Magnetic state of K_0.8Fe_1.6Se₂ from a five-orbital Hubbard model in the Hartree-Fock approximation

Motivated by the recent discovery of Fe-based superconductors close to an antiferromagnetic insulator in the experimental phase diagram, here the five-orbital Hubbard model (without lattice distortions) is studied using the real-space Hartree-Fock approximation, employing a 10×10 Fe cluster with Fe vacancies in a √5×√5 pattern. Varying the Hubbard and Hund couplings, and at electronic density n=6.0, the phase diagram contains an insulating state with the same spin pattern as observed experimentally, involving 2×2 ferromagnetic plaquettes coupled with one another antiferromagnetically. The presence of local ferromagnetic tendencies is in qualitative agreement with Lanczos results for the three-orbital model also reported here. The magnetic moment ∼3μ_B/Fe is in good agreement with experiments. Several other phases are also stabilized in the phase diagram, in agreement with recent calculations using phenomenological models.