Modified Curie-Weiss law for <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>j</mml:mi><mml:mi>eff</mml:mi></mml:msub></mml:math> magnets

In spin-orbit-coupled magnetic materials, the usually applied Curie-Weiss law can break down. This is due to potentially sharp temperature dependence of local moments. We therefore propose a modified formula suitable for analysis experimental susceptibility. show octahedrally coordinated materials ${d}^{5}$ filling that Weiss constant obtained from improved in excellent agreement with calculated microscopic exchange interactions. Reanalyzing measured susceptibility several Kitaev candidate resolves apparent discrepancies between various experiments regarding magnitude and anisotropies underlying couplings.