Abstract
1. Introduction
In general, spintronics such as spin valves must involve in their structures multiple layers of magnetic thin films[
Liberating the degrees of freedom of lattice-matching as well as the demanding growth conditions had never been so facile until the day when graphene was exfoliated by simply using a scotch tape[
Magnetic vdW materials, by definition, are consisted of 2D layers with spontaneous spin polarization below their magnetic critical temperatures. A long-range spin ordering was long believed not to be available in 2D as predicted by the early theories based on the isotropic Heisenberg models[
Compared to their 3D counterparts, the advantages of magnetic 2D materials can be twofold. Firstly, 2D magnets can be seamlessly assembled with different rotational angles between the layers of different kinds, thus allowing band engineering through tunable moiré super lattices. Secondly, many of the 2D magnets can be implemented into in-plane field effect transistors, as they are semiconducting with Fermi levels gate tunable. It thus largely increases the opportunities for future spintronic applications, in terms of not only the variety of materials but also the emergent physical phenomena that arise from the novel 2D nanostructures. In this review, we will briefly describe the state-of-the-art spintronics based on 2D vdW materials and offer a new perspective by drawing a potential roadmap of vdW spintronics for the next decade.
2. The ‘pre-history’ of vdW magnets spintronics
Many of the vdW materials are non-magnetic. However, even in the early days of the 2D materials research, spin-related phenomena have already attracted vast attentions. Indeed, spin injection from magnetic electrodes into non-magnetic substrates can work as an in-plane configuration of spin valves[
Figure 1.(Color online) (a, b) Typical spin valve devices made of graphene[
Except for direct spin injections, MoS2 was also found to be a candidate for spin manipulations in an optical manner due to the spin-valley locking in this specific type of 2D material[
As stated above, before the isolation of 2D intrinsic vdW magnetic materials, spin-related phenomena have been widely studied in a broad range of vdW materials, which can be categorized in the volume of the ‘pre-history’ of intrinsic vdW magnets.
3. Exfoliated intrinsic vdW magnets spintronics
2D vdW magnets are not at all new materials, while they were just for unknown reasons not quite exfoliated before 2017. Actually, the bulk forms of layered magnetic compounds have been well characterized in terms of crystallographic and spin structures[
To date, experimental examinations together with theoretical predictions show that most spin exchange interactions in the 3D scenario (including direct Ising and XY interactions, and other indirect interactions) can prevail down to the 2D limit[
The successful exfoliation of vdW magnets is just the beginning of the game, like every new topic in condensed matter physics — people have to find new physics, as well as new applications out of them. In the coming sections, we will discuss a couple of examples of such efforts.
4. Spin valves based on exfoliated vdW magnets
Spin valves have planar configurations as indicated in Section 1 and in Fig. 2(a), as well as vertical configurations that sometimes take the advantage of electron tunneling by sandwiching a tunneling insulator between two ferromagnetic (FM) layers, as shown in Fig. 2(b). This FM-insulator (I)-FM structure is well known as tunneling magnetoresistance (TMR)[
Figure 2.(Color online) (a, b) Schematics of configurations for 2D spin valve devices, and (c) 2D spin filter tunnel junction (sf-TJ). (d–f) The first spin valve demonstrated using 2D vdW magnetic (Fe-doped TaS2) materials[
The first attempt of the vertical spin valve using vdW magnetic materials was realized in 2015, with two pieces of few-layered Fe-doped TaS2 as the FM layers, while the tunnel layer was oxides formed naturally between their interface (Figs. 2(d) and 2(e)[
Magnetic vdW materials can be easily exfoliated on a desktop and assembled into spintronic devices, which are free of epitaxial technology and in principle can be mass-produced by means of chemical vapor deposition (CVD) etc. It thus opens up a totally new page and shall give a profound impact on the future of the spin-valve industries. Nevertheless, the experimentally-demonstrated spin valves are, so far, working far below room temperature. The transfer/stacking method is yet to be optimized for batch production, which will also be discussed from a broad perspective in the last section of this review.
5. vdW magnetic tunnel junctions
Besides the FM–I–FM configuration, metal–FM–metal configuration, which has the insulating FM sandwiched between two metals, has also been used as spintronic device, often referred to as spin filter tunnel junction (sf-TJ), as shown in Fig. 2(c). As given in Table 1, many of the vdW magnetic materials (including CrI3, CrBr3 and etc.) have semiconducting gaps, and usually are quite insulating at low temperature, thus providing a unique chance for the study of 2D vdW sf-TJs and related tunneling physics.
