The monolayer \(\hbox {C}_4 \hbox {N}_3\) BN, short for s-triazine g- \(\hbox {C}_4\hbox {N}_3\) with B and pyrrolic N tailored at vacant sites, is investigated under in-plane strain, with a focus on its magnetism. Our density functional study shows that the half-metallic ground state and ferrimagnetic order persist up to considerable strain from −7% to 10%, and temperatures as high as 900 K. At the critical uniaxial strain of approximately −9.30% and 10.90%, or symmetric biaxial strain of −7.62% and 17.38%, this ferrimagnetic half-metal turns into ferromagnetic semiconductors, all with the magnetization 1 \(\upmu \) B per unit cell. We present a simple explanation for that metamagnetism in the system by means of spin charge transfer and by counting its atomic valencies in the unit cell. Our finding adds to and enriches physicochemical understanding of the magnetism in carbon nitride-based half-metals.
