B4CN3 and B3CN4 monolayers as the promising candidates for metal-free spintronic materials

The search for candidates of spintronic materials, especially among the two-dimensional (2D) materials, has attracted tremendous attentions over the past decades. By using a particle swarm optimization structure searching method combined with density functional calculations, two kinds of boron carbonitride monolayer structures (B₄CN₃ and B₃CN₄) are proposed and confirmed to be dynamically and kinetically stable. Intriguingly, we demonstrate that the magnetic ground states of the two B _x C _y N _z systems are ferromagnetic ordering with a high Curie temperature of respectively 337 K for B₄CN₃ and 309 K for B₃CN₄. Furthermore, based on their respective band structures, the B₄CN₃ is found to be a bipolar magnetic semiconductor (BMS), while the B₃CN₄ is identified to be a type of spin gapless semiconductor (SGS), both of which are potential spintronic materials. In particular, carrier doping in the B₄CN₃ can induce a transition from BMS to half-metal, and its spin polarization direction is switchable depending on the doped carrier type. The BMS property of B₄CN₃ is very robust under an external strain or even a strong electric field. By contrast, as a SGS, the electronic structure of B₃CN₄ is relatively sensitive to external influences. Our findings successfully disclose two promising materials toward 2D metal-free spintronic applications.