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[8, 9]. In addition, Zhong et al. find that the monolayer CrI3 possesses a giant magneto band-structure effect, i.e. the change of spin orientation can significantly modify the band structure of this material. Here, rotating the magnetic moment of CrI3 from out-of-plane to in-plane will cause a direct-to-indirect bandgap transition (Fig. 1(f)), which can result in a magnetic field controlled photoluminescence. They also find a significant change of Fermi surface with different magnetization directions, giving rise to giant anisotropic magnetoresistance. Moreover, the spin reorientation is found to modify the topological states. Their work opens a new paradigm for spintronics applications. Xu et al. have studied phonon edge states and phonon Berry Phase with the monolayer CrI3 as a plateform through first-principles calculations. They find that the phonon Berry phase is quantized to ± π at two inequivalent valleys and the phonon edge states terminated at the projection of phonon Dirac cones (Figs. 1(d) and 1(e)). This work extend the knowledge of valley physics, providing wide applications of topological phonons.
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, charge doping[14, 36], electric field[13, 17, 37] and strain[17, 38] on its physical properties, such as electronic structure, magnetic anisotropy, Tc and so on. Wang et al. find that the I vacancies on the surface of monolayer CrI3 can not only can enhance the intrinsic ferromagnetism of monolayer CrI3 but also induce switchable out-of-plane electric polarization (Fig. 2(a)), and the I vacancies do not break the semiconducting nature of CrI3. In addition, they also find that the polarization direction can be reversed by switching the position of I vacancices and this method is also applicable to many other metal trihalides. This work provides a new way for people to realize and regulate the electric polarization in low-dimensional systems, which is meaningful for engineering the multifunctional nanodevices. According to the study of Wu et al., now we have already known that electron or hole doping can transform CrI3 from semiconductor to metal, and improve the stability of its FM ground state, see Figs. 2(b) and 2(c). In addition, the electronic structure and the magnetism regulation of the 2D ferromagnet can also be realized by the strain engineering and the external electric field, see Figs. 2(d) and 2(e). For example, Gong et al. found that by applying a vertical electric field, the bilayer VS2 can convert from A type antiferromagnet to half metal. Fig. 2(f) shows the side view of bilayer VS2. The regulation of the electric field on the energy levels shows the opposite trends in different spin channels, leading to the gradual decrease of the gap of spin-α states and the opening of the gap of the spin-β states. When the electric field reaches a certain value, the band gap of one spin state is zero, while the other spin state is still an insulated state. Thus, the 100% spin polarization current will be generated in bilayer VS2, see Fig. 2(g). Their research is very meaningful for designing the spin field effect transistor. What’s more, Wu et al. also point out that the energy difference between FM state and AFM state directly corresponds to the Tc. From this we can conclude that the above methods can certainly change the Tc of the material. And in experiment, Zhang et al. have proved that the Tc of Fe3GeTe2 can be increased to room temperature by electric field. In addition, the regulation of 2D ferromagnetism by external electric field and electrostatic doping has been realized experimentally.
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.
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[39–41] such as structure search, machine learning and the like. Moreover, we can further screen 2D FM materials with excellent performance from the database through formula screening, geometry screening, manual screening and DFT calculations (Fig. 3(a)).
At present, people have gradually established a 2D material database, and are still trying to discover new 2D ferromagnets from theory and experiment. As shown in Fig. 3(b), Sun et al. theoretically find a class of new 2D ferromagnets, CrOCl and CrOBr monolayers, which can be obtained by exfoliating from the layered AFM bulk materials. Also, Lei et al. successfully synthesize vdW magnetic compound VI3 in experiment, and theoretically predict that the monolayer VI3 possesses the ferromagnetic-insulator property and are feasible to be exfoliated from the bulk (Fig. 3(c)). It can be seen that the combination of theoretical prediction and experimental verification will greatly accelerate the research process of 2D FM materials, and there will be more and more 2D FM materials be made in the future.
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), Cr2Ge2Te6 (Tc = 28 K), Fe3GeTe2 (Tc ≈ 19 K), which will limit the application of these materials in pragmatic spintronic devices. Therefore, it is urgent to find 2D FM materials with the higher Tc and the method of raising the Tc. The Journal of Science has published 125 of the most important issues, one of which is whether it is possible to produce a room-temperature FM semiconductor. Recently, some research progress has been made in these areas.
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, see Fig. 4(a). Kan et al. propose a method to raise the Tc. As shown in Fig. 4(b), they found that in double-orbital model, the ferromagnetism of the material is closely related to the virtual exchange gap. Thus, we can reduce the exchange gap by equivalent alloying the material, which can greatly enhance the FM coupling. They also validated the proposed theory with CrI3, CrGeTe3 and so on, and the results show that their Tc is increased by 3–5 times by alloying. In addition, Yang et al. also raise a method to achieve high Tc. They design a special organometallic frameworks connected by antiaromatic rings, which can enhance the Tc of 2D FM semiconductors much higher than room temperature, see Figs. 4(c)–4(e).
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[19, 20].