Gadolinium Halide Monolayers: A Fertile Family of Two-Dimensional 4f Magnets

Two-dimensional (2D) magnets have great potentials for applications in next-generation information devices. Since the recent experimental discovery of intrinsic 2D magnetism in monolayer CrI3 and few-layer Cr2Ge2Te6, intense studies have been stimulated in pursuing more 2D magnets and revealing their intriguing physical properties. In comparison to the magnetism based on 3d electrons, 4f electrons can provide larger magnetic moments and stronger spin–orbit coupling but have been much less studied in the 2D forms. Only in very recent years have some exciting results been obtained in this area. In this mini-review, we will introduce some recent progress in 2D Gd halides from a theoretical aspect. It is noteworthy that 4f and 5d orbitals of Gd both play key roles in these materials. For GdX2 (X = I, Br, Cl, F) monolayers and related Janus monolayers, robust ferromagnetism with large exchanges comes from the 4f7 + 5d1 hybridization of Gd2+. The spatially expanded 5d electrons act as a bridge to couple localized 4f spins. For GdX3 monolayers, the intercalation of metal atoms can dope electrons into Gd's 5d orbitals, which leads to numerous intriguing physical properties, such as ferroelasticity, ferromagnetism, and anisotropic conductance. In brief, Gd halides establish an effective strategy to take advantage of f-electron magnetism in 2D materials.