Tuning $J_{\rm eff}$=1/2 magnetism and anisotropy in Sr$_2$IrO$_4$ through Ru substitution

Iridates can hosts a spin-orbit entangled ground state with $J_{\rm eff}$=1/2 magnetic moments. Mapping of these pseudo-spins onto several distinct crystal structures has led to the uncovering of a variety of novel properties, such as Kitaev physics and Weyl semimetals. Sr$_2$IrO$_4$ is a canonical example of a $J_{\rm eff}$=1/2 material and much interest has focused on the predictions of unconventional superconductivity driven by analogies to cuprates. In particular, proximity to a superconducting phase was strongly suggested by the magnetic excitation spectra which revealed striking similarities to La$_2$CuO$_4$ and was able to be described within a pure Heisenberg model consisting of spin-1/2 on a square lattice. We have investigated the series Sr$_2$Ir$_{\rm 1-x}$Ru$\rm _x$O$_4$ with neutron and resonant x-ray scattering and found an evolution of magnetism from ab-plane to c-axis aligned spins [1]. Despite this change the $J_{\rm eff}$=1/2 moments remain robust. By probing the excitations we observed a large spin-gap (40 meV) that indicates an alteration of the magnetic anisotropy associated with the parent. [1] S. Calder et al., PRB 92, 165128 (2015). [2] S. Calder et al., arXiv:1610.03399 (2016).