the reach of isolated 2D atomic layered materials, and for manipulating spin degree of freedom at the limit of few atomic layers, which empower next-generation spintronic and memory devices. However, to date, the electronic properties of 2D ferromagnetic heterostructures still remain elusive. Here, we report an unambiguous magnetoresistance behavior in CrI3/graphene heterostructures, with a maximum magnetoresistance ratio of 2.8%. The magnetoresistance increases with increasing magnetic field, which leads to decreasing carrier densities through Lorentz force, and decreases with the increase of the bias voltage. This work highlights the feasibilities of applying two-dimensional ferromagnetic vdW heterostructures in spintronic and memory devices.