First-principles calculations of two-dimensionalmagnetic materials

Abstract

Two-dimensional (2D) magnetic materials have attracted intensive research interest owing to their extraordinary electronic properties, abundant tunable functionalities and promising device applications. Early experiments attempted to extrinsically introduce magnetism into 2D nonmagnetic materials, for instance, by defect engineering or magnetic proximity. These extrinsic engineering strategies usually lead to weak magnetism and are difficult to control experimentally and thus unfavorable for applications. The discovery of intrinsic ferromagnetism in 2D insulators CrGeTe3 and CrI3 has stimulated great research effort in the field. Besides, different kinds of 2D magnetic materials have been synthesized recently, including itinerant ferromagnets Fe3GeTe2, VSe2 and MnSe x , antiferromagnets NiPS3 and FePS3, and intrinsic magnetic topological material MnBi2Te4. 2D magnetic materials can be used to construct van der Waals heterojunctions to realize novel physical phenomena such as multiferroics and quantum anomalous Hall effect, as well as to develop spintronic devices such as magnetic tunneling junctions and spin-field-effect transistors. On the other hand, first-principles calculations have been widely applied in the study of condensed matter physics and material science. Density functional theory (DFT) in the framework of Kohn-Sham scheme is one of the most popular methods for first-principles calculations. The critical issue of DFT is to use suitable exchange-correlation functional for achieving computational efficiency and accuracy simultaneously. Commonly used exchange-correlation functional, such as local density approximation and generalized gradient approximation, can successfully describe various properties of most compounds comprised of main group elements, such as atomic and electronic structures. However, these local or semi-local functional cannot provide an accurate description of localized or strongly correlated electronic states, including open-shell d and f orbitals. More advanced methods, such as the DFT+ U and hybrid functional methods, are generally desired to study 2D magnetic materials. In this review, we firstly introduced general theoretical background and computational methods on magnetism, including theory of magnetic exchange interactions and magnetic anisotropy. Magnetic exchange interactions determine magnetic orders, which are classified into local magnetic exchange and itinerant magnetic exchange according to electronic properties of the system. Magnetic anisotropy is contributed by shape anisotropy and magnetocrystalline anisotropy, which are originated from magnetic dipolar interaction and spin-orbit coupling, respectively. Then we presented theoretical progresses on the study of 2D magnetic materials, focusing on four representative material systems: (1) 2D magnetic insulators CrGeTe3 and CrI3; (2) 2D ferromagnetic metal Fe3GeTe2; (3) 2D intrinsic magnetic topological material MnBi2Te4; (4) 2D high-temperature quantum anomalous Hall insulator LiFeSe. Through these example studies, we systematically introduced fundamental mechanisms of exchange coupling and magnetic anisotropy, feasible ways to tune electronic and magnetic properties, as well as new physics introduced by the interplay of magnetism and topology. Finally we offered an outlook on future first-principles research of 2D magnetic materials.