Rational Design of Two-Dimensional High-Temperature Ferromagnet from HCP Cobalt

Abstract

Cobalt has the highest Curie temperature (T$_c$) among the elemental ferromagnetic metals and is in a Hexagonal Close-Packed (HCP) structure at room temperature. In this manuscript, HCP Co could be thinned to the thickness of several (n) unit cells along the $c$ axis and then passivated by halogen atoms, thus being named Co$_{2n}$X$_2$ (X = F, Cl, Br, and I). For Co$_2$X$_2$ and Co$_3$X$_2$, all of them are not only kinetically but also thermodynamically stable from the viewpoint of the phonon spectra and molecular dynamics. Moreover, we find that similar to the parent bulk phase, two-dimensional (2D) Co$_2$F$_2$, Co$_2$Cl$_2$ and Co$_3$X$_2$ (X = Cl, Br, and I) still show ferromagnetic metal within the Stoner model but Co$_2$X$_2$ (X = Br, and I) show ferromagnetic half-metal with the coexistence of the metallic behavior for one spin and the insulating behavior for the other spin. Taking into account the spin-orbit coupling (SOC), the easy-magnetization axis is within the plane where the magnetization is isotropic, making them look like 2D XY magnet. Applying a critical biaxial strain could lead to the easy-magnetization axis changing from the in-plane to out-of-plane direction. Finally, we use classical Monte Carlo simulations to estimate T$_c$ which are as high as 957 and 510 K for Co$_2$F$_2$ and Co$_2$Cl$_2$, respectively, because of the direct, strong exchange interaction between the nearest Co atoms. Different from being obtained by mechanical or liquid exfoliation from van der Waals layered structures, our study opens up new possibilities to search novel 2D ferromagnets from the elemental ferromagnets and renders opportunities for realizing realistic ultra-thin spintronic devices.