Direction control of the easy magnetization axis in the magnetic GdN and GdNX (X = F, Cl) monolayers

Two-dimensional (2D) multiferroics integrating ferromagnetism/antiferromagnetism and ferroelasticity hold potential for applications in information storage and magnetic response and have attracted intensive attention recently. The large magnetic moment originating from the half-filled 4f orbitals and the spontaneous structural polarization make monolayer (ML) GdN a potential magnetoelastic multiferroic material. First-principles calculations were performed to thoroughly examine the magnetic, ferroelastic, and magnetoelastic properties of MLs GdN and GdNX (X = F, Cl). Our calculations show that ML GdN is an antiferromagnetic (AFM) semiconductor, and MLs GdNX are ferromagnetic (FM) half-metals. The occurrence of FM Gd-F/Cl-Gd super-exchange interactions and the change of average Gd-N-Gd angles from 140.8° to 115.9° in GdNF and 118.2° in GdNCl triggered by the addition of F and Cl result in ferromagnetism in MLs GdNF and GdNCl, respectively. The large magnetic anisotropy energies (MAEs) demonstrate the strong preference of long-range AFM and FM ordering in MLs GdN and GdNX. Moreover, MLs GdN and GdNX exhibit in-plane MAEs with the same magnitude but opposite sign during the ferroelastic transition. The moderate ferroelastic energy barriers imply the reversible ferroelastic switching in MLs GdN and GdNX at room-temperature and the reversible switching of the in-plane easy magnetization axis. Surprisingly, ML GdN exhibits a negative Poisson's ratio (NPR), which is advantageous to its application in ferroelastic devices. These predictions underscore the potential of MLs GdN and GdNX as promising candidates for the next generation of multifunctional nanodevices.