Two-dimensional (2D) magnetic materials are essential to developing high-performance spintronic devices. Recent experimental discoveries of several atomic thin 2D ferromagnetic materials have stimulated great interest in further exploring this fascinating class of materials. Here, combining an advanced crystal structure search method and extensive first-principles energetic and dynamic calculations, we have identified a planar ${\mathrm{CoB}}_6$ monolayer as a stable 2D ferromagnet. We show that the ferromagnetic ground state of the ${\mathrm{CoB}}_6$ monolayer remains robust in the ambient environment, and the magnetic stability and moment can be remarkably enhanced and tuned by external strain. Moreover, we propose feasible synthesis routes for the the newly predicted ${\mathrm{CoB}}_6$ monolayer, either by Co atom adsorption on the recently proposed ${\delta }_4$ boron sheet or by direct chemical growth. The present results establish a fundamental material and physics basis for synthesis and characterization of the ${\mathrm{CoB}}_6$ monolayer among the emerging 2D ferromagnetic materials.

• Received 7 September 2018
• Revised 20 November 2018

DOI:https://doi.org/10.1103/PhysRevB.99.045445