Two-dimensional (2D) ferromagnetic semiconductors are considered to be one of the most promising candidates for spintronics. Nonetheless, the 2D ferromagnetic semiconductors with large perpendicular magnetic anisotropy and high Curie temperature (T_c) are rarely reported. Here, through first-principles calculations, we predict a series of ferromagnetic semiconductors monolayer GdX₂ (X = Cl, Br, I) by utilizing the rare-earth Gd with large spin-orbit coupling effect. Monte Carlo simulations manifest that the monolayer GdX₂ show high T_c beyond 220 K, showing the promising applications in spintronics devices. Most interesting, three monolayer GdX₂ show different magnetic anisotropy, among which the easy axis of monolayer GdCl₂ is along the [001] direction, while monolayer GdBr₂ and GdI₂ possess in-plane magnetic anisotropy. Based on the analysis of the contributions from different atomic orbitals to magnetic anisotropy energy (MAE), it is found that the competition between the contributions of Gd-p orbitals, Gd-d orbitals, and p orbitals of halogen atoms to MAE transfers easy axis from out-of-plane to in-plane from Cl to I. The lattice mismatch between 2D materials and substrate definitely affects the properties of 2D materials, −5 % ∼ +5 % strain have been applied on the monolayer GdX₂. The MAE of monolayer GdCl₂ can increase from 111 to 190 μeV/f.u. under −5 % compressive strain. Besides, the easy axis of monolayer GdBr₂ transfers from in-plane to out-of-plane over −3 % compressive strain. Furthermore, all the monolayer GdX₂ have excellent thermal and dynamic stabilities. The present study is useful to design functional 2D materials with large perpendicular magnetic anisotropy and high T_c for the next generation spintronics.

二维 (2D) 铁磁半导体被认为是自旋电子学最有希望的候选者之一。尽管如此，具有大垂直磁各向异性和高居里温度（T _c）的二维铁磁半导体很少被报道。在这里，我们通过第一性原理计算，利用具有大自旋轨道耦合效应的稀土 Gd预测了一系列铁磁半导体单层 GdX ₂ (X = Cl, Br, I)。蒙特卡罗模拟表明，单层 GdX ₂显示出超过 220 K 的高 T _c，显示了在自旋电子器件中的有前途的应用。最有趣的，三个单层 GdX ₂表现出不同的磁各向异性，其中单层GdCl ₂的易轴沿[001]方向，而单层GdBr ₂和GdI ₂具有面内磁各向异性。通过分析不同原子轨道对磁各向异性能（MAE）的贡献，发现Gd- p轨道、Gd- d轨道和卤原子p轨道对MAE的贡献之间的竞争转移易轴从面外到面内，从 Cl 到 I。2D 材料和基板之间的晶格失配肯定会影响 2D 材料的性能，-5 % ∼ +5 % 应变已应用于单层 GdX ₂. 在-5%压缩应变下，单层GdCl ₂的MAE可以从111增加到190 μeV/fu。此外，单层 GdBr ₂的易轴在 -3% 的压缩应变范围内从面内转移到面外。此外，所有单层GdX ₂都具有优异的热稳定性和动态稳定性。本研究有助于为下一代自旋电子学设计具有大垂直磁各向异性和高 T _{c 的}功能性二维材料。