Susceptibility anisotropy and its disorder evolution in models for Kitaev materials

Mott insulators with strong spin-orbit coupling display a strongly anisotropic response to applied magnetic fields. This applies in particular to Kitaev materials, with alpha-RuCl3 and Na2IrO3 representing two important examples. Both show a magnetically ordered zigzag state at low temperatures, and considerable effort has been devoted to properly modeling these systems in order to identify routes towards realizing a quantum spin liquid. Here, we investigate the relevant Heisenberg-Kitaev-Gamma model primarily at elevated temperatures, focusing on the characteristic anisotropy between the in-plane and out-of-plane uniform susceptibility. For alpha-RuCl3, we find that the experimentally observed anisotropy, including its temperature dependence, can be reproduced by combining a large off-diagonal Gamma(1) coupling with a moderate g-factor anisotropy. Moreover, we study in detail the effect of magnetic dilution and provide predictions for the doping evolution of the temperature-dependent susceptibilities.