Abstract
1. Introduction
Because of the prediction of Landau, people are always uncertain of the existence of two-dimensional (2D) materials before graphene was exfoliated successfully[
The appearance of van der Waals heterostructure (vdWH) engineering is accompanied by the in-depth study of 2D materials[
In this review, we summarize recent researches on 2D FM materials and 2D FM vdWHs, including the intrinsic physical properties of 2D FM monolayers and 2D FM vdWHs, as well as the regulation of physical properties by some external conditions such as defect, doping, strain, and electric field. We also discuss the developing trend of 2D FM monolayers and vdWHs. We believe that it can help readers to further understand 2D FM materials.
2. Research about 2D FM materials
2.1. Intrinsic physical properties of 2D FM materials
We first show the crystal model of CrI3 and Cr2Ge2Te6 in Fig. 1(a). It can be seen that the atomic structure of CrI3 is simpler, which means that it has higher symmetry. And Figs. 1(b) and 1(c) display the band structures of monolayer CrI3 and Cr2Ge2Te6, respectively, from which we can see that these two monolayers are both ferromagnetic semiconductors. Also, their Curie temperature (Tc) and magnetic anisotropy have been systematically studied[
Figure 1.(Color online) (a) The structure diagram of monolayer CrI3 (left) and monolayer Cr2Ge2Te6 (right). Reprinted with permission from Refs. [
2.2. Regulation of physical properties of 2D FM materials
In addition to study the intrinsic physical properties of two-dimensional ferromagnet, it is also very important to study the effects of external conditions such as the defect[
Figure 2.(Color online) (a) The I-vacancies models of monolayer CrI3. Reprinted with permission from Ref. [
Through these studies, we can see that FM materials under 2D limit possess unusual properties in many aspects. I believe this will attract more and more attention, and there will be more groundbreaking and meaningful results in the future.
2.3. More 2D FM materials with higher Tc
Compared with the past, people simply obtained a kind of new material or a class of new materials by atomic substitution, but now people have begun to search for new 2D materials quickly and systematically using high-throughput calculation methods[
Figure 3.(Color online) (a) A schematic diagram to illustrate the search procedure for 2D FM materials. Reprinted with permission from Ref. [
At present, people have gradually established a 2D material database[
Although there are many 2D FM materials predicted by theory, very few have been successfully prepared in experiments, and most of the Tc are much lower than room temperature, such as monolayer CrI3 (Tc = 45 K)[
In theory, Zhao et al. systematically screen out five high temperature 2D FM materials (LaCl, YCl, ScCl, LaBr2, and CrSBr with Tc > 200 K) using the first-principle calculation methods [
Figure 4.(Color online) (a) The diagram of
In experiments, it has been mentioned that different research groups have tried to increase TC of materials through some external conditions. At the same time, people are trying to explore new 2D intrinsic ferromagnetic materials with room temperature. For example, VSe2 and MnSe2 monolayers have been successfully obtained in experiments, and the results show that they have high Tc with 330 K for VSe2 and 300 K for MnSe2[
3. 2D FM vdWHs
Not only 2D ferromagnets themselves have many novel physical properties, but also many novel physical phenomena will appear when they are stacked with other nonmagnetic 2D materials, such as proximity induced spin polarization[
3.1. Spin-polarized band structures
When the ferromagnetic materials and non-magnetic materials are stacked forming the vdWHs, due to the spin-polarized electronic structure of magnetic materials, the heterostructures will also possess different electronic states in different spin channels. As shown in Fig. 5(a), the Mg(OH)2/VS2 vdWH possess the spin-polarized band structure with the conduction-band maximum (CBM) are located at K and M point in the spin-up channel and spin-down channel, respectively. Obviously, the band gaps of different spin channels are different with 0.14 eV for spin up and 0.57 eV for spin down. Moreover, it can also be seen from the band alignment that there are different band offsets in different spin channels, see Fig. 5(b), which means that there are different separation possibilities for electrons and holes in spin-up and spin-down channels. In addition, Xiong et al. also found that the external electric field can induce the different band-alignment transition in different spin channels[
Figure 5.(Color online) (a) The spin-polarized band structure of Mg(OH)2/VS2 heterostructure. (b) The band alignment and work function of the heterostructure, referring to the vacuum level (
3.2. Valley polarization and quantum anomalous Hall effect
More and more attention has been paid for the valley-physics properties of graphene and monolayer transition metal dichalcogenides (TMDC) MX2 (M = Mo, W; X = S, Se, Te). Compared with graphene, the monolayer MX2 have stronger spin orbit coupling (SOC) effect, which leads to obvious energy level splitting at K and K’ valleys, and these two valleys are energetically degenerate because of the time-reversal symmetry. Also, the broken inversion symmetry features two inequivalent valleys with different angular momenta, as shown in Fig. 6(a). Based on the above factors, we can implement the valley hall effect in monolayer MX2 where carriers in different valleys flow to opposite transverse edges when applying an in-plane electric field[
Figure 6.(Color online) (a) The energy diagram of the monolayer WSe2 at the K, K’ valleys.
With the emergence of 2D FM materials, people began to study the valley polarization in 2D/2D systems. Recently, Liu et al. have studied the effect of stacking mode on valley polarization in WSe2/CrI3 vdWH[
We also note that a new type of multiferroic vdWHs has recently been proposed[
4. Conclusion
In conclusion, we summarize the recent theoretical advances on intrinsic 2D FM materials and vdWHs, including the intrinsic electronic structure, magnetism, Tc, valley polarization, band alignment and so on. These studies show that 2D FM materials and vdWHs possess many novel physical properties, and they are promising materials to be applied in spintronic and valleytronic nanodevices. Moreover, many methods, such as vacancy, doping, strain and external electrical field, can be used to regulate the electronic structure, magnetism, Tc, valley polarization, and band alignment of 2D FM materials and vdWHs, indicating the properties of 2D FM materials and vdWHs are highly tunable. However, although many 2D FM materials have been predicted in theory, very few of them have been successfully prepared in experiments. In addition, most Tc of 2D FM materials are much lower than room temperature, which limit the applications of these materials in practice. Therefore, it is necessary to find the 2D FM materials with the higher Tc or find the method of raising the Tc. In short, although we have made many breakthroughs in this field recently, there are still many problems waiting for us to find and solve.
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