Prediction of two-dimensional $π$-electron half-metallic ferrimagnets
We propose a strategy to obtain conducting organic materials with fully spin-polarized Fermi surface, lying at a singular flat band, with antiferromagnetically coupled magnetic moments that reside in pi-orbitals of nanographenes. We consider a honeycomb crystal whose unit cell combines two different molecules with $S=1/2$: an Aza-3-Triangulene, a molecule with orbital degeneracy, and a 2-Triangulene. The analyzed system is half-metallic with a ferrimagnetic order, presenting a zero net total magnetic moment per unit cell. We combine density functional theory calculations with a Hubbard model Hamiltonian to compute the magnetic interactions, the bands, the intrinsic Anomalous Hall effect, and the collective spin excitations. We obtain very large intermolecular exchange couplings, in the range of 59 meV. Based on the spin excitation dispersion, we estimate thermal stability in the range of 100 Kelvin. When the magnetization is off-plane, intrinsic spin orbit coupling in graphene opens up a topological gap that, despite being very small, leads to a quantized Hall conductance in the tens of mK range, but is thermally smeared above 1 Kelvin.