Coupled charge-spin-orbital state in Fe- or Co-doped Sr2IrO4

Abstract

We studied the electronic structure of Fe- or Co-doped Sr${}_{2}$IrO${}_{4}$, using density functional calculations including spin-orbit coupling and electron correlation. Our results show that upon the doping ($x\ensuremath{\le}$ 0.5), a Fe${}^{3+}$-Ir${}^{5+}$ or Co${}^{3+}$-Ir${}^{5+}$ charge state appears on the Ir${}^{4+}$ matrix. Contrary to the effective $j=1/2$ state of the Ir${}^{4+}$ ion due to a strong spin-orbit coupling, the Ir${}^{5+}$ ($5{d}^{4}$, ${t}_{2g}^{4}$) ion could form a closed ${j}_{\mathrm{eff}}=3/2$ subshell and hence behave like a singlet. The Fe${}^{3+}$ and Co${}^{3+}$ ions are both in a high-spin state, with their respective $S=5/2$ and 2, and the Co${}^{3+}$ ion also carries a big orbital moment. This coupled charge-spin-orbital state, assisted by spin-orbit coupling and electron correlation, gives rise to an insulating behavior of Fe- or Co-doped Sr${}_{2}$IrO${}_{4}$ and accounts for their varying magnetic moments.