Novel Magnetism from the Spin-Orbit-Coupling Strength Alternation*

We show that bringing close two magnetic ions with strong and weak spin-orbit coupling, respectively, can yield strong ferromagnetic anisotropy from antiferromagnetic superexchange. We applied this novel exchange anisotropy generating mechanism to explain the unique magnetism of the copper-iridium oxide Sr$_3$CuIrO$_6$ containing chains of alternating Cu(II) and Ir(IV) ions. The calculated large-gap spin excitation spectrum agrees well with the Ir $L_3$ edge resonant inelastic x-ray scattering experiment. Our findings point to novel magnetic behavior to be expected in mixed $3d-5d$ transition-metal systems via exchange pathways that are absent in pure $3d$ or $5d$ systems. Reference: W.-G. Yin et al., PRL 111, 057202 (2013).