Theoretical simulation and design of two-dimensional ferromagnetic materials

Spintronic devices, using the spins of electrons as information processing, have generated world-wide interest. Just as graphene, transition-metal dichalcogenides, and black phosphorus revolutionized condensed matter and materials engineering, the discovery of two-dimensional (2D) van der Waals (vdW) magnetic materials is expected to open a new horizon in material science and enable the potential development of spintronics. In fact, 2D magnetism has been investigated for decades while the experimental validation was unable to achieve till recently. The recent exciting 2D ferromagnetic breakthroughs, such as monolayers CrX3 (X = Cl, Br), monolayer Fe3GeTe2, and bilayer CrGeTe3 from their vdW bulk down to atomically thin, have also pushed forward researches on novel magnetic properties and creative concepts. In contrast to the traditional magnetic thin films, 2D vdW ferromagnetic materials (FM) largely decouple from the substrates, allow electrical control and are open to chemical functionalization. Without clear targets or guidelines, traditional trial-and-error experiments face the fundamental challenges of long time and high costs. Computational simulations, which serving as an important first step in exploring possible applications of new materials, can not only predict novel 2D materials but also suggest their possible synthesis routes. Many interesting cases, such as the growth of 2D borophene (B) and tellurene (Te), thermoelectricity in tin selenide (SnSe), ferroelectricity in tin telluride (SnTe), and high carrier mobilities in black phosphorene, have been confirmed by experiments, showing the accuracy of computational methods and their ability in facilitating experimental exploration in 2D space. Compared to other computational methods, the first-principles method, which has been the most widely used tools in designing new materials, only require a few basic physical constants and the atomic position coordinates. It is valuable mainly in two important aspects: (1) It can be used to predict and design new materials with novel properties, and (2) it provides understanding of the physics underlying the properties of new materials to replace the expensive and time-consuming physical test. Therefore, first-principles method based on density functional theory is effective for investigating new materials. In fact, the rapid development of 2D magnetic materials benefits from theoretical simulation. For example, the recent star ferromagnetic bilayer CrGeTe3, monolayer CrI3, and monolayer Fe3GeTe2 were also first predicted theoretically, and they have recently been experimentally made, which shows the strong power of first-principles calculations in designing these spintronics materials. In this review, we highlight the overall picture of recent progress, current challenges, and future prospects on theoretical design of FM materials. We hope this review provides basic understanding the importance of first-principles calculations in facilitating new discoveries and the accurate characterization of 2D FM materials. To achieve this, we first give the reason why ferromagnetic order exists in 2D space at finite temperature theoretically. Then, we summarize the discovery processes and magnetic properties of recent landscape of several 2D ferromagnetic semiconductors, metals, and half-metals, using 2D CrI3, CrGeTe3, Fe3GeTe2, and FeCl2 as the examples, respectively. Finally, we highlight the existing problems of designed 2D FM materials and propose possible directions in computational simulations for further development. Of course, this review cannot cover all 2D FM materials, and readers can also refer to other recent reviews and references therein for more low-dimensional FM materials.