Indeed, several experiments have confirmed that multilayered-CrX3 (X = I, Br, Cl) sf-TJs have extremely large magnetoresistance up to ~ 106%[
In addition to the applications stated above, sf-TJs made of vdW magnetic tunnel layers can serve in a new kind of vertical spin-related field effect transistor. In this scenario, transparent few-layered graphene gate electrodes are often equipped within the structure illustrated in Fig. 3(a), in the sense that optical probe can penetrate the graphene gate without catching any parasitic magnetic signals but the ones from the vdW magnetic tunneling layer itself. During measurements, gate voltages can be seamlessly applied, pumping electron in and out of the band structure of the target vdW magnet and sequentially affecting the magnetic parameters including coercivity, Curie temperature, and etc.[
Figure 3.(Color online) (a) Schematics of CrI3 sf-TJ[
It is noteworthy that, so far, investigations on sf-TJs that utilize antiferromagnetic vdW layers as tunnel barrier remain scarce. To some extent, both sf-TJs (Fig. 2(c)) and TMRs (Fig. 2(b)) can be used as logic unit and magnetic sensors[
6. Planar vdW magnetic field effect transistors
Not quite similar to the sf-TJ with insulating vdW magnetic tunnel layer mentioned above, a real planar structure that mimics the famed metal-oxide field effect transistor (FET) can also be achieved in vdW magnets. This will require naturally a semiconducting channel, or at least a conducting channel whose electron density of states can be gate tuned, via electrostatic or liquid gate techniques.
A series of vdW materials Cr2M2Te6 (M = Si, Ge) compounds were reported to be intrinsic magnetic semiconductors, with reported band gaps varying in the range of 0.4–1.2 eV[
Figure 4.(Color online) Optical image of several versions of spin-FETs based on magnetic vdW materials (a) semiconducting CrSiTe3[
It was known that even conventional metallic thin Fe, Co films can manifest tunable magnetic parameters via ionic gating[
Up to now, studies on vdW magnetic semiconducting FETs are still on the go. There is plenty of room to improve the performances, including the working temperature, and the interface between FM and AFM vdW semiconductors, and etc.. While vdW FM semiconductors are of great interest for planar FETs, it is noteworthy that there is the family of vdW AFM semiconductors, which are also promising for planar FETs, as well as opto-electronic applications.
7. Current-driven switching of vdW magnets
About three decades after the theoretical predictions[
When interfaced with a magnetic layer, the SHE in the non-magnetic layer with large SOC can exert an orbit torque (SOT) on the magnetic layer that can switch the direction of magnetization, thus giving rise to a current-driven spin flip. During the switching process, a small external magnetic field colinear with the current is required[
Indeed, the reports in 2019 indicate that it is absolutely feasible to replace the conventional magnetic film with vdW ferromagnets in the SOT structures[
Figure 5.(Color online) (a, b) Schematic and optical image of a typical Pt/FGT device[
At the current stage, SOT devices using vdW magnets as a platform are still far from mature to meet the applicational standard, such as free of an external magnetic field, low critical switching current density, and etc. Apparently, room temperature operation is needed, and the SOC layers are so far still conventional metallic thin films. There is thus a long way to go to address the above points to push the vdW SOT devices towards real applications.
It is worth noting that spin Hall effects can be also observed via magneto-optical Kerr measurements as experimentally evidenced in semiconducting TMDs[
8. Other possibilities and an outlook of vdW spintronics
In the previous sections a couple of typical vdW spintronic devices such as spin valves, spin filter tunnel junctions, and planar spin FET have been introduced, and we now come to a brief discussion on several other configurations of spintronic devices using vdW magnets as a platform. In Fig. 6, a variety of nanostructures for vdW spintronics are illustrated. It can be seen that spin-related electronic devices can in principle be built via a mechanical stacking method, giving rise to possible applications such as 2D heterostructure of multi-ferroics, vdW magnetic recording, and topological magnetic states, etc. Up to now, the emerging phenomena in 2D vertical multi-ferroics as well as topological magnetic states in the vdW systems are attracting great interests. We now propose a roadmap for exfoliated spintronics, as indicated in Fig. 7. In short, depending on the development of vdW materials, the future trend of the vdW spintronics can be classified into fundamental- and application-oriented directions.
Figure 6.(Color online) Illustration of different nanostructures for vdW spintronics.
Figure 7.(Color online) A roadmap for the exfoliated spintronics.
For fundamental research, the future tasks will be looking for new emerging phenomena including topological magnetic states (Skyrmion[
For application research, the top priority will be to find room temperature vdW magnets. And the mission for future application shall include spin torque transfer (or spin Hall related) devices, spin diodes, spin valves, vdW magnetic semiconductors (as compared to the bulk diluted magnetic semiconductors[
Finally, we would like to recall the fact that, in principle, vdW magnetic materials are compatible with mass production processes such as CVD methods[
Acknowledgment
This work is supported by the National Key R&D Program of China (No. 2017YFA0206302), and is supported by the National Natural Science Foundation of China (Grants No. 51627801). G.Y. thanks the finical supports from the National Natural Science Foundation of China (Grants No. 11874409). T. Yang acknowledges supports from the Major Program of Aerospace Advanced Manufacturing Technology Research Foundation NSFC and CASC, China (No. U1537204).
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