Electron Correlations,Spin-Orbit Coupling,and Antiferromagnetic Anisotropy in Layered Perovskite Iridates Sr_2IrO_4

The effects of electron correlations and spin-orbit coupling on the magnetic anisotropy in the antiferromagnetically ordered 5 d perovskite iridates Sr_2IrO_4 is investigated theoretically using a microscopic model includes a realistic five-orbital tight-binding Hamiltonian, atomic spin-orbit coupling, and multi-orbital Hubbard interactions. Hartree-Fock approximation is applied to obtain the ground state properties with varying spin-orbit coupling and electron correlations.We demonstrate that the interplay between the atomic intraorbital Coulomb repulsion and the Hund's rule coupling leads to a remarkable variability of the resulting magnetic anisotropy at a constant nonzero spin-orbit coupling. At the same time, the preferred direction of the ordered antiferromagnetical moment remains unaltered upon changes in the strength of spin-orbit coupling.