Impact of local lattice disorder on spin and orbital orders in Ca _2-xSrxRuO₄

We have studied relationship between local lattice disorder, Ru 4d spin-orbit interaction, and global spin/orbital orders of Ca _2-xSrxRuO4 in the Sr- and Ca-rich regions of its phase diagram by using unrestricted Hartree-Fock calculation on a 8 × 8 RuO₄ lattice model. The calculations show that the local elongation of RuO ₆ octahedron in an antiferromagnetic insulator Ca₂RuO ₄ induces local orbital change with making the Mott gap narrower. On the other hand, local compression of RuO₆ octahedron in a paramagnetic metal Sr₂RuO₄ induces global antiferromagnetic or ferromagnetic states. This result is consistent with a recent systematic μSR study by Carlo et al. which has revealed the static antiferromagnetic order at low temperature in the Sr-rich region.