Two-dimensional inverse double sandwich CoB₇: strain-induced non-magnetic to ferromagnetic transition

A full-scale structural search was performed using density functional theory calculations and a universal structural prediction evolutionary algorithm. This produced a lowest energy two-dimensional (2D) CoB₇ structure. The CoB₇-1 global minimum structure has unusual inverse double sandwich features. The structure consists of two outer borophene layers which are held together with a layer of Co atoms. This is a B-Co-B sandwich structure. Density functional theory calculations predict that CoB₇-1 has good thermodynamic stability, kinetic stability, and mechanical stability. The overall bonding framework is able to survive molecular dynamics annealing up to 1000 K for 10 ps. The DFT results predict that this structure is very stable and should be amenable to experimental synthesis. Surprisingly, we found that this material can change from a non-magnetic ground state to a ferromagnetic state under the influence of biaxial strain. This unusual property of having a magnetic transition while at the same temperature leads us to conclude that this new 2D CoB₇ crystal may have potential applications in future nano-magnetic devices at or near room temperature.